1. Intro_to_Amateur_Astronomy[8]

1. Intro_to_Amateur_Astronomy[8]
θωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψ
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IntroductiontoAmateurAstronomy
TCAAGuide#1
CarlJ.Wenning
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomersAllRightsReserved
1
IntroductiontoAmateurAstronomy
TCAAGuide#1
VERSION1.3
JULY5,2016
ABOUTTHISGUIDE:
This Introduction to Amateur Astronomy guide – hopefully only the first of several such TCAA guides – was created after
severalyearsofthinkingaboutthequestionofwhymorepeopledon’tbecomeamateurastronomers.Thisremainsamystery
for most amateur astronomers and astronomy clubs, but especially for the TCAA where we have a friendly and stable
organization,outstandingresources,agoodobservingsite,asolidwebpresence,agoodnewsletter,membershipbrochures,
regular publicity, and plenty of member education and public outreach. Despite these facts, membership in the TCAA has
beenroughlystableataround40-50memberssincethestartoftheclubin1960.
Whyshouldthisbewhenwehaveametropolitanareaofover100,000people?Thereappearstobetwocontributingcauses:
(1) people no longer understand the concept of a hobby so tied up are they with electronic diversions, and (2) given the
recenttechnologicaladvancesintelescopesandimagingequipmentmanypeopleareflummoxedbywhattheyneedtoknow
inordertopursueamateurastronomyasahobby.Thisguidehasbeencreatedinresponsetothelatterimpediment.
Thisguideisanintroductiontothebasicknowledgewithwhichawould-beamateurastronomershouldbefamiliar.Whileit
isnotasubstituteforlearningmoreextensivelyaboutthescienceofastronomy,itprovidesthebasicinformationoneneeds
tobridgethegapfromneophytetoanobservervestedwiththeknowledgeofwhatittakestoproperlyviewtheheavens
using basic equipment. As such, this is not a reference work. It is not intended to be the answer to all questions that an
amateur astronomer might have. It merely provides sufficient information to get one started in the field of amateur
astronomy.
The author gratefully acknowledges the assistance of the following TCAA members who either provided guidance or
conductedaneditorialreview:TonyCellini,Dr.AllanSaaf,KenKashian,JimGibbs,andespeciallyGeoffHughes.
ABOUTTHEAUTHOR:
Dr.CarlJ.Wenningisawell-knownCentralIllinoisastronomyeducator.Hestartedoffviewingtheheavenswiththeaidofhis
grandfatherinthesummerof1957.Sincethattimehecontinuedviewingthenightskyfornearlysixdecades.HeholdsaB.S.
degree in Astronomy from The Ohio State University, an M.A.T. degree in Planetarium Education from Michigan State
University, and an Ed.D. degree in Curriculum & Instruction with a specialization in physics teaching from Illinois State
University.
Dr.WenningwasplanetariumdirectoratIllinoisStateUniversityfrom1978to2001.From1994-2008heworkedasaphysics
teacher educator. Retiring in 2008, he continued to teach physics and physics education courses for an additional seven
years. He also taught astronomy and physics lab science almost continuously at Illinois Wesleyan University from 1982 to
2001.HefullyretiredfromIllinoisStateUniversityin2014afternearly40yearsofuniversity-levelteaching.
Carl became associated with the TCAA in September 1978 – shortly after he was hired to work at Illinois State University.
Today he is an Astronomical League Master Observer (having completed 14 observing programs to date) and received the
2007NCRALRegionAwardforhiscontributionstoamateurastronomy.HeisalifelonghonorarymemberoftheTCAAandisa
memberofitsG.WeldonSchuetteSocietyofOutstandingAmateurAstronomers.
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TABLEOFCONTENTS
PREFACE:WHATITTAKESTOBECOMEANAMATEURASTRONOMER…………………………………………...…………………………..…………………….PG.4
INTRODUCTORYTOPICS:
1.
MOTIONSOFTHESTARS…………………………………………..…………………………..…………………………..……………………………….….PG.7
2.
SKYMAPSANDCONSTELLATIONS…………………………..…………………………..…………………………..……………………………………....PG.8
3.
HOWTOFINDNORTHINTHESKY…………………………..…………………………..…………………………..……………………………………....PG.8
4.
MAGNITUDES……………………………………………………………………………………………………………………………………………………….PG.9
5.
MOTIONSOFTHESUN…………………………..…………………………..…………………………..…………………………………………………..PG.10
6.
MOTIONSANDPHASESOFTHEMOON…………………………..…………………………..…………………………..…………………………..….PG.10
7.
ANGULARMEASURESINTHESKY…………………………..…………………………..…………………………..………………………………….....PG.11
8.
THECELESTIALSPHERE…………………………..…………………………..…………………………..…………………………………………….…...PG.11
9.
ALTITUDEANDAZIMUTH…………………………..…………………………..…………………………..……………………………………………....PG.11
10. RIGHTASCENSIONANDDECLINATION…………………………..…………………………..……………..………………..………………………..….PG.12
11. TELESCOPESVERSUSBINOCULARS…………………………..…………………………..…………………………………………………………….......PG.12
12. USINGBINOCULARS……………………………………………………………………………………………………………………..…………….……….PG.13
13. HOWTELESCOPESWORK……………………….…..…………………………..…………………………..…………………………………….…….....PG.14
14. THREEPOWERSOFATELESCOPE…………………………..…….……………………..…………………………..………………………………...…..PG.15
15. LIMITINGMAGNITUDEOFATELESCOPE……………………………………………………………………………………………….…………………..PG.16
16. COMMONTELESCOPETYPES…………………………..…………………………..…………………………..……………………………………….…..PG.19
17. COMMONTELESCOPETRAITS…………………………..…………………………..…………………………..…………………………………………..PG.21
18. TELESCOPEMOUNTSANDPIERS…………………………..…………………………..…………………………..…………………………………..…..PG.23
19. HOWTOBUYATELESCOPE…………………………..…………………………..…………………………..……………………………………………..PG.24
20. TELESCOPEEYEPIECEBASICS…………………………..…………………………..…………………………..……………………………………..……..PG.25
21. EYEPIECEFIELDORIENTATION…………………………..…………………………..…………………………..…………………………………..……..PG.27
22. FINDERSCOPES…………………………..…………………………..…………………………..………………………………………………..…………..PG.27
23. STARHOPPING…………………………..…………………………..…………………………..……………………………………………………………..PG.29
24. LIGHTPOLLUTION,TRANSPARENCY,ANDSEEING…………………………..…………………………..………………………………….…..……..PG.29
25. DARKADAPTATION…………………………..…………………………..…………………………..………………………………………………..……..PG.31
26. THEARTOFASTRONOMICALOBSERVING…………………………..…………………………..…………………………..…………………………...PG.31
27. OPTIMIZINGOBSERVATIONSOFDEEPSPACEOBJECTS…………………………………………………………………………………………….……PG.34
28. THEUSEOFOBSERVINGFILTERS…………………………………………………………………………………………………………………….……….PG.38
29. RECORDINGYOUROBSERVATIONS…………………………..…………………………..…………………………..……………………………….…..PG.39
30. MOONPHASESANDTHEIREFFECTONOBSERVING…………………………..…………………………..………………..…………………………..PG.39
31. MOONPHASEVERSUSBRIGHTNESS……………………………………………………………………………………………………….……………….PG.41
32. DEALINGWITHWEATHER…………………………..…………………………..…………………………..………………………………………………..PG.42
33. CELESTIALOBJECTSTOOBSERVE…………………….……..……………………………..…………………………..…………………………….……..PG.44
34. ASTRONOMICALLEAGUEOBSERVINGPROGRAMS…....…………………………..…………………………..…..…………………………..………PG.45
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ADVANCEDTOPICS:
35. CLEANINGYOUROPTICS…………………………………………………………………………………………………………………………………….…PG.45
36. CHECKINGTHECOLLIMATIONOFYOURTELESCOPE………………………………………………………………………………………………………PG.46
37. CHECKINGYOUROBJECTIVE’SOPTICS………………………………………………………………………………………………………….………..…PG.47
38. BALANCINGYOURTELESCOPE…………………………..……………………………………………..…………………………………………..………..PG.48
39. POLARALIGNINGANEQUATORIALMOUNT…………………………..…………………………..……………………………………………………..PG.49
SUGARGROVEOBSERVATORY:
40. OPERATINGTHE10’ASHDOME……………………………………………………………………………..………………………..……………………PG.51
41. OPERATINGTHEASTRO-PHYSICS1100MOUNT…………………………………………………………………………………………..…………….PG.51
42. FINDINGOBJECTSWITHTHESGOTELESCOPES….…………………………..………………………………………………………..…………………PG.53
43. OPERATINGSGO’SVISUALANDH-ALPHATELESCOPES……………………………………………………..………………………….…………….PG.54
44. IFSOMETHINGGOESWRONGWITHTHEA-P1100……………………………………………………………………………………………….……PG.54
45. PERSONAL,OBSERVATORY,ANDSGNCSAFETY…………………………………………………………………………………………………….……PG.55
46. RESERVINGSGOFORANOBSERVINGSESSION……………………………………………………………………………………………………………PG.56
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PREFACE:WHATITTAKESTOBECOMEANAMATEURASTRONOMER
SincethefoundingoftheTCAAin1960,amateurastronomyhaschangeddramatically.Atthattimethenorminamateur
astronomywasa2.4”refractorora4.25”reflector.Whentelescopeswereturnedtotheheavens,objectsviewedconsisted
thesolarsystembodies,doublestars,andfewofthebrighternebulas.Sincethattimetherehasbeenatremendousgrowth
inthesizeofavailableequipment,andthenumberofaccessorieshasexpandeddramatically.Amateurastronomytodayis
muchakintoprofessionalastronomyof justadecadeorsoago.Asaresult,it’smuchhardertobridgethegapfromnonastronomer to amateur astronomer today. Thinking about this topic for a while, I have concluded that the following are
thingsthatpeopleneedtoknowandbeabletodoinordertowearthelabel“amateurastronomer”proudly:
ó Possess a general knowledge of basic astronomy.
wheretofindsolarsystemanddeepspaceobjectsamong
Consider the etymology of the word ‘amateur’. This a
them.
th
French word with a late 18 century origin. It is derived ó Understand the effect of light pollution on viewing
from the Latin words amare ‘to love’ and amator ‘lover’,
celestialobservations.Lightpollution–theilluminationof
as well as from the Italian word amatore. No one can be
the sky by either natural or artificial sources of light –
saidtobeagoodamateurastronomerwhodoesnot‘love’
produces a bright sky and reduces the contrast between
the things of the firmament. Love is most frequently
the sky and objects located in it. With increasing light
demonstrated by spending time with the object of one’s
pollution most celestial objects become less visible. The
love.Therefore,toqualifyasanamateurastronomerone
limitingconditioninthiscaseisduringthedaytimewhen
mostcertainlywillspendlotsoftimegettingtoknowthe
the sun “pollutes” the sky with light making it appear
subject matter. We must be careful to understand that
bright and blue and reducing the contrast so much that
“throwing money” at amateur astronomy such as buying
objects cannot be seen without a telescope. While light
an expensive telescope with little knowledge or
pollutioniscausedbyartificiallightsourcessuchasstreet
understanding of how to use it is an illegitimate way to
lamps, advertising displays, stadium illumination, and so
becomeanamateurastronomer.Peopleoftenspendlots
forth, amateur astronomers must also realize that the
of money buying a telescope, but do not have the ‘love’
moon’spresencealsocanhaveadeleteriouseffectupon
necessary to employ it properly. Having lots of fancy
celestialobservations.Objectsreadilyobservedonadark
equipment does not guarantee that one knows how to
nightoftencannotbeseenatallonnightswhenthemoon
useit,whattolookfor,andthemeaningofwhatisseen
is located nearby and shining brightly. The effect of light
when observing with it. It’s not unlike purchasing a
pollution on limiting magnitude should be understood as
musicalinstrumentthatoneneverlearnstoplay.Owning
well.
amusicalinstrumentdoesnotamusicianmake!Reading ó Understand the need for dark adaption. People living in
systematicallyintheareaofastronomycanhelpaperson
town stepping out for a few minutes at night hardly see
obtain a general knowledge of basic astronomy. This
any stars. This is due to the facts that they are not darkmeans picking up a recent book about astronomy and
adaptedandthatthecityskyislightpolluted.Unlessthe
reading it from cover to cover. Library books and used
eyehasaminimumofabout20minutestoadapttofaintcollegetextbooksinastronomyarenothardtocomeby.
light conditions, it’s hard to see well at night. Dark
ó Know how to use a sky map to find constellations.
adaption is accomplished in two ways: (1) the pupil
Observerswillhaveahardtimelovingtheskyiftheydon’t
dilates, and (2) the retina produces rhodopsin (‘visual
know it intimately. Each and every amateur should be
purple’) that sensitizes the eye’s rods and cones to light.
familiar with the major constellations and, as time goes
Withouttheseadaptions,it’shardtoseeatnight.Wouldon, get to know the fainter constellations (e.g., Lynx,
beamateurastronomersneedtoknowthattogetagood
Sagitta, Camelopardalis, Leo Minor, and Delphinus) and
viewoftheskyonemustgoouttoandremainforawhile
asterisms of the sky (e.g., Summer Triangle, Keystone,
underdark-skyconditions.SugarGroveNatureCenterisa
Teapot, Job’s Coffin, and Northern Cross). Many people
good location for doing so, and budding amateur
areoverwhelmedwiththeconfusingmassofstarsseenin
astronomersshouldknowthatthiscanbeagoodplaceto
a dark clear sky on a good night. Many, too, are the
viewthenightskylocally.
peoplewhoareflummoxedwhentheylookataskymap ó Understand the effects of seeing and transparency on
andseeonepairofdirectionsreversed,notunderstanding
celestial observations.Notallnightskiesareequal.Poor
that the map is “correct” only when held up to the sky.
seeing–causedbyturbulenceinEarth’satmosphere–can
There are numerous free applications that can be used
make it difficult if not impossible to see fine details in
with cell phones and tablets that not only show the
objects being viewed telescopically. This shimmering of
constellations, but that properly associate and orient the
theairisresponsibleforthetwinklingphenomenonthatis
screen images with the portion of the sky to which the
sowellknownbyamateurastronomers.Onsomenights,
observer is directing attention. These all can help the
viewingcelestialobjectsismuchlikewatchingbirdsfrom
would-beamateurastronomergettoknowthestarsand
thebottomaripplingswimmingpool!Lowtransparency–
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poor sky clarity – also can make it more difficult to view
objects. The sky is sometimes clear, often translucent
(such as when one can see the moon or planets but not ó
the stars), and frequently opaque (as when the sky is
overcastwithclouds).
ó Knowwhatconstitutesagoodtelescope.Thisonecanbe
tricky because there are so many telescope types and
configurations, as well as other complicating factors –
reflector,refractor,catadioptric,magnification,eyepieces,
finders, Dobsonian, equatorial, push to, goto... Also
consider the fact that many of the telescopes out there
aretoys(junkwouldbeabetterterm).Whenpeoplebuy
toy telescopes they are quickly disillusioned and soon
dropoutofamateurastronomyaltogether.Thisispartof
the problem with ‘throwing money at amateur
astronomy’.Inordertobeagoodamateurastronomer,it
takesmorethanjustagoodtelescope.Ittakesknowledge
of basic astronomy, the constellations, the effects of
observing conditions on viewing, what can be observed,
where it is located, and so forth. Purchasing a telescope
without first going through the anticipatory steps
circumventstheprocessofbecomingalegitimateamateur
astronomer.It’snowonderthatthatsomanypeoplefail
in their aspirations to become amateur astronomers and ó
why we find so few of them in today’s world. (This
coupledwiththefactthatpeoplehavealternativewaysof
dealing with boredom, a failure to understand what a
hobbyentails,andalackofreadyaccesstoadarkskysite
arealsocomplicitinthisproblem.)
ó Know how to use a good telescope properly. Consider
the myriad of telescope-related terms in addition to the
ones listed above: primary, secondary, collimation,
eyepiece, finder, Telrad, laser pointer, polar alignment,
slowmotioncontrol,slew,Barlow,apparentfieldofview,
truefieldofview,resolvingpower,light-gatheringpower,
eye relief, right ascension, declination, polar axis, polar
alignment… The list goes on and on. No wonder people
areconfusedevenwhentheypurchaseagoodtelescope!
Again, it takes time and effort – persistence – getting to
knowwhattolookforinagoodtelescopeandhowtouse
the telescope well. Observer knowledge also should
include how to safely observe the sun telescopically (or
visually) as it is the only object in the sky that can be
harmfultothehumaneyeifviewedimproperly.
Know what to observe.Atelescopeuser,nomatterhow
wellqualifiedintheuseofthetelescope,willuseittono
avail unless he or she knows what to observe. The
repertoryofanyobservershould,attheminimum,include
listings of deep space and solar system bodies. These
include nebulas, clusters, galaxies, binary stars, carbon
stars, quasars, black hole candidates, asteroids, planets,
comets, the moon, the sun, and so forth. Every visual
observer should know about and participate in the
Astronomical Leagues’ myriad of observing programs. By
completingthese,theobservercanavoidlookingatjusta
few showcase objects that experienced observers have
cometoknowalltoowell.Usingtheseobservinglistswill
openuptheuniversetoawould-beobserver.Itisthrough
completion of observing programs that visually oriented
amateur astronomers can really excel. Every visual
observer should have as a personal goal viewing every
meaningful object within the visual range of his or her
telescope. Such a goal can lead to views of considerably
morethan1,000differentobjectsevenforamodestsize
(8”–11”)telescope.
Know how to find faint objects in the sky. While
constellations, asterisms, and the moon and planets are
interesting to observe, they are not the end-all, be-all as
far as celestial observations are concerned. Many people
want to see the faint star clusters (globular and open),
nebulas (emission, reflection, dark, and planetary),
galaxies, and everything else that populate the regions
betweenthestarsofthevariousconstellations.Theseare
typically not visible without the use of a good telescope
(thoughtherearenumerousexceptionssuchastheOrion
Nebula, the Andromeda Galaxy, and the Pleiades star
cluster for instance). Once one gets to know the star
patterns,onecanuseagoodskymaporapptofindthese
celestialinterlopers.Byusingaskymapwiththelocations
of,say,thebrighterMessierobjects,onecanusetheeye
orasetofbinocularstoseekthemout.Insodoing,they
learn the process of ‘star hopping’, moving from one
recognizablestargroupingtoanotherinanefforttotrack
downthedesiredobjectforviewing.Thisskillcanthenbe
appliedtotheuseofagoodtelescope.
Even if an amateur astronomer were to know all this, there seems always more to be learned in this rapidly advancing
hobby. As with any hobby, it takes time and effort to learn and understand how to do things right and perhaps more
importantly,appreciatethemandtheirmeaningwithinthebiggerpicture.Astronomyisanall-encompassingsubjectwherethe
individualisnotrequiredtoownexpensiveequipment.Rather,itissufficienttojusthaveaninterestinlearningaboutthesky,
theobjectsinit,andusetheirmindtocomprehendit.ThisTCAAGuidewillhelpyoutolearnwhatittakestobeanamateur
astronomerintoday’sworld,andmovebeyondmereinfatuationtobecomeafull-fledgedamateurastronomer.
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1. MOTIONSOFTHESTARS
Thesun,moon,stars,andplanetsallmoveinthesky
with the passage of time. Each of the movements is
predictable,thoughsomemotionsaremorecomplexthan
others. Let’s examine these motions so you have a better
understandofhowthesky“works”.
Thestarsappeartoriseandsetdaily.Theydothisasa
resultofEarth’srotation.AsEarthspinsfromwesttoeast,
the stars move from east to west across the sky. We can
characterize this motion by drawing paths on a celestial
sphere–alargeimaginaryglobeofindeterminatesizethat
iscenteredonEarth.Theonlypartofthecelestialsphere
we can “see” is that half which is above the horizon, the
circle centered on the observer where the sky and land
appear to meet. The remaining celestial hemisphere is
belowourhorizon.
Thecelestialsphere,likethesphereofEarth,haspoles
andanequator.Theseareknownasthenorthcelestialand
southcelestialpolesandthecelestialequator.Ifonewere
to project Earth’s pole and equator into the sky, they
would “touch” the celestial sphere as shown in the figure
below. The celestial poles appear directly overhead at
Earth’s poles and the celestial equator appears directly
overhead for observers viewing from anywhere along
Earth’sequator.
Stars rise on the eastern half of the sky, reach their
highest point in the heavens when they cross over the
meridian (an imaginary circle passing from north to
overheadtosouth)goingfromeasttowest,andsettingin
the western half of the sky. As they move across the sky,
stars trace out circles on the celestial sphere as shown
below.
When watching stars rise in the east in mid northern
latitudes, they move to the upper right. Stars head down
tothelowerrightastheysetinmidnorthernlatitudes.In
thesouththestarsappeartomovelefttorightexecuting
an arch. Stars in the northern sky move in a
counterclockwisedirectionaroundtheNorthStar.
Note that if a star rises due east, it sets due west.
Pathsofallstarsareparalleltooneanother.Starsrisingin
the northeast set in the northwest. Stars rising in the
southeast set in the southwest. Stars close enough to the
northcelestialpole(NCP)willinfactneverriseorsetbut
are continuously in the northern sky. These are the north
circumpolar stars. Stars close to the south celestial pole
(SCP) never rise or set and are continuously below the
southern horizon. These are the south circumpolar stars.
Starsthatriseandsetarecallequatorialstars.
One’s latitude, φ, determines whether stars are
circumpolar or equatorial. The elevation of the NCP is
always equal to the observer’s latitude in the northern
hemisphere. The elevation of the SCP is always equal to
thenegativeoftheobserver’slatitude(φisconsidered<0
inthesouthernhemisphere).
Foranorthernhemisphereobserver,thefarthernorth
a star is from the celestial equator, the longer it is in our
sky.Similarly,thefarthersouthastarisfromthecelestial
equator,theshorteritisinoursky.
Note that because of the latitude, stars do not rise
straitupintheeastandsetstraightdowninthewest.That
only occurs at the equator where φ = 0°. What would
happen if the observer’s latitude were +90° (at the North
Pole)?
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7
2.SKYMAPSANDCONSTELLATIONS
A sky map show the stars and constellations as they
wouldappearoveraparticularlocationatagivendateand
time. Sky maps can be used within one hour of these
prescribed times to find constellations and bright stars in
the nighttime sky. Go outside and, with a red filtered
flashlight if necessary, identify objects in the sky using a
skymap.Itisbesttoviewtheskyfromadarklocation.
Look carefully at the sky map example below. The
center of the map represent the zenith – the overhead
Note the patterns – constellations and fragments of
constellations called asterisms – that fill this
representationofthenighttimesky.Youwillobservethat
some dots representing stars are larger than others. This
indicatesthatstarsappearwithdifferentbrightness.Large
dotsrepresentbrightstars;smalldotsrepresentdimstars.
“Pointed disks” – the diameter of which is related to
brightness–representplanetswhenpresent.Theskymap
shows the sky for a mid northern latitude during spring
2016.
point in the sky. The outer circle represents the horizon.
Along the horizon you will find the directions NORTH,
SOUTH,EAST,andWEST.Atfirsttheirpositionsinrelation
tooneanothermightseemincorrect;noteespeciallyhow
east and west appear reversed from that of a traditional
landmap.Youmustrememberthatskymapsaredrawnto
representtheskyandnottheearth.Whenholdingthesky
mapoverheaditcanbeorientedsoastomatchtheactual
directions.
Because dark adaption is necessary for viewing the
fainter stars and Milky Way, be certain to use a dim, redfiltered flashlight to view the sky map at night. This will
allow you to move quickly between map and sky without
havingtoreadapt.Ifyouuseabrightwhiteflashlight,your
dark adaption will be destroyed and you’ll have a hard
timeseeingtheconstellationsdepictedontheskymap.
3.HOWTOFINDNORTHINTHESKY
Whenonewatchesthenorthernstarsoverthecourse
ofanighttheytraceoutentirecirclesinthenorthernsky.
The stars appear to move in a counterclockwise fashion.
TheNorthStar,whosenameisPolaris,islocatedveryclose
to the north celestial pole (within about 2/3 of 1°) and is
therefore the one star about which all others appear to
move. To find north, draw a line straight down to the
horizonfromPolaris.
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IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomersAllRightsReserved
Contrary to popular belief, the North Star is not the
th
brighteststarinthesky.Infact,itisthe49 brighteststar
intheskywith48otherstarsbeingbrighter.Itisconfused
withbeingbrightestbecauseitisimportantandtherefore
famous. The brightest star in the sky is the star Sirius
locatedamongthestarsofCanisMajor,theBigDog.
Asnotedabove,findingthedirectionnorthiseasyto
doonceonefindstheNorthStar.Theproblemis,whichof
the many stars visible at night is the North Star and how
does one confirm its identity? The way to find it is to use
thepointerstarsoftheBigDipper.Extendalineoutofthe
bowl about a dipper’s length and you’ll run into a star. If
this star is the star at the end of the handle of the Little
Dipper, you can rest assured that you have found the
NorthStar.
Again, as misleading as the image above might be,
remember,theNorthStarisonlyamoderatelybrightstar
andbynomeansthebrighteststarinthesky.
4.MAGNITUDES
Apparent magnitude:Astronomersusethesystemof
magnitudestodescribetheapparentbrightnessofstarsas
seen from Earth. The first to use magnitudes was the
Greek astronomer Hipparchus. In his very rudimentary
classificationsystem,thebrighteststarswereclassifiedas
first magnitude (m = 1). The second brightest stars were
classified as second magnitude (m = 2) and so forth. The
dimmeststarsvisibletotheunaidedeyeunderaverydark
skywereclassifiedassixthmagnitude(m=6).
In 1856,Hipparchus’ rudimentary system was
formalized by the astronomer Norman Pogson who
defined a first magnitude star to be 100 times brighter
than a sixth-magnitude star, thereby establishing the
logarithmic scale westill in use today. This scale implies
thatastarofmagnitudemisroughly2.512timesasbright
as a star of magnitudem+1. This figure, 2.512, is the fifth
root of 100. With the introduction of photometry, a
method that could actually count photon by photon the
lightreceivedfromastar,awaybecameavailabletomore
accuratelydefinethemagnitudesofstars.Underthisnew
systemSirius,theDogStar,thebrighteststarappearingin
ournightsky,hasanapparentmagnitudeof–1.4.
At this time, Pogson used
the star Vega as a standard
benchmark for 0 magnitude,
and the brightness of stars
were thereby measured in
relationtothisstar.(Todaywe
use the “north polar
sequence”ofsome30starsto
definethebenchmark.)
Because astronomer can
nowadays
measure
the
amountoflightreachingEarth
fromastar,wedefinetheapparentmagnitudeintermsof
flux (𝐹), a number proportional to the photon count, as
follows:𝑚! = −5𝑙𝑜𝑔!""
10,𝑚! = −2.5𝑙𝑜𝑔!"
!!
!!,!
!!
!!,!
or more commonly in base
where𝐹! istheobservedfluxof
a given star and𝐹!,! is the flux of the reference star of
magnitude0suchasVega.Invertingtheaboveformulafor
a magnitude difference of 𝑚! − 𝑚! = ∆𝑚 implies a
brightness factor of 𝐹! 𝐹! = 100∆! ! = 10!.!∆! ≈
2.512∆! . With this latter formula, we can compare the
apparent brightness of different objects. In the current
evening sky (June 10, 2016), Mars (Ares) shines at
apparentmagnitude-1.80.ThestarAntares(rivalofMars)
shines at +1.07. How many times brighter is Mars in
comparison with Antares? 𝐹! 𝐹! = 2.512(!!!!!) implies
𝐹!"#$ 𝐹!"#$%&' = 2.512(!!.!"!!.!") = 2.512!.!" = 14.1. Thatis,Marsissome14timesbrighterthanAntaresonthe
dateinquestion.Amagnitudedifferenceof2.87wouldnot
suggest this comparison directly; therefore, amateur
astronomers should know how to make use of these
formulas.
Magnitudeadditionisabitcomplex.Still,ifonewants
to determine the apparent brightness of a pair of double
stars, one can add their magnitudes in the following
manner: 𝑚! = −2.5𝑙𝑜𝑔!" (10!!!∗!.! + 10!!!∗!.! ) where
mfistheresultingmagnitudeafteraddingthebrightnessof
starswhosemagnitudesarem1andm2.
Absolute magnitude: Flux decreases with distance
accordingtotheinverse-squarelaw.Thatis,ifyoudouble
!
thedistanceofagivenstar,itwillappear(1 2) or1/4as
bright as before. Hence, a particular apparent magnitude
couldequallywellrefertoastaratonedistance,orastar
4 times brighter at twice that distance, etc. When one is
interested in theintrinsicbrightness of an astronomical
object,thenonerefersnottotheapparentmagnitudebut
its absolute magnitude. The absolute magnitude, M, of a
star or astronomical object is defined as the apparent
magnitude it would have as seen from a distance of 10
parsecsorabout32.6lightyears.
Distancemodulusformula:Absolutemagnitude,M,is
related to apparent magnitude, m, by the following
expression:𝑚 − 𝑀 = +5𝑙𝑜𝑔!" 𝑑 − 5 where distance, d, is
expressed in parsecs. This expression assumes no
interstellar dimming. The value of𝑚 − 𝑀 is known as the
distance modulus, and the entire expression is known as
thedistancemodulusformula.Iftwoofthethreetermsin
the formula are known, then the third can be solved for
uniquely. If, for instance, the parallax of a star can be
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9
found through observation (which is readily done for
nearby stars), then from knowledge of its apparent
magnitude its absolute magnitude can be found. If, for
instance, a star’s absolute magnitude can be found using
spectral analysis and the Hertzsprung-Russell diagram,
thenknowledgeofitsapparentmagnitudecanbeusedto
determineastar’sdistance.
N.B. Magnitudes are a bit more complicated than
presentedhere.Apparentmagnitudeisafunctionofcolor
and the presence of interstellar darkening and reddening.
Cosmologicalredshiftalsocanaffectapparentmagnitude.
Bolometric magnitude is a magnitude that takes into
account the full range of a star’s emission, not just in a
particular wavelength region. Keep in mind that this is
merely a short introduction to magnitude systems and in
nowayistobeconsideredafulltreatiseonthematter.
5.MOTIONSOFTHESUN
Thesunrisesintheeasternskyeachmorningandsets
in the western sky each evening. The spinning of Earth
causes this motion. As Earth turns from west to east, the
sunappearstomovefromeasttowestacrossthesky.The
path that the sun takes across the sky changes each day
duetothe23.5degree“tilt”ofEarth’saxisofrotation.On
the first day of our summer, the sun rises roughly in the
northeastandsetsroughlyinthenorthwest.Atmiddaythe
sun is high in the southern sky but never passes directly
overhead. (That can occur only for people living between
the tropics of Cancer and Capricorn.) On the first days of
autumnandspringthesundoestendtorisedueeastand
set due west, and is located about half way up in the
southernskyatmidday.Onthefirstdayofwinterthesun
rises roughly in the southeast and sets roughly in the
southwest. At midday it is low in the southern sky. The
changing elevation of the midday sun and changing
directions of the rising and setting sun is due not only to
Earth’saxisbeinginclinedtoitsorbit,butbecauseEarthis
inorbitaroundthesunaswell.
As Earth moves around the sun, the sun appears to
shift about 1 degree eastward among the background of
stars each day. (This is why we have 360 degrees in a
circle.)Thiseastwardmotionofthesuncannotbedirectly
observed due to the bright sky, telltale evidence can be
found in the drift of the constellations. Each evening the
constellations appear to rise and set about 4 minutes
earlier. This occurs because of the sun’s daily motion
among the background stars. It takes Earth about 4
minutestoturnthroughoneadditionaldegreeeachdayto
bringthesunbacktothesamelocationinthesky.
Theexactpathofthesuntracedoutamongthestars
isknownastheecliptic.Theconstellationsthroughwhich
the sun appears to move over the course of the year are
known as the zodiac. The sun, moon, and planets are
always located among the stars of the zodiac because all
planetsorbitthesuninapproximatelythesameplane.
6.MOTIONSANDPHASESOFTHEMOON
The moon’s motion is very similar to that of the sun.
However, its eastward motion among the stars is much
more rapid and easily discerned from night to night.
BecauseEarthspinsonitsaxis,themoonrisesandsetsas
doesthesun.However,becausethemoonorbitsEarthina
period of about a month, its eastward motion among the
backgroundstarsoccursatarateofabout13degreesper
day. Hence, from one night to the next, the moon’s
eastward progression among the stars of the zodiac is
easily seen. The moon’s rapid eastward motion accounts
forthefactthatitrisesandsetsabout50minutesearlier
eachdayonaverage.
AsthemoonorbitsEarth,weseemoreandthenless
of its sunlit surface over the course of a month. This
accountsforthephasesofthemoonasshownhere.When
themoonisinthesamedirectionintheskyasthesun,its
farsurfaceislitwhereasitsnearsurfaceisdark;thisgives
usthenewmoon.Asthemooncontinuestoorbitwesee
moreandmoreofitssunlitsurface.Whenthemoonis¼in
itsorbitaroundEarth,itisatthefirstquarterphase.Half
10
of the moon’s visible surface is lit – that nearest the sun.
Eventuallythemoonisroughlyoppositethesuninthesky,
and its surface appears fully illuminated. The reason we
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don’t have eclipses of the sun at new moon phase and
eclipses of the moon at full moon phase is because the
moon’s orbital plane is inclined by just over 5 degrees to
Earth’sorbitalplane.Themoonpassesaboveorbelowthe
sunanewphaseandaboveorbelowEarth’sshadowatfull
phase.
7.ANGULARMEASURESINTHESKY
The distances between objects or the sizes of
objectsintheskyaremeasuredinangles.Forinstance,
thesunandmoonbothappeartohaveanangularsize
ofabout½of1°.TheBigDipperisabout25°longfrom
thetipofthehandletothetipofthebowl.
Because humans are scaled roughly the same, it is
possible and literally very “handy” to measure angular
distanceswiththeoutstretchedarm.Fingersinvarious
combinations are good approximations of angular
measuresasshownintheimagebelow.
Formeasuringangulardistancesmorethan15°,say
30°, two spans of fingers covering the distance in two
equal15°lengthsisanadequateapproximation.
8.THECELESTIALSPHERE
Thecelestialsphereisanimaginarysphereoflargebut
indeterminate size with Earth located at its center. The
poles of thecelestial sphereare aligned with and are
located directly over the poles of Earth.
Thecelestialequator lies along thecelestial spherein the
same plane as Earth’s equator. There are many other
reference points and circles on the celestial sphere with
whichallamateurastronomersshouldbefamiliar:
Zenith–Thepointonthecelestialspheredirectlyoveran
observer’shead.
Nadir–Thepointonthecelestialspheredirectlyunderan
observer’sfeet.
Meridian – A circle splitting the sky into eastern and
westernhalves.Thecircleisfixedrelativetotheobserver
and passes from the north point on the horizon (N),
throughthezenith,tothesouthpointonthehorizon(S).It
isthemeridiantowhichAMandPMrefer.Antemeridiem
(Latin)is“beforethemeridian”andreferstothemorning
hours. Post meridiem (Latin) is “after the meridian” and
refers to the afternoon hours. When the sun is on the
meridianitislocalsolarnoon.Objectsareattheirgreatest
altitude in the sky when they cross the meridian (transit)
goingfromeasttowestduetoEarth’srotation.
Celestial Equator – A projection of Earth’s equator into
space. When an object is on the celestial equator, it is
locateddirectlyoverEarth’sequator.
Ecliptic–Thecircleonthecelestialspherethatrepresents
thepathofthesun’smotionoverthecourseofayear.The
motion is only apparent being caused by Earth’s orbital
motionaroundthesun.Theeclipticisinclinedsome23°44’
ofarctothecelestialequator.
Horizon–JustlikeEarth’sequator,Earth’shorizoncanbe
projected onto the sky for use with an alternative
coordinatesystem.
The problem with
this
coordinate
system is that it
moves with the
observer. Celestial
coordinates are
roughly fixed on
the sky but for
slow precession of
the First Point of
Aries, γ, among
the background of
stars.
9.ALTITUDEANDAZIMUTH
Statingthelocationsofobjectsintheskycanbedone
inavarietyofways.Forinstance,aplanetis3degreesto
the west of the head of Leo the Lion. But what happens
when there are no nearby convenient markers or if
someone doesn’t know the constellations as is often the
case?Thenthebestthingtousetoidentifythelocationof
anobjectintheskyisitsaltitudeandazimuth.
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11
Altitude refers to the angular distance of an object
above the horizon. An object with 0° altitude will appear
on the horizon. An object at the zenith (the overhead
point)willhaveanaltitudeequalto90°.Anobjecthalfthe
wayupintheskywillhaveanaltitudeof45°,1/3way30°,
2/3way60°andsoforth.
Azimuth refers to the angular distance of an object
from the direction north measured eastward around the
horizon. The azimuth of an object that is due north is 0°,
east 90°, south 180°, west 270°, and so forth. The
numbering continues northward reaching 360° at the
northpointwhichisthesameas0°.
Thelocationofanobject1/3wayupinthenorthwest
wouldbeasfollows:altitude30°,azimuth315°.Whilethis
isaconvenientsystemfor
findingobjectsinthesky,
it is time dependent. As
Earth rotates and objects
rise in the eastern sky,
move left to right across
the southern sky, and
then set in the western
sky, their altitudes and
azimuthschange.
10.RIGHTASCENSIONANDDECLINATION
Theearliestmethodusedtofindobjectsintheskywas
with the use of constellations. Star patterns were
identified as various types of objects such as gods,
humans, and animals. A celestial object might be located
“below the belt of Orion” or “near the head of Cygnus.”
Theselocationdescriptionsweretooimprecisefortracking
moving objects such as planets (wanderers). Over the
course of time, a celestial coordinate system known as
rightascensionanddeclinationwasdeveloped.
Rightascension(R.A.)anddeclination(Dec.)aretothe
celestial sphere as longitude and latitude are to Earth’s
globe.JustlikeEarth,theskyhastwopolesandanequator
– celestial poles and equator as compared to terrestrial
polesandequator.Thesky’scoordinatesareprojectionsof
earth’scoordinatesystemontothecelestialsphere.
R.A.isanalogoustoEarthlongitude.Itisexpressedin
h
hours (1 = 15° at the celestial equator, C.E.), and is
measured eastward along the C.E. from the First Point of
Aries (γ – sometimes called the Vernal Equinox), the
positionofthesunonthedateoftheMarchequinox.On
the March equinox the sun is directly on the celestial
h m s
equator and has a R.A. of 0 0 0 by definition. An object
h m s
90°eastofγwillhaveanR.A.of6 0 0 .HoursofR.A.are
used instead of degrees because meridians of R.A.
converge at points away from the C.E. and meet at the
h
celestial poles. The span of 1 of R.A. is therefore not
h
always 15°. The span of arc for 1 of R.A. equals15°𝑐𝑜𝑠𝛿
whereδrepresentsthedeclination.
Dec.isanalogoustoEarthlatitude.Dec.ismeasuredin
degrees north (+) or south (−) of the C.E. The Dec. of the
north celestial pole (NCP) is +90°; the Dec. of the south
celestialpole(SCP)is−90°.PointswithequalDec.valueslie
equidistant from the celestial equator along parallels of
Dec.
Consider how R.A. and Dec. can be used to uniquely
determinethepositionofanobjectinthesky.Saythatan
h m s
objecthasthefollowingR.A.(α)andDec.(δ):α=5 0 0 ,δ
12
= +55°. Can you find the location of that object on the
celestialsphere?Thereisonlyonesuchlocation,anditis
shownintheimagebelow.
Telescopes, especially computer “go to” telescopes,
willusetheR.A.-Dec.coordinatesystemtofindobjectsin
the sky. Fortunately, amateur astronomers don’t often
havetoworkwiththissystemanylonger.Notsolongago
telescopes had setting circles that used R.A. and Dec. but
no longer as many found their use rather confusing in
practice.
Someoldertelescopesevenusedhourangles(H.A.)to
findcelestialobjectswhereH.A.=localsiderealtime(l.s.t.)
minusR.A.=l.s.t.–α.DoyoufindR.A.andDec.somewhat
confusing?Justwaituntilyoudelveintosiderealtimeand
you’llreallygettounderstandthemeaningoftheword.
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11.TELESCOPESVERSUSBINOCULARS
Many would-be amateur astronomers are surface-brightness objects and/or objects of large angular
disappointed when I suggest that the first thing they size are best viewed using low magnifications. There are
should consider purchasing for sky watching is a good set several reasons for this. But first, let’s consider a number
of binoculars and an observing guide. They are clearly of of well-known celestial objects that are best viewed by
theopinionthat“highpower”isallthatoneneedstosee northern hemisphere amateurs using low-power
thingsinthesky.Thefactofthematteristhatmanylow- binocularsorspottingscopesratherthantelescopes.
• TrifidNebula
• WitchHeadNebula
• Pleiades
• Barnard’sloop
• CaliforniaNebula
• TriangulumGalaxy
• RhoOphiuchicomplex
• NorthAmericanNebula
• OrionNebula
• Hyades(starcluster)
• BeehiveCluster
• PerseusDoubleCluster
• AndromedaGalaxy
• Scutumstarcloud
• ComaStarCluster(Mel110)
• VeilNebula/Cygnusloop
• RosetteNebula
• AlphaPerseiCluster
• HelixNebula
Whilemostbinocularsandspottingscopesdon’thave producelowermagnifyingpowers.Eyepieceswith82°and
thebenefitoflargeaperture,theydohaveonethingthat 100°fieldsofviewshowaconsiderablywidertruefieldof
iscriticaltoobservinglow-surface-brightnessobjects–low viewthatwillourexamplePlössleyepiece.Unfortunately,
magnification. This might seem counterintuitive, but these multi-element wide-field eyepieces are quite
considerthefollowingtwofacts:
expensive – some ranging in the hundreds of dollars.
Lowmagnificationbinocularsandspottingscopescan Lower magnification (longer focal length) eyepieces can
provide a much wider field of view than will a telescope. have the same effect, but the size of the exit pupil
ThefieldofviewofsayaCelestron5”f/10spottingscope becomes so large that all the light gathered by the
is much larger than that provide by a Celestron 11” f/10 telescopecannotenterintothehumaneye.Thelowerthe
telescope given the same eyepiece. Let’s say we have an magnifying power, the wider the cone of light exiting the
18mm eyepiece with an apparent field of view of 52° – eyepiece. This results in vignetting which effectively
typicalofaninexpensivePlössleyepiece.Firstconsiderthe reducestheapertureofatelescope.
resulting magnifications of the two instruments using this
Low magnification binoculars and spotting scopes
particular eyepiece which are calculated by dividing the typically provide a much brighter image than will a
focallengthoftheobjective(F)bythefocallengthofthe telescope. This statement might be surprising to a good
eyepiece(f):
many amateur astronomers. With larger aperture
telescope lenses and mirrors, how can this be? The
M11”=F/f=2,794mm/18mm=155.2X
explanation has to do with the effect of magnification.
Consideratypicalhumaneyefullydilatedtosay8mm.In
M5”=F/f=1,270mm/18mm=70.6X
comparison, an 11” (280mm) mirror can collect gather
2
about 1,225 (35 ) times more light than can the fully
Next consider the true field of view given by the dilated eye. However, when an image is magnified, the
following formula: true field of view of an eyepiece (T) surface brightness of the thing observed is reduced. For
equals its apparent field (A) divided by resulting instance, with my CPC 11” I most commonly observe at
magnification.
87x. An object 87 times higher and 87 times wider will
haveasurfacebrightnessofonly1/7,569thatofthenon2
T11”=A/M=52°/155.2X=0.34°
magnifiedimage(87 ).Theobjectistherefore1,225/7,569
timesasbrightasobservedwiththeunaidedeye!Thatis,
T5”=A/M=52°/70.6=0.74°
the object is only 16.2% as bright as the real thing when
observedwiththistelescope.Considertheeffectofa7x50
Hence, the field of view is more than twice as wide set of binoculars on apparent image surface brightness.
2
(withanarea4.7Xasmuch)inthelower-powertelescope. With each lens gathering 39 times ((50mm/8mm) ) the
So, larger objects can more easily be observed in lower- amountoflightastheeye,eachobjectivemakesadiffuse
powerinstrumentsduetotheirlargerfieldsofviewgiven celestial object 39 times brighter than seen with the
the same eyepiece. Now, it is quite true that using unaidedeye.Atamagnificationof7,theobjectisreduced
eyepieces that provide different magnifications and inbrightnessto1/49thenakedeyeview.Thiscombination
apparent fields of view will yield different true fields of results is a comparative surface brightness of 39/49 or
view. For instance, longer focal length eyepieces will 79.6%. Hence, in comparison with the telescope
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13
combination cited, the binoculars will produce an object
nearly five times brighter in terms of surface brightness.
This is critical when observing low surface brightness
objectssuchasthoseinthelistabove.
Of course, this presentation is rather simplistic
because it does not take into account such things as
secondary aperture blocking, reflection of light by
eyepieces,utilityofhighermagnificationsinseeinggreater
detail,andsoforth.Nonetheless,itdoesshinelightonwhy
higher-power telescopes are not always the best
instruments for viewing objects of the heavens. This goes
to explain why so many amateurs have telescopes of
different apertures and designs as well as a variety of
binoculars. No one telescope is suitable for observing
everythinginthesky.
Ihopethatthishelpsournoviceamateurastronomers
gain a better understanding of the recommendation of
binoculars and observing guides as the first place to start
when purchasing astronomical viewing equipment. There
hastheaddedbonus,too,thatbinocularscanbeusedfor
much more than astronomical viewing should interest in
skywatchingwane.
12.USINGBINOCULARS
Binocularsconsistoftwoopticaltubeassemblieseach
calledamonocular.Let’ssayforthesakeofthediscussion
that you own a 6x42 pair of binoculars. First off, what do
these numbers mean? The 6 refers to magnifying power.
The 42 refers to a 42mm aperture. Binoculars come in a
variety of types such as 7x35, 7x50, 10x50, and 15x70.
Typicallythelargertheaperturethebetterwhenitcomes
observing the dim objects of the night sky. Higher
magnificationcanhelp,but“easydoesit.”Binocularsthat
magnify 10x are a lot harder to use than those that
magnify 7x. This is because your hand shaking is also
magnified. Binoculars of 15x and 20x almost always need
tobemountedforeffectiveuse.
Lots of people have a hard time focusing binoculars
because, quite frankly, they don’t know what they are
doing. There are two focusers on most quality binoculars.
There is a center focus (sometimes called the Instafocus)
between the two optical tube assemblies. This focuses
both monoculars at the same time. One of the two
eyepieceswillhaveanindependentfocususuallyinscribed
with the symbols and word +2, +1, 0, -1, and -2 diopters.
The diopter focus is needed because most peoples’ eyes
are not the same. When used without the usual
eyeglasses, viewers might have to adjust the two
monoculars separately. If the user’s eyes are identical,
thenthediopterfocuswillread0diopterswhenproperly
focused.Again,forthesakeofthediscussion,let’ssaythat
thediopterfocusisonrightmonocular.
Closing your right eye and looking at a very distant
object, focus your left monocular using the center focus.
Then,openingyourrighteyeandNOTtouchingthecenter
focus,usethediopterfocustobringtheimageintheright
monocular into clear view. Both monoculars should now
be well focused. Once this latter adjustment is made, the
observer can use the center focus to adjust both
monoculars when observing objects near and far, both of
which will have their own focus setting. A clear image
shouldbepresentforeacheye.
13.HOWTELESCOPESWORK
Telescopes are a mystery to those who have never
studied them. This lack of understanding arises from the
factthatmostpeopledon’tknowhowlensesandmirrors
work. Before we begin our “study” of the telescope, let’s
try to understand the concept of pinhole projection. We
will then turn our attention to the simplest form of
telescope – the refractor. Once we understand how a
refractor works, it’s easy to generalize our knowledge to
othertelescopes.
The best way to gain an understanding of how a
refracting telescope’s main lens (the objective lens)
produces an image for examination, is to examine a
pinhole camera. Suppose a tiny box has a pinhole on one
side and a sheet of wax paper has replaced the opposite
side. Say you are standing in a darkened room during the
daytime. If you were to see a tree outside your window,
youcouldholdyourpinholecamerauptothewindowand
14
an inverted image of the tree would appear on the wax
paper. It would be easy to see because you would be
viewing it from within the confines of your much darker
room.
The reason the tree appears inverted (flipped top to
bottom and side to side) is because of the point-to-point
mappingofthetreeontothewaxedpaperscreen.Thered
lines in the image here show only two rays that would
entertheboxthroughthepinhole.Becauselighttravelsin
astraightline,aninvertedimageisproduced.Becausethe
pinhole is tiny, only a faint image is produced. If a larger
pinholeiscreatedoppositetheviewscreen,theimagewill
bebrighterbutalsoblurrierduetothefactthatthereisno
longer a point-to-point mapping between the tree and its
projection.
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point mapping is maintained despite the fact that the
apertureisnolongerapinhole.
Onedifferencefromthepinholethoughisthatthereis
a particular distance beyond the lens that an image will
form. This is the so-called image plane. So, a lens will
project an image of an object on the side of the lens
opposite the object whose image is being created. The
image literally “hangs in space” and an eyepiece can be
usedtoviewit.
Eyepieces are little more than glorified magnifying
lenses.Recallthatmagnifyinglensesproduceerectimages
whenheldclosetoobjects.Themagnifyingglassdoesnot
produce an inverted image; it only makes the object look
bigger. So, when an eyepiece is brought together with an
objective lens that produces a projected image, the
eyepiece produces an enlarged view. Nothing could be
simpler!
Theobjectivelensofarefractingtelescopeisinsome
ways like a pinhole. A lens will produce a brighter image
thoughbecauseitislarger.Theshapeandrefractiveindex
of the lens will bend rays, however, so that the point-to
14.THREEPOWERSOFATELESCOPE
When people purchase telescopes, the only power
they typically think about is magnifying power. Actually,
that’sprobablytheleastimportantofthethreepowersof
the telescope! Here I summarized the three powers, and
providesomeguidanceforpurchasingasuitabletelescope
andeyepieces.
1. Light Gathering Power – This power gives an
indication of how much light a telescope will gather
intoone’seye.LGPisoftengivenincomparisontothe
light-gathering ability of the human eye dilated to a
certaindiameter.Likeabucket’sabilitytocollectrain
drops in comparison with a test tube, the larger the
apertureofatelescope(thesizeofitsmirrororlens),
the greater will be the LGP of a telescope. The LGP
ratio between a telescope (t) and eye (e) is easily
expressed in terms of a ratio of the light-gathering
areasofthetelescopeandeye:
𝐿𝐺𝑃! 4𝜋𝑟!!
2𝑟! !
𝑑! !
=
=
=
𝐿𝐺𝑃! 4𝜋𝑟!!
2𝑟!
𝑑!
As this equation implies, the LGP of a telescope
with respect to the human eye increases with square
of the diameter of the telescope objective (lens or
mirror). For instance, an 11” telescope (36.4mm)
telescope will gather nearly 1,600 times more light
than will a human eye dilated to 7mm
2
(280mm/7mm) .
This formula also can be used to compare the
differences between telescopes as well. An 8”
2
aperturetelescopewillgatherfourtimes(2 )asmuch
lightaswilla4”.A16”telescopewillgather16times
2
(4 )asmuchlightasa4”telescope.
The limiting magnitude, LM, of a telescope (the
faintest star a telescope can show under ‘average’
conditions depending on a number of factors such as
magnification,M,diameterofobjectiveinmm,D,the
transmissionfactor,t,whichisusually0.85–0.9,and
even sky darkness which is associated with the
magnitudeofthefaintestnaked-eyestarvisible,m)is
givenbythefollowingapproximation:
2.
𝐿𝑀 = 𝑚!"#$% !"! − 2 + 2.5𝑙𝑜𝑔!" (𝑀 ∙ 𝐷 ∙ 𝑡)
An 11” telescope (D = 280) on an average night
(mnakedeye=5.0)usedatamagnificationof87,andwith
t = 0.9 will have a limiting magnitude of about 13.8.
Thisis8.8magnitudesfainterthantheLMoftheeye,
andabrightnessratioof3,300times.
Resolving Power – Resolving power is the ability of a
telescope to show fine detail. Ignoring, momentarily,
the blurring of an image caused by atmospheric
turbulence (seeing) and assuming no optical
imperfectionsofthetelescope,theabilitytoseparate
two stars with a telescope is given by Dawes limit as
follows
116
𝛼=
𝐷
where α is the minimum separation between two
distinguishablestarsexpressedinarcseconds,andD,
the diameter of the telescope objective expressed in
mm. The theoretical resolving power of my 11” CPC
telescope is 116/280 = 0.4 arc seconds – about the
bestthatcanbeexpectedgiveneventhebestseeing
conditions(addressedelsewhere).
Thisequationshowsthat,allelsebeingequal,the
largertheaperture,thebettertheangularresolution.
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15
3.
Note that the resolution is not dependent upon the
magnificationofatelescope.Telescopesmarketedby
giving high values of the maximum power often
deliver very poor images for a variety of reasons, not
the least of which is due to imperfections in the
optical system. As the aperture of telescopes
increases,theresolutioncandropbecauseitisfurther
limited by the turbulence of the atmosphere. Larger
aperture telescopes gather light rays from a larger
cross-section of the sky, and this increases the
turbulence in the telescopic image degrading
resolution.Sometimes,forinstance,thebestviewsof
planetscanbeseenintelescopesofsmalleraperture–
say 4.25 to 8 inches in diameter. The best views of
Jupiter the writer have ever seen were in a 6”
reflecting telescope due primarily to the limits
imposed by seeing. Aperture in relation to resolving
then is a double-edged sword. The greater the
aperture, the greater the theoretical resolving power
but also the greater the problems with seeing; the
smallertheaperturethelessthetheoreticalresolving
powerbutalsothesmallertheproblemswithseeing.
Thisexplains,inpart,whymanyamateurastronomers
havetelescopesofdifferentaperture.
MagnifyingPower–Magnifyingpowerdescribeshow
much larger an object looks in an eyepiece in
comparisontotheobjectwhenseenwiththeunaided
eye.Atamagnificationof10X,forinstance,andobject
willappeartobe10timeshigherand10timeswider
in comparison to the view without magnification.
Magnifyingpower,M,dependsupontwoaspectsofa
telescope: the objective’s focal length (F) and the
eyepiece’sfocallength(f).Thatis,
𝐹
𝑀= 𝑓
Consider once again an 11” telescope whose
objectivehasafocalratio(f/)of10,andanapertureof
D = 280mm. The focal length is then found by
multiplyingtheaperturebythef/number.Thatis,
𝐹 = 𝐷 ∗ 𝑓/= 280𝑚𝑚 ∗ 10 = 2800𝑚𝑚
The magnification using an eyepiece with, for
example, a 32mm focal length eyepiece is then
calculatedfromtheearlierequationasfollows:
𝑀 = 𝐹/𝑓 = 2800𝑚𝑚/32𝑚𝑚 = 87.5𝑋
As state earlier, magnifying power is among the
leastimportantfactorsintheuseofatelescope.Why
should this be? It’s because by merely swapping one
eyepiece for another, magnification can be changed.
Longer focal length eyepieces produce lower
magnifications (and typically wider angular fields of
the sky) whereas shorter focal length eyepieces
producehighermagnifications(andtypicallynarrower
angular fields of the sky). Because telescope
eyepiecesarequitevariedintermsoffocallengthand
apparentfieldofview(andmanyotherfactors),these
areaddressedinaseparatepublication.
15.LIMITINGMAGNITUDEOFATELESCOPE
With the recent acquisition of an 18-inch Obsession
telescope,Ihavehadtheopportunitytocomparehowthis
telescope and my 2006 Celestron CPC 11-inch telescope
performintermsoflimitingmagnitude–themagnitudeof
the faintest stars visible at zenith through a telescope. I
haveduringthepastmonthtakenseveralopportunitiesto
comparetelescopicviewssidebyside,andhavecomeup
withanumberoffindings.Whileinvestigatingthelimiting
magnitude of my 18-inch (for the purpose of generating
betterstarmaps),Iwasmildlysurprisedtofindoutthata
largenumberoffactorsaffectthelimitingmagnitudeofa
telescope. I’d like to share some of my thoughts and
reflectionsdealingwithlimitingmagnitude.
My first real question after acquiring the Obsession
was, “What is this telescope’s limiting magnitude?” More
technically speaking, how faint a star can I see at zenith
with the telescope under varying conditions? I have been
stunnedbytheobserveddifferencesbetweenthe11-and
18-inch telescopes. Typical views through the 11-inch
telescope match very nicely the star maps generated by
my iPad’s SkyVoyager (recently renamed SkySafari)
16
program. That program shows stars down to about 12th
magnitude. When looking at the same star field with the
18-inch, however, the difference is amazing! While only a
few stars might be found in a given 11-inch field, many
timesmorestarscanbeseeninthesame18-inchfieldof
view.
Clearly, the larger a telescope’s objective lens or
mirror, the more light it is able to gather into the
observer’seye.Consideringtheobjectiveonly,theamount
of light that it can gather is directly proportional to its
surface area. The ratio of areas tells the number of times
morelightalargerobjectivecangatherincomparisontoa
smaller objective. Consider the relative light gathering
powers(LGP)ofmy11-and18-inchmirrors:
𝐿𝑃𝐺!"
18 !
=
= 2.68
𝐿𝐺𝑃!!
11
The 18-inch objective (not considering the secondary
obstructionandotherfactors)gatherssome2.68timesas
much light compared to the 11-inch objective all other
things being equal. This aperture difference alone will
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provide views of stars just over one magnitude fainter
1.01
(2.512 = 2.68). Clearly, this doesn’t entirely account for
the differences observed between my two telescopes.
Many other considerations also apply, and it is these that
accountforthemajordifferencesinwhatIhaveobserved.
Typeoftelescope:Reflectingtelescopeshaveamirror
for an objective. Most reflectors (but not a Shiefspiegler
for instance) have a secondary mirror that blocks a
significantamountoflightfromhittingtheprimarymirror.
Mirrorsaren’tperfecteither;theydon’treflectallincident
light. These factors work together to reduce the limiting
magnitude. (Recall that the fainter the object the higher
the magnitude.) The refractor has a lens as its objective
and is free from a central obstruction. Still, refracting
telescopes can backscatter a significant amount of light
from their surfaces if suitable anti-reflective coatings are
not in place. Lenses can also absorb some of the incident
light. The Schmidt-Cassegrain has a lens-and-mirror
combination. It is subject to all these problems of
reflectorsandrefractors.
Mirrorreflectivity/lenstransmittance:Thereflectivity
of the mirror and the transmittance of a lens will place a
cap on limiting magnitude. Both mirror and lens coatings
and optical cleanliness can affect limiting magnitude. For
instance,oldpurealuminumcoatingsonmirrorshadonly
an88%reflectivity.Twomirrors(primaryandsecondary)in
series would have an effective reflectivity to only 77%
2
(0.88 ).Modern“enhanced”coatingonprimarymirrorsis
typically 95% reflective and on secondary mirrors 98%
reflective with overall reflectivity of 93% (0.95*0.98).
Similar considerations must be taken into account for
refracting telescopes with and without anti-reflective
coatings on critical surfaces. Also of concern with
refractors is the clarity of the optical glass used to
formulate the objective lens. The same is true with
eyepieces.Thisarticleassumestheenhancedreflectivityof
mirrorcoatingsandtheuseofantireflectivecoatings.Itis
assumedthateyepiecesdonotplayadirectroleinterms
oflightreflectionandabsorption.Limitingmagnitudeswill
be lower by approximately 0.2 magnitudes than those
stipulated in this article if modern reflective coatings are
not used on the surfaces of objectives and secondary
mirrors (if employed). Poorly maintained (e.g., dirty or
oxidized optical coatings) will further reduce the limiting
magnitude of a telescope. “Clean optics” are assumed for
thepurposeofthisarticle.
Magnification:Theeffectofmagnificationonlimiting
magnitude is surprisingly great. My recent experiences
with observations of the planetary nebula Pease 1 in
globular cluster M15 show that magnification is a major
consideration. Higher magnification (e.g., 230X) with the
18-inchanda9mmeyepieceshowsdisproportionallymore
stars in the same field of view than are visible at lower
magnification (e.g., 52X) with the same telescope using a
40mm eyepiece. The higher magnification reduces the
brightnessofthebackground,makingfainterstarsvisible.
It’s the higher contrast that makes the difference. Under
stable atmospheric conditions, stars approximate point
sources and cannot be magnified in size significantly; the
background sky can be magnified, however, spreading its
light over a wider surface area of the pupil and therefore
reducingitsintensity.Thisincreasesthecontrastbetween
sky and star. Greater contrast means greater visibility.
(That’s why you don’t see stars during the daytime even
though present in the sky – the contrast is too low.) So,
limiting magnitude is clearly dependent upon
magnificationaswellasaperture.
Atmospheric seeing: Seeing – essentially the
turbulence of the atmosphere – can influence limiting
magnitude. Seeing can be measured by determining the
diameterofastarimage.Stars,whilelargeobjects,areso
distant that they normally appear only as point sources.
The Earth’s atmosphere can play havoc with starlight,
making the images much larger. Point sources are not
affectedbydimmingasaresultofmagnification;thesame
cannot be said when stars appear as disks. Disks of light
can be magnified, thereby reducing their surface
brightness. The more turbulent the atmosphere is, the
greater will be the size of stellar disks. Stellar disks can
easily vary from 0.5 arc seconds under ideal seeing
conditions to several seconds of arc under poor seeing
conditions.Poorseeingdecreasesthelimitingmagnitude.
Sky darkness: Pursuing a knowledge of limiting
magnitude of a telescope further makes one realize that
sky darkness will also help to determine the number of
stars visible in a telescope. This is analogous to the
experience where more stars are visible in the sky on
nights when it is especially dark. Anyone who has viewed
the sky from both urban and country settings will clearly
have a grasp on this. In an urban setting it is not
uncommon to find a limiting magnitude at zenith of 3 or
lower. Poor nights in the countryside will have a limiting
magnitude of perhaps 4.5, typical nights of perhaps 5.5,
and optimum nights of perhaps 6.5. A brighter sky will
reducelimitingmagnitude.
Sky transparency: Sky darkness and sky transparency
are not to be confused. A sky can be extremely dark and
yetstarscannotbeseeniftheskyisnottransparent(e.g.,
overcastwithclouds).Highhumidity,hazefromforestfires
and volcanic eruptions, dust from farm work, and thin
layersofcloudscaneasilyaffectskytransparency.Forthe
purposeofthisarticle,hightransparencyisassumed.Low
skytransparencywillreducethelimitingmagnitude.
Zenith distance and extinction coefficient: The path
length that starlight must traverse through the Earth’s
atmosphere depends upon zenith distance. The closer to
the horizon one observes, the greater the amount of
atmospheric extinction one experiences. We are all
familiarwiththefactthatthesuncanappearappreciably
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17
dimmer when near the horizon than when higher in the
sky. That dimming results from the increased path length
thatlightmusttravelthroughtheatmospheretoreachthe
eye.Overhead,thepathlengthisunity.Atthehorizonlight
must travel through as much as 5 times the amount of
atmosphere before reaching the eye. This added path
length causes dimming which is related to the extinction
coefficient.Typically,extinctioncoefficientsrangefrom0.2
to 0.6 magnitudes per unit air mass. Observing closer to
the horizon will reduce limiting magnitude. Atmospheric
extinction accounts for the dimming of the sun near
sunriseandsunsetandcanamounttoseveralmagnitudes
iftheskyisnottransparent.
B-V color index (CI) of a star: CI in this article is
definedasblueminusvisualmagnitude(B-V).Thebluera
star, the smaller the value of CI is. The CI of stars varies
considerably and affects visual acuity. We all know, for
instance,thatstarscomeinarangeofcolorsfromblueto
white to yellow to orange to red. This color can affect
one’s ability to see a faint star. We all know that we are
relatively insensitive to red light (hence, the use of red
lightatnight)andthemuchhighersensitivitytothebluegreen portion of the spectrum (whose light can destroy
dark adaption). Consider the following color indices:
Regulus, bluish B7 spectral type, CI = -0.11; Sirius, whitish
A0spectraltype,CI=0.0;Sun,yellowG2spectraltype,CI=
0.63;andBetelgeuse,redM2spectraltype,CI=1.85.The
humaneyeismostsensitivetotheyellow-greenportionof
the spectrum. Hence, observing faint stars outside this
optimum color range will result in a reduced limiting
magnitude.
Dark adaption: This article assumes complete dark
adaption and good eyes…properly focused star images,
etc. Dark adaption will allow for the eyes’ pupils to dilate
and for the chemical rhodopsin to form in the retina that
sensitizes it to faint light. Clearly, people who are dark
adaptedwillseemorestarsthansomeonewhoisnotdark
adapted – despite the fact that poorly dark adapted
individualswilloftenclaimthattheskyismuchdarkerthan
itappearstoaproperlydarkadaptedobserver.Oncefully
dark adapted, an observer is more likely to see the sky
glowinadditiontothebrighterstars.
Experienceofanobserver:Eventheexperienceofan
observer can affect limiting magnitude. Experience
observers will use averted vision effectively. This helps to
seedimstars,butthisisaqualitativeparameterandisnot
dealtwithfurtherinthisarticleorthesubsequentlimiting
magnitudecalculations.
Limiting magnitude calculations: So, the limiting
magnitude of a telescope is not a simple thing to
determine.Itdependsonlotsofopticalfactors,observing
conditions, and observer characteristics. Using the
followingwebsitewhosecodewaswrittenbyLarryBogan
(1998), I have been able to develop a data set for my 18inch telescope to which I have made adjustments to
includethenewestantireflectiveopticalcoatings.
http://www.nature1st.net/bogan/astro/optics/maglimit.html
Table1belowshowswhatIhavecalculatedtobethe
limiting magnitudes of my Obsession 18-inch telescope
dependingonvaryingconditions:
Magnification
52X
230X
PoorConditions*
12.7
15.1
TypicalConditions*
13.7
15.9
OptimumConditions*
14.7
16.6
Table1.LimitingmagnitudesofObsession18-inchtelescopeundervaryingconditions
So,whatdoesthismeanintermsofMilkyWaystarsin Obsession,ontheotherhand,canshownearly380million
the observable range? Consider Table 2. Under typical stars at high power under typical conditions – more than
conditions,myCPC1100canrevealabout5.3millionstars 70timesasmany!
at low power under typical conditions. My 18-inch
Magnitude
Range
NumberofStarsinRange
CumulativeNumberofStars
12
+11.50to+12.49
3,481,113
5,304,685
13
+12.50to+13.49
10,126,390
15,431,076
14
+13.50to+14.49
29,457,184
44,888,260
15
+14.50to+15.49
85,689,537
130,577,797
16
+15.50to+16.49
249,266,759
379,844,556
Table2.Numbersofstarsvisiblebymagnitude
The next time someone asks you the limiting old tried and true formulas (method 1: ML = 3.7 + 2.5 *
2
magnitudeofatelescope,becertaintotellthemthatthe Log10(D )whereD=apertureinmmandtakenfromVisual
18
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Astronomyforthe Deep Sky byRogerN.Clark;method2:
ML=9.5+5.0*Log10(D)whereD=apertureininchesand
taken from The Observational Amateur Astronomer by
Patrick Moore) aren’t really very accurate. For instance,
method 1 gives 17.0 for my Obsession telescope and
method 2 gives 15.8 under who knows what conditions.
Clearly, it is difficult to say precisely what the limiting
magnitude of any telescope actually is without a detailed
analysis such as that provided by Bogan and slightly
modifiedtoaccountfornewantireflectivemirrorcoatings
and such. For additional information about limiting
magnitude, see the article by Bradley Schaefer who first
calculated the limiting stellar magnitude an observer can
expect under various conditions with various types and
sizesoftelescopes.TheprocessisfullydescribedinSky&
Telescopemagazine,November1989,page522.
----------
*“Poorconditions”consistofa35degreezenithdistance,
4.5zenithlimitingmagnitude,extinctioncoefficientof0.6
magnitudes per atmosphere, dirty optics, seeing 2 arc
seconds,andsizeofeyepieceexitpupilislessthansizeof
observer’s pupil. “Typical conditions” consist of a 35
degree zenith distance, 5.5 zenith limiting magnitude,
extinction coefficient of 0.4 magnitudes per atmosphere,
moderately clean optics, seeing 1 arc second, and size of
eyepiece exit pupil is less than size of observer’s pupil.
“Optimal conditions” consist of a 35 degree zenith
distance, 6.5 zenith limiting magnitude, extinction
coefficient of 0.2 magnitudes per atmosphere, very clean
optics, seeing 0.5 arc second, and size of eyepiece exit
pupil is less than size of observer’s pupil. These
calculations also assume an “average” observer, neither
expert nor novice, with well-adapted eyes and a properly
focused telescope. Of course, the color of a star will also
makeadifference.Calculationsarebasedonthepresence
ofhighlydetectableA0starswithacolorindexof0inthe
fieldofview.
16.COMMONTELESCOPETYPES
Telescopescomeinavarietyoftypeswithamyriadof
variations as shown in the figure below. The three most
common telescope types used by amateur astronomers
today, however, are the classical refractor (A), the
Newtonian reflector (B), and the catadioptric of the
Cassegrain design (C). Each type of telescope has
advantages and disadvantages, and those looking to
purchaseatelescopeshouldbefamiliarwitheachinorder
tomakethebestpossiblechoice.
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19
TelescopeType
A)Refractingtelescopes…
aretheprobablythemostcommontelescopearound
generallyspeaking.Theyuselensesinsteadofmirrors
andtheeyepieceislocatedatthebottomofthe
telescope.Theirdesignissimilartobinocularsandmost
spottingscopes.Astronomicalrefractorstypicallywill
haveadiagonalprismormirrorjustinfrontofthe
eyepiecetochangethepathoflightby90°makingit
possibletolookdownratherthanupwardthroughthe
eyepiece.Withtheuseofadiagonalprismormirror,
imagesaresemi-invertedduetotheuseofanodd
numberofreflectingsurfaces.Theobjectivelensmustbe
eitherachromaticorapochromatictoavoidchromatic
aberration.Asingleelementlensisunsatisfactoryfor
astronomicalapplicationsbecauseoftheresulting
chromaticaberrationthatproducescolorfringesaround
celestialobjects.
B)Reflectingtelescopes…
useamirrorinsteadofalenstogatherlightandforman
image.Theeyepieceislocatedatthesideofthemain
tubenearthetop.Dependingonthetypeofmountused,
theeyepiececansometimesappearbelowthetelescope
tubenecessitatingtheobservertorotatethetelescopein
itsmounttobringtheeyepiecetothetop.Theproblem
isacuteifanequatorialmountisused.Imagesare
alwaysfullyinverted–flippedtoptobottomandleftto
right–duetotheuseofanevennumberofreflecting
surfaces.Thesetelescopesarecommonlyavailablewith
computerizedaltazimuthmounts(abovefromleftto
right),manualGermanequatorialmounts,and
Dobsonianpush-tomounts.Mirrorsmustbeparabolized
20
Advantages
Disadvantages
Easytouseduetothe
simplicityofdesign.
Excellentforlunar,planetary,
andbinarystargazing.
Sealedtubeprotectsoptics
andreducesimage-degrading
aircurrents.
Rugged,needslittleorno
maintenance.
Itssealedtubepartially
protectstheobjective’s
surfacefromcontaminants.
Oftencomewith“erector
prism”thatcanreorient
imagesofterrestrialobjects
sotheycanbeseenproperly
(aswithbinocularsora
spottingscope).
Generallyavailableonlyin
smallapertures,typically2¼
to6inches,duetohigh
expenseofproduction.
Highqualityrefractorscost
moreperinchofaperture
thananyotherkindof
telescope.
Smalleraperturesmean
poorerviewingoffaint
objectssuchasgalaxiesand
nebulae.
Heavier,longer,andbulkier
thanreflectorand
catadioptrictelescopesof
equalaperture.
Objectivelensissubjectto
dewandfrostwithoutthe
presenceofadewshieldor
lensheater.
Mosttoytelescopesareof
thisdesign.Caveatemptor–
letthebuyerbeware.
Readilyavailableinlarger
aperturesandatlowcost
comparedtoothertypes.
“Lightbuckets”usuallyhave
largeraperturesproviding
excellentviewsoffaint
galaxiesandnebulae.
Shortfocallengthsystems
candeliverlargerfieldsof
viewandbrighterimages.
Areflectorcoststheleastper
inchofaperturecomparedto
refractorsandcatadioptrics
becausemirrorscanbe
producedatalowercostthan
lenses.
Theenclosedobjectivemirror
isgenerallynotsubjectto
deworfrostwhileobserving.
Generally,notwellsuited
forterrestrialapplications
duetoinvertedimages.
Thetubeisopentotheair,
whichmeansdustonthe
opticsevenifthetubeis
keptunderwraps.
Reflectorsaremoresubject
toopticalmisalignmentthat
anyothertypeoftelescope
andtheyrequireperiodic
collimation.
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toatleast“¼wave”inordertoavoidobviousspherical
aberrationwhichresultsintheimpossibilityof
completelyfocusingonanextendedobject.
C)Catadioptrictelescopes…
useacombinationofmirrorsandlenses.Twoofthe
morepopulardesignsaretheSchmidt-Cassegrain(left)
andMaksutov-Cassegrain(right).ThetermCassegrain
referstothefactthatthelightpassesthroughaholein
theobjectivemirrorandlightexitsthebackofthe
telescoperatherthantheside.Whenthesetelescopes
areusedinconjunctionwithanaltazimuthmount(left)
theeyepieceinthesameorientationtothegroundno
matterwhereintheskythetelescopeisaimed.When
usedwithaGermanequatorialmount(right),thisisno
longerthecase.Diagonalprismsormirrorsarealsoused
forvisualobservingtoallowtheobservertolookdown
ratherthanupwhenpeeringthroughtheeyepiece.Due
touseofanoddnumberofreflectingsurfacesinsuch
cases,imagesaresemi-inverted.
Mostversatiletypeof
telescopewithexcellent
lunar,planetaryanddeep
spaceobservingplus
terrestrialviewingand
photography.
Readilyaccommodatesa
telecompressorthatpermits
theusertochangethe
effectivefocalratio.
Bestnearfocuscapabilityof
anytypetelescope.
First-ratefordeepsky
observingor
astrophotography.
Closedtubedesignreduces
imagedegradingaircurrents
oncethetelescopeasawhole
hasreachedthermal
equilibriumwiththeair.
Compactanddurable.
Moreexpensivethan
reflectorsofequalaperture.
Correctorplatethatholds
thesecondarymirroris
subjecttodewandfrost.
Thesetelescopesalmost
alwaysneedtobeusedin
conjunctionwithadew
shieldorlenswarmer.
17.COMMONTELESCOPETRAITS
Regardless of the design of telescope you own –
refractor, reflector, Cassegrain, Schmidt-Cassegrain,
Maksutov,Dall-Kirkham,ormanyothers–therearethree
basic types of telescopes: reflectors, refractors, and
composite telescopes. Composite telescopes are a
combination of reflector and refractor. (See the handout
COMMONTELESCOPETYPES.)
Regardless of telescope type, there are certain
telescopetraitsorotheraspectsthatarecommontomost
telescopesdesigns.
Aperture–Thediameter,D,ofatelescope’sobjective
mirror or lens constitutes its aperture. Because aperture
determines to a large extent light-gathering power and
theoreticalresolvingpower,itisoneofthemostimportant
traitsofatelescope.
Focal length – The distance between the objective
mirror or lens and its focal plane is known as the focal
length, F. The focal length is directly proportion to a
telescope’s magnification and true field of view with a
giveneyepiece.Longerfocallengthtelescopesyieldhigher
magnificationsandsmallerfieldsofview.
Focal ratio – The quotient of the focal length, F, and
the diameter of the objective, D, constitutes the focal
ratio.Forinstance,atelescopemighthaveafocalratioof
6.Thisisexpressedasf/6.Thisdoesnotmean“fdividedby
6”;thefmerelystandsfor“focal”andthe/infersaratio.
Expressedmathematically,focalratiof/=F/D.BothFand
D must be expressed in the same unit of measure to
accurately determine the focal ratio. Consider the
following example: F = 2800mm; D = 280mm; f/ = F/D =
2800mm/280mm=10.Hencethisisanf/10system.What
is the significance of the focal ratio? This ratio is used to
describethe“speed”ofatelescope.Iff/<6,thetelescopic
is considered “fast.” Such telescopes are ideal for lowpower, wide-field viewing and for photographing dim
objects. If f/ > 8, the system is considered “slow”. These
telescopesaregoodforworkingwithbrightobjectswhere
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21
highmagnificationisdesiredandnarrowfieldsofvieware
acceptable such as in viewing the sun, moon, or planets.
Focal ratios between 6 and 8 are considered generalpurposetelescopes.
Barlowlenses–Barlowlensesareconcavelensesthat
will effectively increase the focal length (and therefore
focalratio)ofatelescope.ABarlowlensisplacedbetween
the objective and the eyepiece in order to increase the
magnification of the system. It is typically attached to an
eyepiece.Forinstance,atypical2XBarlowlenswilldouble
the magnification of a given telescope eyepiece
configuration. Barlow lenses effectively double the size of
one’s eyepiece collection. If you had 40mm 32mm and
18mmeyepiecesforexample,addinga2XBarlowtoyour
collectionwouldbelikeowningthreeadditionaleyepieces
of 20 mm, 16mm, and 9mm focal lengths. A Barlow is
much more cost effective, usually costing less than the
priceofoneeyepiece.
Telecompressors – Many are the times when owners
of high focal ratio telescopes (e.g. f/10 such as with most
Schmidt-Cassegrain designs) are desirous of viewing with
lowerpowerandwiderfieldsofview.Thisismadepossible
withtheuseofatelecompressor.Telecompressorsarethe
oppositeofBarlowlenses.Theyareconvexlenses.Rather
than extending the effective focal length of a telescope,
theycompressit.Typicaltelecompressorlensesreducethe
focal ratio by a factor of 0.67. For example, an f/10
telescope effectively becomes an f/6.7 telescope with the
useofatelecompressor.ABarlow lens can be thought of
as a teleextender. An f/4.5 telescope effectively becomes
anf/9telescopewiththeuseofa2XBarlowlens.
Refractors with achromatic and apochromatic lenses
– Quality refracting telescopes will have objective lenses
that are either achromatic or apochromatic. Achromatic
means that the optical elements of the objective lens
(typically 2 elements) are matched in such a way as they
provide good color correction at two points on the
spectrum. Such lenses are less expensive than
apochromatic lens (typically 3 elements) that are color
corrected at three points on the spectrum. Apochromatic
lensesaresuperiortoachromaticlenses,buttheyalsocost
considerably more. Apochromatic lenses give the truest
colors to observed objects, while achromatic lenses do so
less well. Lenses without chromatic (color) correction will
producedimagesofstarswithcolorfringes.Toytelescopes
with plastic lenses are not commonly corrected for
chromatic aberration. Manufacturers of such telescopes
getaroundthisproblembyproducingtelescopesoflonger
focallength(largerfocalratio)wherechromaticaberration
islessofaproblem.
Reflectors with parabolized mirrors – Parabolization
ensures that light from all parts of the mirror produce all
parts of an image on the same focal plane. Cheap toy
reflecting telescopes commonly have spherical mirrors
rather than parabolic mirrors. At shorter focal ratios
especially(f/<10),sphericalmirrorswillnotproduceclear
22
images.Whenexaminedwithaneyepiece,variouspartsof
the image (center versus edge) come and go out of focus
as the eyepiece is racked in and out. Cheap reflecting
telescopesgetaroundthisbyhavingfocalratiosoff/≥10.
Athigherfocalratiosthedifferencebetweenparabolicand
spherical lenses pretty much vanishes. This is why
purveyorsoftoytelescopespromotehighermagnification
viewing.Theirgreaterfocalratio(and,hence,longerfocal
length)instrumentshavelessdifficultywiththeproblemof
sphericalaberration.
Catadioptric telescopes – These telescopes have a
front“correcting”lenslikearefractorandamirrorsystem
like a reflector. Perhaps the most popular type of
catadioptric is the Schmidt-Cassegrain. These telescopes
are very popular, very powerful. They tend to be longer
focallength,typicallyf/10.
Corrected reflectors – Many more advanced systems
are compounds with mirror systems capturing and
focusinglight,andthenlenssystemsmakingcorrectionsto
thefocalplane,suchaswithacorrectedDall-Kirkham.
Telescopestubesandtrusses–Tubeortrusssystems
help hold the optical components in place. They vary in
rigidity, thermal stability, and stray light trapping. Tubes
are solid cylinders and can be heavy. Truss systems
typicallyhave8strutsthatcanletstraylightin.Tubescan
becardboard,compositematerials,metal,etc.Trussesare
typically built with metals. Ideally tubes and trusses have
low coefficients of thermal expansion. High quality
thermallystabletubesandtrussesarecomposedofcarbon
fiber.Closedtubesystems(refractorandcatadioptric)can
sufferfromthermalcurrentsinthetubesasthenighttime
temperaturesdrop.
Light trapping – The insides of telescope tubes are
painted flat black. Higher quality tubes have flocking
paper.Thestrutsofatrusssystemwillmostcommonlybe
coveredwithablackshroud.Refractorsoftenhaveseveral
internal baffles. All these materials effectively capture
stray light that has a strong tendency to reduce image
contrast.
Paint – Traditionally portable telescopes have been
paintedwhitetomakethemmorevisibleatnightsoasto
help observers avoid bumping into them. Such a highly
reflective color can also help keep a telescope cooler if
usedfordaytimeobservations.
Dewshields–Higherqualityrefractorstypicallyhavea
section of larger diameter “tube” extending past the
objectivelensostensiblytokeepdewfromcondensingon
the lens. Catadioptric telescopes will often be outfitted
with a flexible, wrap around dew shield to keep the
corrector plate from dewing. Closed-tubed reflectors
already protect their objective from dewing and further
precautionsarenotneeded.
Secondary mirrors – Non-refractor telescopes have a
secondarymirrorredirectthelightformingtheimagetobe
observed.IntheNewtonianversion,thesecondarymirror
is usually held in place by four thin metal vanes that
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produce the “X” diffraction spikes often seen in
astronomical pictures. The absence of the “X” means the
telescope is either a refractor or a catadioptric. In some
configurations the secondary mirror reflects the focused
lightat90degreestotheside.Inothers,itreflectsitback
down toward the primary mirror through a hole in the
centerofthemirror.
18.TELESCOPEMOUNTSANDPIERS
Ifyouareinthemarkettopurchaseatelescope,you
might want to seriously think about the type of mount
and pier that will come along with the telescope of your
choice. They are not all equal. Keep in mind that we are
not talking about “junk” toy telescopes here with their
over-powered telescopes, flimsy mounts, and spindlylegged tripods. There are several legitimate types of
mount/piercombinationsforyouasaseriousobserverto
considerwhenyoubuyaqualitytelescope.Butfirst,let’s
notethedistinctionbetweentheterms‘mount’and‘pier’
–termsthatconfusesomepeople.Amountisthedevice
thatisdirectlyconnectedtothetelescopeandrestsupon
a pier. A pier is a single column or a
tripodthatsupportsthemount.Themountallowsforthe
telescope’s motion; the pier should be rock solid and
should not flex. There are two fundamental types of
telescope motion just as there two fundamental types of
mounts.
Altazimuth mounts – These mounts –
altitude/azimuth–allowtelescopestohorizontally(sideto
side or in azimuth) with one axis and vertically (up and
down or in altitude) in the other axis. This seems like a
pretty reasonable way to mount a telescope until one
realizes that stars don’t move in this fashion. Here in mid
northernlatitudesthestarsintheeasternskyrisemoving
to the right as they do so. When in the south, the stars
move from left to right. When in the west the stars set
moving to the right as they do so. In order for an
altazimuth mount to track the stars, they need to be
moved in both axes at different and changing rates
dependingwhereintheskythetelescopeispointed.These
are serviceable mounts, but the best among them are
driven by computer-controlled stepper motors. These
motorscandirecttheaimofthetelescopeatvariablerates
andinchangingdirections.
Equatorial mounts – Are a simple solution to the
problems of altazimuth mounts. The have two axes of
movement90degreesoffsetfromtheotherjustasinthe
caseofthealtazimuthmount.However,ratherthanhaving
thehorizontalrotationaxisorientedvertically,itisinclined
in such a way that is parallel to Earth’s rotation axis. Its
right ascension or polar axis is aimed toward the north
celestialpoleneartheNorthStar.Thisallowsthetelescope
tobeslewedeastandwestacrossthesky.Sooriented,the
telescope can follow celestial objects with the motion of
oneaxisonly.Onlythisaxisneedbemotorizedinorderfor
ittofollowcelestialobjects,andthespeedanddirectionof
motion is constant. The other axis, the declination axis,
allows the telescope to be slewed north and south in the
sky.
Now there are variations on these two types of
mounts,andthereisevenabitofoverlapinthedefinitions
of these two types under certain circumstances as well.
Consider “what if” either of the two above mounts was
used at either the North or South Pole of Earth. At these
locations,analtazimuthmountwouldbenodifferentfrom
anequatorialmountasthecelestialpolewouldbelocated
overhead!Themount’srightascensionorpolaraxiswould
becomeanazimuthaxis,anditsdeclinationaxiswouldits
altitudeaxis.
Thepriorimagesshowwhatisknownasaforkmount.
Due to problems with fork mounts and long telescope
tubes (e.g., one can’t above all the way up to the zenith
withthealtazimuthmountnorallthewaytothecelestial
polewiththeequatorialmount),mosttelescopesthatuse
a type of azimuthal mount today use a style designed by
JohnDobsonoutinCaliforniayearsago.Theadvantageof
theDobsonmountisthatthetelescopeiskeptlowtothe
groundmakingitaccessibletothesmallestofviewers.It’s
great for public observing sessions. Another advantage is
that this style is easily and cheaply constructed. Many
serviceable telescopes use this sort of mount.
Unfortunately,thisstyleofmountisnotreadilymotorized
on the cheap. Yes, there are Dobson mounts with
computer-controlled drive motors, but they tend to be
rather expensive. Other variations of the fork mount can
beseeintheCelestronCPCseriesoftelescopes.
Much more readily motorized are the equatorial
mounts.Thesecomeinseveraldifferenttypesaswell,two
of which are worth mentioning. The yoke mount (an
exampleofwhichisshownonthenextpage)israrelyseen
in the world of amateur astronomy nowadays. As the
picture illustrates, the yoke has a real propensity for
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23
getting in the way
whentryingtoview
through
the
eyepiece.
This
design is perfectly
acceptable for large
telescopes such as
the 200” Mount
Palomar telescope,
but that’s because
theobservercansit
inacageandviewfromwithinthetelescopeitself(though
this isn’t done today given recent with advances in
observingequipment).
An alternative to the yoke mount is the German
equatorial mount. This mount is commonly found in
amateur telescopes today. The telescopes in the TCAA’s
Prairie Sky Observatory, for instance, are all on German
equatorial mounts. Experts like these mounts, though
many novices find them rather objectionable. In order to
viewtheeasternsky,thetelescopemustbelocatedonthe
western side of the right ascension or polar axis. When
switchingviewstothewesternsky,thetelescopemustbe
“flipped”totheeasternsideoftherightascensionorpolar
axis. Another objectionable aspect of this mount is that
there are large counterweights on the declination axis to
counterbalance the weight of the telescope. This is
becausethemotorsdrivingthepolaraxisdon’tneedtolift
thetelescopeduetothecounterweights.
Apiersupportsthemountthatsupportsthetelescope
thatallowsittomovearoundthesky.Thereareessentially
two types of piers in use today – vertical columns and
tripods. Vertical columns are typically massive and are
meant to stay in one place. Tripods are lighter and are
designed for use in mobile situations. A generation ago it
waspossibletofindverticalcolumnswiththreehorizontal
supportlegsassociatedwithportabletelescopes;that’sno
longer the case today. Good piers are heavy and solidly
built; poor piers such as found with toy “junk” telescopes
arelightandtypicallyquitespindly…
19.HOWTOBUYATELESCOPE
Buying a telescope is an exciting prospect.
Unfortunately,thepurchaseanduseofapoorqualitytoy
telescope will result in difficulty, frustration, anger, and
ultimately a loss of interest in sky viewing. It is important
therefore to know how to purchase the right type of
telescope before doing so. Avoid throwing money at the
problem, and just buying the first thing that looks
interesting – even if it is expensive. Many are the people
who have purchased expensive, seemingly sophisticated
telescopes only to end up with a useless piece of junk
ratherthananinstrumentthatcanservetheirinterestfor
a lifetime. The only legitimate way to enter amateur
astronomyistospendtimelearningaboutthesubjectand
carefully reviewing what the experts have to say about
which are the best telescopes to buy. This essay will give
some basic information, and will reference a number of
otherpublicationswrittenfortheTCAA’sUniverseSampler
IIclass.Pleasebecertaintoincludesuggestedreadingsas
youendeavortopurchaseyour“dream”telescope.
Terrestrialorcelestialtelescope?Telescopesdesigned
forskywatchingareoften,butnotalways,differentfrom
those designed for terrestrial use. Telescopes come with
two basic types of mounts. Terrestrial telescopes will
always come with non-motorized altazimuth mounts
where the movement is up and down and left and right.
Unfortunately,celestialmotionsarenotassimpleasthat.
Astronomicaltelescopesaredesignedtotrackthemotion
of the stars and will either come with computer assisted
altazimuth mounts or equatorial mounts whose main
rotationaxisisalignedwiththatofEarth.Readaboutthese
mountsinthesectionTELESCOPEMOUNTSANDPIERS.
Reflector, refractor, or another design? Reflecting
telescopes have mirrors that gather light and create an
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imageforviewingwithaneyepiece.Refractingtelescopes
usealenstodothesame.Combinationsofreflectorsand
refractors – catadioptric telescopes available in various
designs – use combinations of lenses and mirrors to
produce images for viewing. Reflectors are relatively
inexpensive in comparison to refractors. A high quality 8inchreflectingtelescopemightcostintherangeof$1,000$2,000;thesamesizerefractingtelescopecouldeasilyrun
$10,000 or more. When making a mirror for a reflecting
telescope, there is only one optical surface to finish. The
lens of a high-quality, color corrected (apochromatic)
refractor will have 6 surfaces to finish, and the finishes
much exactly match the adjoining lens’ surface. The
Schmidt-Cassegraintelescope(SCT)–acatadioptric–uses
asinglethinlensasacorrectorplateandamodifiedmirror
to accomplish the same task. The advantage of the SCT
design is that one can compress a large telescope into a
relatively small space. For example, a classic 8” f/10
reflector would have a tube over 7 feet long. A SCT with
the same optical characteristics would be less than 2 feet
long.
Howbigamirrororlens?Whileonecansavemoney
by purchasing a small aperture telescope, it would
constitute a waste of money not to buy the largest
aperture one can afford – up to a point. People who
purchase small telescopes at the outset often come to
regretitbecausebeforelongtheycatch“aperturefever.”
Larger telescope do, in general, provide greater benefits.
(SeethehandoutCOMMONTELESCOPETRAITSfordetails.)Asa
general guideline, I suggest a telescope of at least 8”
aperturefortheseriouswould-beamateurastronomer.If
youcanaffordit,an11”aperturetelescopewouldbeeven
better. As an Astronomical League Master Observer with
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over50yearsviewingexperience,Iamveryhappywithmy
11”SCT.Goinglarger–inlightpollutedIllinois–resultsin
diminishing returns. If one views from the rural skies of
Illinois, the sky is not adequately dark to take full
advantage of a larger aperture. While large telescope
mirrors do collect more light, they also collect more sky
light, much of which is produced by artificial illumination.
As a result of this problem, I recently sold a very high
quality 18” telescope for which I paid nearly $10,000
becausetheviewsitprovidedrelativetomy11”telescope
were not all that much better – certainly not worth the
troubleofdraggingitoutintothecountrysideandsetting
itup.
What about eyepieces? Telescopes will sometimes
come with several eyepieces providing a variety of
magnifications and fields of view. These are commonly
inexpensive but serviceable. Still, experienced observers
willtypicallywanttoreplacetheseeyepieceswithonesof
higher quality. Higher quality eyepieces most commonly
willprovidelargerfieldsofviewatthesamemagnification.
Certainothereyepiecewillprovidemuchbettereyerelief
that makes them more useful for those who wear
eyeglasses.Yetothersareparfocalthatmeanstheycanbe
exchanged with one another without requiring significant
refocusing.Aclusterofperhapsfivegoodqualityeyepieces
along with a Barlow lens should be purchased with a
quality telescope. Eyepiece focal lengths should span the
range from minimum to maximum useful magnification.
See the following section, TELESCOPE EYEPIECE BASICS, for
additionalinformation.
I’m not quite ready to purchase a telescope…That’s
perfectly fine, and might well reflect that fact that you
needtospendtimelearningmoreaboutthingsofthenight
skyandtheworkofamateurastronomers.
Caveatemptor–letthebuyerbeware.Again,buyan
honest-to-goodness optical instrument and avoid
purchasing a toy telescope. If you are not yet ready to
purchase a quality telescope, then consider starting off
with a good set of binoculars. (See BINOCULARS VS.
TELESCOPES to gain a better understanding of the
difference.)
20.TELESCOPEEYEPIECEBASICS
Recall how a telescope works. An objective lens or
mirrorproducesarealimagethatisthenexaminedwitha
sophisticated magnifying glass known more commonly as
an eyepiece. Eyepieces come in many different varieties,
each with its own characteristics – pro and con.
Nonetheless, there are several traits that all eyepieces
haveincommon,andsoourintroductiontounderstanding
eyepieceswillbeginhere.
Therearefivephrasesthatcanbeusedinassociation
witheveryeyepieceusedwithatelescope.Thesephrases
arefocallength,apparentfieldofview,truefieldofview,
exitpupil,andeyerelief.
Focal Length – The focal length of an eyepiece is the
distance from the principal plane of the objective to the
focal plane of an eyepiece where parallel rays of light
convergetoasinglepoint.Itisfixedforagiven(non-zoom)
eyepiece.Asaresult,mostamateurastronomerswillhave
a variety of eyepieces of different focal lengths ranging
from about 6mm to 40mm most commonly. Because
magnificationisdefinedasfocallengthoftheobjective,F,
divided by the focal length of the eyepiece, f, different
magnifications can be obtained using different eyepieces.
Thatis,M=F/f.
ApparentFieldofView–FOVAisacharacteristicofan
eyepiece.Fieldofviewissimplyhowwideanangleaneye
can perceive looking through an eyepiece. While the
humaneyecanspanoffieldofviewofnearly180degrees,
eyepieces lenses – because their fields of view are
confinedbythenarrowtube-typicallyrangefromabout
30°forinexpensiveeyepiecestoasmuchas110°fortruly
expensiveeyepieces.
True Field of View – FOVT is a function of any
eyepiece-telescopecombination.Ittellstheangularsizeof
theportionofskythatcanbeviewedthroughaneyepiece
when used with a particular telescope. It is magnification
specificandistypicallybetweenonetenthofadegreeand
two degrees (about four times the angular size of the full
moon). FOVT can be approximated from the following
formula:FOVT=FOVA/M.Theformulaisaccurateto4%or
better up to 40° apparent field of view, and has a 10%
errorfor60°.
1¼”vs.2”Eyepieces.Eyepieces(andsometelescopes)
come with three draw tube or barrel diameters – 0.965”,
1¼” and 2”. (The smallest standard barrel diameter is
usually found in toy store telescopes and will not be
addressedhere.)Qualityeyepiecesareonlyofthe1¼”and
2” varieties. A 2” eyepiece is larger, heavier, and more
expensive than a 1¼” eyepiece of the same focal length.
Why then do some amateur astronomers purchase the
larger-diameter eyepieces? It all has to do with the true
fieldofview.A2”eyepiececanprovidealargerFOVTthan
a 1¼” eyepiece. For an eyepiece designed with a given
apparentfieldofview,thebarreldiameterwilldetermine
themaximumfocallengthpossibleforthateyepiece.This
issobecausenofieldstop(theinnerdiameterofthedraw
tubeinmostcases)canbelargerthanthebarrelitself.For
example,aPlössltypeeyepiecewithaFOVAequalto45°in
a1¼”barrelcannothaveamaximumfocallengthgreater
than35mm.Anylongerfocallengthwouldrequireawider
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25
barrel or the view is restricted, effectively making the
apparentfieldofviewlessthan45°.Amateurspurchase2”
eyepiecesbecause,dependingupontheopticaltype,they
can obtain larger true fields of view for a given
magnification.
Eyepiece Types – There are many eyepiece designs
commonly used with telescopes these days. Each has its
benefitsanddetractions.Theydifferintermsofprice,eye
relief, apparent field of view, achromatic characteristics,
andfieldflatnesstonamejustafew.Whilethedifferences
aretoocomplextodealwithhere,thetypesofeyepieces
commonlyavailabletodayareasfollows:
Orthoscopicor“Abbe”
Monocentric
Erfle
Negativeor“Galilean”
Convexlens
Huygens
König
RKE
Nagler
Ramsden
Kellneror“Achromat”
Plösslor“Symmetrical”
Eye Relief – Eye relief is the distance one needs to
placetheeyefromtheeyepiecetoseeitsfullfieldofview.
Shorteyereliefeyepiecesrequireanobservertoplacethe
eye close to the outermost lens of the eyepiece. This can
be a problem with lower-cost eyepieces, especially when
they have short focal length. Shorter focal length usually
translatestolesseyerelief.Thisisn’talwaysaproblem.An
observerwhowearseyeglassesfornearorfar-sightedness
can remove them if desired to look through a telescope.
Thefocusofthetelescopecanbeadjustedtocompensate
for these eyes defects. However, if an observer wears
eyeglassesforastigmatism,thenheorshewillneedtouse
eyeglasseswhenlookingthroughthetelescopetoobtaina
sharpfocusacrossthefieldofview.Shorteyereliefmakes
ithardertogetone’seyecloseenoughtotheeyepiecefor
viewing–especiallyifusingglasses.Thisiswherethewise
choiceofeyepiecescomesintoplay.Eyereliefisafunction
ofeyepiecetypeandfocallength.Someeyepiecesprovide
moreeyereliefthandoothers.Withoutglasses,10-20mm
ofeyereliefisgoodforcomfortableobserving.However,if
you need glasses while observing (or simply prefer not to
havetokeepremovingthem),useeyepieceswithatleast
17-20mmofeyerelief.
Exit Pupil–EPisthediameterofthelightbeamthat
emergesfromaneyepiece.Thepupilofyoungadult’sfully
dark-adapted human eye is about 7mm diameter (and
decreases to about 5mm with increasing age). So, if an
eyepiecehasanexitpupillargerthan7mm,itpassesmore
light than the eye can intercept and vignetting of the
26
image results. When the exit pupil approaches 1mm
diameter,solittlelightispassingthroughtheeyepieceas
to make viewing almost futile. Exit pupil is a function of
magnification.Theseaspectssuggest,then,thatthereare
upperandlowerlimitstomagnification,andsothereare.
Exitpupil(inmm)canbedeterminedfromthediameterof
thetelescopeobjective(alsoinmm)andthemagnification:
EP = D/M. For example, an 11” (280mm) diameter
telescopeusedat87Xwillproduceanexitpupilof3.2mm
regardlessofthetypeofeyepieceused.Themaximumand
minimum eyepiece focal lengths used with any telescope
arethensetbythesecriteria.M=D/EP
Mmin=280mm/5-to-7mm=40X-to-56X
Mmax=280mm/1mm=280X
Parfocal Eyepieces – Parfocal eyepieces come as a
matched set from a manufacturer. When Parfocal
eyepieces are switched out to obtain different
magnifications, the image stays pretty much in focus.
Thereisinevitablyasmallamountoffocuserror,butitis
minimal.Sucheyepiecesaremoreofaconveniencethana
necessity.
BarlowLens–Theauthorwouldberemissifhewere
nottomentiontheBarlowlens.The“Barlow”isaconcave
lens that effectively doubles (or sometimes triples
dependingontype)thefocallengthofanobjectivelensor
mirror. In doing so, a Barlow will increase the effective
magnificationofagiveneyepiecewithoutreducingitseye
relief significantly. A Barlow is sometimes used with a
lowerpowereyepieceasawayofincreasingmagnification
withoutdecreasingeyerelief.
Magnificationandfieldofviewareimportanttraitsin
the selection of an eyepiece for viewing. Higher
magnificationmeansasmallerfieldofviewwiththesame
typeofeyepiece.Thefirsttwoimageshereshowtheeffect
of increasing the field of view at the same magnification.
Thethirdimageincomparisonwiththefirstimageshows
the effect of using a higher power eyepiece with a wider
fieldofview.
FindingtheTrueFieldofView–Withthemotordrive
of your telescope turned off place a star on the celestial
equator at the edge of a field of view. Position it so that
the star drifts directly through the center of the field.
Repositionthestarjustoutsidethefieldofview.Timehow
long it takes the star to move entirely across the field of
view.BecauseEarthturnsatarateofabout15°perhour
and 1° every four minutes, the true field of view can be
determined by the time of passage. Say the passage
requires 45s or 0.75 minutes. The field of view is then
calculatedfromthestandardformula“amountequalsrate
timestime.”
FOVT=1°/min.x0.75min.=0.75°or45’ofarc
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21.EYEPIECEFIELDORIENTATION
It’s not uncommon for the novice sky watcher to be
surprised to find out that his or her telescope produces
“upside down” images. Why does this occur? After all,
binoculars and eyepieces both have objectives and
eyepieces too. The reason binoculars produce upright or
erect images is because they have prisms (roof or porro)
between their objectives and eyepieces. These prisms
invert images naturally produced by the objective lenses.
When viewed with eyepieces, the images appear as they
wouldtotheunaidedhumaneye,merelylarger.
Inspacethemeaningofupordownarelost.Thereisno
up or down in space so it doesn’t matter which way an
image appears in the eyepiece. Nonetheless, it is good to
know how the introduction of additional mirrors in basic
telescope designs affects the images. The table below
providesaquicksummaryofwhathappenswithdifferent
typesoftelescopesandvariouscombinationsoflensesand
mirrors. Additional mirrors normally come into play with
theadditionofadiagonal(mirrororrightangleprism)just
beforetheeyepiece.
22.FINDERSCOPES
Traditionally, finder ‘scopes are typically small
telescopes that are attached to and optically aligned with
the main telescope. They are used to aim the main
telescope. Finder scopes are of short focal length, low
magnifying power, and wide field of view. Most include
crosshairs.Thesesmallauxiliarytelescopesareusedtofind
FinderA)Straightthroughfinder
objects and center them in the field of view. Once this is
done, the main telescope should be pointing at or very
closetotheobjectthattheobserverintendstoview.
Todaythisisawidevarietyoffinders,andnotallare
telescopes.Notallhavemagnification.Shownbelowarea
number of finder telescopes, each with its pros and cons.
FinderB)Withrightangleprismandilluminatedcrosshairs
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27
FinderC)Telradwithzero-powerheadsupdisplay
FinderD)Reddotfinder
Finder A. This is an all-too-commonly-found ‘cheap’
finder.Ittypicallyisa6X30monocular,andisusuallyfound
on toy telescopes. It provides 6X magnification and has a
30mm aperture. While marginally adequate, it lacks basic
features such as illuminated crosshairs. Focusing is often
difficult (adjustment at the objective lens rather than the
eyepiece),andthesmallaperturemakesitdifficulttosee
any but the brighter stars. Another drawback is its poor
ability to align with the main telescope. The three
setscrews on the side of the finder provide only a
minimum of adjustment. This often makes it difficult to
keep the finder properly aligned with the main telescope.
Images are inverted, which is commonly the case with
standardtelescopes.Thisisnotaproblem,butitdoestake
some getting used to. Push up to go down and left to go
right. I do not consider this a ‘serious’ finder scope. Still,
higher quality versions of this type of finder can be
consideredquitehelpful.
FinderB.Thisisamoreserioustypeoffinder,butitis
also fraught with difficulties in this configuration. It has a
largeraperturethatwillrevealmorestarsandmighteven
show the object you intend to observe. The illuminated
reticle is a definite plus. This unit is much more
substantially built and provides greater latitude for
alignmentwiththemaintelescope.Oncealigned,thisunit
probablywillstayaligned.Theproblemreallybeginswhen
looking through this telescope. If you thought that an
invertedfieldofviewwasdifficultinthecaseoftheabove
finder, this one is somewhat worse. Introducing a mirror
makestheeyepiecemoreaccessibletotheobserverasitis
now located at right angles to the telescope.
Unfortunately, introducing the mirror produces a semiinverted field of view. Depending on its orientation with
respect to the sky one might push up to go up and push
righttogoleft.Onlyoneofthetwodirectionsisreversed,
butfiguringoutwhichonecanbearealpain.
Finder C.IusedtoresistusingthisfinderwhenIfirst
started seeing them on other telescopes, but today
wouldn’t observe without it. This is a Telrad finder. It’s a
straight-throughheads-updisplaywithasetoffinderrings
insteadofthetraditionalcrosshairs.AnLEDprojectsthree
redrings(brightnessadjustablewithdimmerswitch)onto
the heads up display. The observer then sees the rings
projectedagainstthesky.Theviewoftheskyiserect(not
inverted or semi-inverted). The advantages are the ultrawidezero-magnificationfieldofviewandthetremendous
eye relief. One can place one’s eye several inches back
from the heads up display and still see the rings. The
disadvantageisthatwithoutalight-gatheringlens,it’shard
to see any but the brighter stars, and is difficult to use
when trying to find faint objects without any brighter
nearby stars. Another draw back is that the heads up
displayisstronglysubjecttodewing.
Finder D. More commonly found on toy telescopes
thesedays,thisistypicallyacheapknockoffversionofthe
Telrad with a considerably smaller heads up display.
Instead of a series of rings, a red dot is projected against
thesky.Tofindobjectsinthemaintelescope,merelyplace
the red dot over the area you intend to observe. In my
opinion, this is not a serious finder either. Still, better
quality versions do exist that some observers find quite
useful.
WhichshouldIchoose?Thefactofthematteristhat
no one finder can satisfy all needs. For years, before I
started using my goto CPC 11” telescope in 2006, I would
useacombinationoffindertypesAandC.Iwouldusemy
Telrad(onthesideofa10”CoulterOdysseytelescope)to
zero in on the area I was planning to observe. Once I did
thatIusedmy7X50straight-throughfindertolocatemore
preciselytheobjectIwasplanningtoobserve.Then,when
I looked through my main telescope, the object I was
seekingwastheninthemaininstrument’sfieldofview.
Ididasyousaid,andstillcan’tseewhatI’mlooking
for.That’sfairlycommon,butdon’tgiveupthesearch.I’ve
hadthishappentomehundredsoftimes.It’simportantto
keep in mind that if you identified the star field properly
anddidthebestyoucouldtocentertheareaoftheobject
you are looking for, then the object is most likely just
outside the field of view of your main telescope. You
shouldexecuteasearchpatterndependingonthetypeof
instrument with which you are observing. There are
typically two or three search patterns you can execute. I
useallofthemdependingontheinstrumentI’musing.All
threepatternsbelowcanbeusedeffectivelywithpush-to
28
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telescope(onethatisnotmotorizedandisfreetomoveup
and down and left to right). If using a telescope that is
motorized,thenthefirsttwosearchpatternscanbeused
mosteasily.Still,withpracticeeventhesectorsearchcan
beusedwithgototelescopes.
How do I align my finder? This is relatively easy, but
it’s a matter of using your finder and main telescopes in
reverse. Chose a bright object like a far away terrestrial
object,themoon,aplanet,orastar.Aimyoutelescopeat
the object and center it without using the finder. Then,
adjustthefindersothattheobjectislocatedinthecenter
of its field of view. Check back and forth between the
finder and the telescope to ensure that they are properly
alignedwitheachother.Onceyougetthetwoinstruments
optically aligned, you’ll not need to follow this procedure
again.
23.STARHOPPING
Star hopping is a procedure that can be used to find objects with
binoculars or telescopes when the object is too dim to be found directly
withtheeye.Theprocessisrelativelysimpleandisillustratedbelow.Let’s
saythatyouwanttofindMessier92(M92)intheconstellationofHercules.
WhileM92isnotpossibletoseewiththeeyeorasmallfindertelescope,it
canbeobservedwiththeuseofthemaintelescope.
Using your finder scope, center the main instrument on the star
labeledπ(justbelowthestarlabeled69).Thefieldofviewofyourfinder
and main telescope are the large and small circles respectively. Note the
arrangement of stars in the telescope’s field of view (large circle), paying
particular attention to the brighter star to the upper right of 69. Slowly
moveyourtelescopesothatthisstarshiftstotheleftsideofthefieldas
showninthesecondlargecircle.Notethe“double”startothelowerright
ofthesecondfieldofview.Moveyourtelescopeagainsothatthebinaryis
nowtothelowerleftofthethirdfieldofview.M92willthenbejusttothe
upperrightofthefieldofviewofthemaintelescope.Movethetelescope
accordinglyandyouwillhaveacquiredM92forviewing.
24.LIGHTPOLLUTION,TRANSPARENCY,ANDSEEING
Notallnightsareequal,eveniftheyareequallydark.
Forinstance,iftheskyisentirelyovercastatnight,itmight
bedarkbutyouwon’tbeabletoseeanycelestialobjects.
Thinlayersofstratuscloudsandatmospherichazedueto
highrelativehumiditycanaffectyourobservingsometimes
forthebetter(suchasforsomesolarsystemobjects)but
most of the time for worse (star clusters, nebulas,
galaxies). Another consideration is the stability of Earth’s
atmosphere.TheturbulenceofEarth’satmospheremakes
it impossible to get a clear view of the sky. So it is with
atmosphericturbulence.Iftheatmosphereisturbulent,we
havepoorseeingandviceversa.Poorseeingisthesource
oftwinklinginstars.Ifthestarsareviolentlytwinkling,you
might consider observing another night if you are seeking
themoststableviews.
All the light that’s visible at night doesn’t come from
the stars, moon, planets, and Milky Way. Too frequently
the night sky is illuminated by wasteful stray light from
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29
st
th
streetlights, parking lot lights, building lights, outdoor 1 magnitude are 100 times brighter than stars of the 6 display advertising, illuminated billboards, and sports magnitude. Each magnitude represents a brightness
5
venues (e.g. driving ranges, stadiums, and arenas). difference of about 2.512 times (2.512 = 100). Sirius, the
Reducing the contrast between the stars and the DogStarinOrion,thebrighteststarinthenightsky(note
th
background sky, light pollution makes it more difficult (if thatit’sNOTPolaristheNorthStar–the49 brighteststar
notimpossible)toseethestarsandMilkyWayatnight.
inthesky)issobrightthatithasamagnitudeofnegative
Just how many stars can be seen at night with fully 1.44. Stars like Vega in Lyra the Harp, Arcturus in Boötes
dark-adapted eyes? That depends almost entirely on the the Bear Drive, and Capella in Auriga the Charioteer are
brightnessofthebackgroundsky.Astheskygetsbrighter, bright enough to qualify as zero-magnitude stars.
fewerstarsarevisible.Atoneextreme,thedaytimeskyis AldebaraninTaurustheBull,SpicainVirgotheMaidenof
sobrightastomakeeventhebrighteststarsimpossibleto the Harvest, and Antares in Scorpius the Scorpion qualify
st
nd
observe without anything other than a high-powered as 1 magnitude stars. Relatively speaking, very few 2 telescope.Atnight,underaverydarkandtransparentsky, magnitude stars and those dimmer are known by their
th
starstothe6 magnitudemightbeobserved.
propernames.
Magnitude is a rating scale used by astronomers to
compare the brightness of stars. Very roughly put, the AWordaboutLimitingMagnitudes
st
brighteststarsaretypicallyof1 magnitude(thoughsome are brighter with 0 and even –1 magnitude). The next
The first column in the table below gives magnitude
nd
rd
th
groupsofincreasinglydimmerstarsareofthe2 ,3 ,4 , names. The second column gives magnitude ranges. The
th
th
th
5 , and 6 magnitude. Stars of the 6 magnitude are the thirdcolumngivesthenumberofstarswithinaparticular
dimmest visible under good dark-sky conditions with fully magnitude range. The fourth column gives the total
dark-adapted vision. This historic six-step magnitude number of stars visible by limiting magnitude – the
system is a qualitative measurement based upon the magnitude of the faintest star visible. The higher the
logarithmic response of the human eye to light. If limitingmagnitude,themorethestarstherearevisibleto
measuredwithamodernlight-sensingdevice,starsofthe theunaidedeye.
Magnitude
MagnitudeRange
No.ofStarsinRange
CumulativeNo.ofStars
–1
–1.50to–0.51
2
2
th
0(0 )
–0.50to+0.49
6
8
st
+1(1 )
+0.50to+1.49
14
22
nd
+2(2 )
+1.50to+2.49
71
93
rd
+3(3 )
+2.50to+3.49
190
283
th
+4(4 )
+3.50to+4.49
610
893
th
+5(5 )
+4.50to+5.49
1,929
2,822
th
+6(6 )
+5.50to+6.49
5,946
8,768
Numbersofstarsvisibleasafunctionoflimitingmagnitude.
On a really dark, clear night with no moon, no light
Limiting magnitude will vary by proximity to outdoor
pollution, and fully dark-adapted eyes, the limiting lightsourcesaswellasweatherconditions.Thecloserone
magnitudeisabout+6.5.Undersuchconditions8,768stars istooutdoorlighting,thelowerthelimitingmagnitudeand
are potentially visible. Because about half of these stars the fewer the number of stars visible. (Recall that the
willbebelowtheobserver’shorizonatanyonetime,and higherthemagnitudenumber,thedimmerthestar.)Asa
those near the horizon will be dimmed by Earth’s result of this, it’s not unusual to find “light domes” from
atmosphere, it’s easy to understand why we will see only nearby towns and cities when observing from a rural
about⅓ofthatnumberorsome2,500–3,000starsatany setting. Few stars might be visible in the light dome but
onetime.Bythetimethelimitingmagnitudereaches+3.5 otherpartsoftheskywillbelesslightpollutedandmore
(say in a small rural town), the number of stars visible starsvisiblethere.Skytransparencyalsoplaysaroleinthe
drops to less than 100. In small cities (Springfield, numberofstarsvisible.Itispossibletohaveapitch-black
Bloomington-Normal,Decatur,etc.)thelimitingmagnitude nightandnotseeasinglestarbecausetheskyiscovered
isoften+2.5,andthenumberofstarsvisibleisreducedto with clouds. The greater the relative humidity and the
only about 30. In larger cities (Peoria, Rockford, and amount of dust suspended in the air, the lower the
metropolitan Chicago) where the limiting magnitude is transparency and the smaller the number of visible stars
often+1orbrighter,thenumberofstarsvisibledropsto7 regardlessofskydarkness.
orfewer.
30
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25.DARKADAPTATION
Dark adaption is necessary for viewing dimmer stars
and other faint objects of the Milky Way. The adaption
process is two-fold. When one enters a darkened place,
the pupil automatically dilates or becomes wider thereby
admitting an increased amount of light to the eye. In
addition,theretina–thelightsensingregionatthebackof
theeyeball–beginsproducingasubstanceknownasvisual
purple or rhodopsin. This substance sensitizes the eye to
light,makingitpossibleforonetoseebetterunderdarker
–butnotpitchblack–conditions.Theremustalwaysbea
smallamountofambientlightinorderforsomeonetosee.
No matter long how long a person sits in a dark cave, for
instance,heorshewillneverbeabletosee.Visionalways
requires the presence of at least a minimum amount of
light.
It takes about 30 minutes for the eyes to become
reasonably well dark adapted, though additional time in
thedarknesswillaidwithimprovingdarkadaptation.
If you intend to observe at night, it is best to avoid
directsunlightduringthedaytimehours.Thiswillenhance
your dark adaptation. Keep in mind, too, that diets
severely deficient in vitamin A (carotene) can result in
nightblindness–theinabilitytoadapttothedarkness.
Also keep in mind that light in colors other than red
willdestroyrhodopsinandruindarkadaptation.Nowhere
isthismoreevidentthanwhenoneleavesamovietheater
on a sunny day. Walking outside can result in a painful
sensation in the eyes. This occurs because the shorter
wavelengthsofsunlight(yellow,green,blue)arebreaking
down the rhodopsin. This results in ionization of the
substrate,anditproducesthesensationofpain.
26.THEARTOFASTRONOMICALOBSERVING
There is more to looking through a telescope than
puttingone’seyeuptotheeyepiece.Havingobservedthe
heavens for more than 50 years now, I can tell you that
therearethingsthatanobservercandotoimprovewhat
onesees.Kindmind,though,thatyou’llneverseeHubble
quality images. Most of us humans will be restricted to
viewing celestial objects from the confines of Earth while
peering through its turbulent atmosphere. Astronomical
viewingismuchakintobirdwatchingconductedfromthe
bottomofaswimmingpool!Theripplesofthewatermake
itdifficulttoseethingsclearly.Bethatasitmay,thereare
many things that one can do to optimize views obtained
throughtheeyepiece.
Plan your observing session. If you set up your
telescope under the stars without a plan for observing,
you’llbesurprisedbyhowlittleyouwillendupseeing.Yes,
there’sthemoonandplanets,butthenwhat?Unlessyou
haveacatalogofcelestialobjectstoobserveinyourhead,
yourviewingwillbequitelimited.I’vebeenobservingthe
sky for over 50 years, and I’d still be limited to viewing a
handful of representative objects were it not for the fact
that I pursue observing programs. Consider pursuing first
andforemosttheMessierobjects.Hereyou’llhavealistof
110 objects you can observe throughout the year. At any
onetimeabouthalfoftheseobjectsareabovethehorizon
at any one time. They generally constitute the best and
brightestclusters,nebulas,andgalaxies.TheAstronomical
League has many such observing programs you should
consider.
Acquire a quality “goto” telescope. Nothing opened
up the sky for me like my CPC 11” goto telescope. I had
observedforyears,butwasgettingtiredofthestruggleto
find “faint fuzzies” using a finder scope all the while
bending and twisting my body like a contortionist. After
seeing the ease with which event faint objects can be
foundusingagototelescope,Ihadtohavemyown.Itwas
the best money I ever spent on amateur astronomy and
accounts–atleastinpart–forwhyIgetouttoobserveso
much. On any given evening I can see several dozen
objectsIhaveneverseenandwouldnothaveseenwereit
notformygototelescope.
Get an observing aid. Even with a goto telescope,
you’ll be pleasantly surprised by how much an observing
application can enhance your experience of looking
through binoculars or telescope. I frequently use my
iPhone, iPod, or iPad while observing. My favorite
application – SkySafari Pro – helps control my telescope
(when I care to do so), but always provides critical
observing information like where a faint object is located
relative to brighter stars. I used to make observing cards,
but today they are instantly available to me through the
SkySafari application. Also, I can find information
immediatelyaboutthesizeordistanceofanobject.
Establish and maintain your dark adaptation. Once
you have achieved dark adaptation, work to maintain it.
Usefaintredlightingtoprovideilluminationasnecessary.
Red light – no matter how intense – does not have
sufficient energy to break down rhodopsin whereas blue
lightdoes.
Seek to observe rather than merely see. There is a
difference between these two acts. One can see things
merely by looking at them. An observer – someone who
lookscarefully–willnotethingsthatthecasualviewerwill
otherwisemiss.Viewingwithintentisthebestwaytosee
fine detail. Knowing what to look for also markedly
improveswhatoneviews.It’snowonderthatexperienced
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31
observerswillseethingsthatcasualobserversoverlook.
Take observing notes. It is helpful to make written
descriptions during observing. Keep track of atmospheric
transparencyandseeing,skydarkness,weatherconditions
(temperature, humidity, and winds), telescope used, the
eyepiece and magnification, filters used, observing
location, date/time, relative difficulty of viewing, and so
forth. Refer back to your notes when re-observing an
object. Compare this time with prior observing sessions
andlearnhowdifferentconditionsaffectwhatyousee.
Makeeyepiecesketches.Youwillbesurprisedathow
much more detail you perceive when you make drawings
attheeyepiece.Whenviewinganobjectpayattentionto
and record such things angular size in relation to the
eyepiece’s field of view, elongation, image brightness,
density, and color. Record field stars in such a way that
depicts star density, counts, and locations accurately.
Consider partitioning the field of view into quarters, and
draweachquarterseparatelyifhelpful.
Know the details for which to look. You will be
amazed at how much detail you can actually perceive if
you know what to look for. For instance, I never really
“saw” the Dumbbell Nebula until I started comparing a
photograph of the object with the view in my eyepiece.
Study your objects patiently and try to see every detail
visibletoyoureye.
Use averted vision. Looking out “the corner of one’s
eye”canimprovewhatoneseeswhenlookingthroughan
eyepiece. The color receptors in the center of the retina
arenotwellsuitedtodimlightconditionsanddon’twork
well at night. This is why things appear in shades of gray
rather than in living color. The eyes’ rods, however,
distributed around the eye but concentrated outside the
fovea where most color perceiving cones are located, are
very sensitive to subtle differences in lighting. Learn how
to avert your vision and use the rods to observe faint
celestialobjects.Avoid,however,avertingyoureyetoward
thebridgeofyournoseastheeyes’bindspots(wherethe
opticnerveconnectstheeyetothebrainisattached)will
thenbeatthecenterofyourfieldofview.
Jiggle the image.A“trick”youcanemployforseeing
finer detail is to “jiggle” your telescope while viewing the
object in question. Tap the side of the telescope, and the
object in the field of view will rapidly oscillate back and
forth. For reasons still somewhat unclear, one can see
more detail in an object in motion than one that is
completelystationary–atleastinatelescope.
Remember transparency and seeing. Recall that not
all nights are the same. When the sky is perfectly clear it
has high transparency. When the sky is completely
overcast,ithaslowtransparency.Observeonnightswhen
theskyisascloudandhazefreeaspossible.Also,despitea
completely transparent sky, the sky can also be very
turbulent with poor seeing. When stars near the horizon
are twinkling violently, keep in mind that seeing is
probably not very good and so objects that tend to show
32
fine detail such as the moon and planets will not likely
showitonnightswhentheseeingispoor.
Use a proper observing
stance. If you prefer to stand
while
observing,
your
approach to the telescope
should
be
carefully
considered.
Ergonomics
suggestsaparticularobserving
stance.Donot,forinstance,lookthroughtheeyepieceas
shownintheillustrationtotheright.Imaginetheneckand
backstrainifapersonweretryingtoviewsomethingmuch
higherupinthesky.Thepersonwouldneedbothtobend
overmoreandthrowtheheadbackfurther.Thiscanlead
toterriblebackandneckpainthatfewobserverscanlong
endure. Consider changing your observing stance. Avoid
lining up your body with the telescope as shown. Rather,
standwithyourbodyata90°angletotheopticalaxisand
turn your head left or right to the eyepiece. Rest your
handsonyourkneesforadditionalsupport.Sopositioned,
anobservercaneasilyspendseveralminutescomfortably
viewingobjectevenwhentheyaremuchhigherinthesky
thanshown.
Employ comfortable seating.Ergonomicsisthename
ofthegamewhenlookingforyetanotherwaytoincrease
observing prowess. Experienced observers know that the
best views to be obtained through the eyepiece are
obtainedwhentheobserveriscomfortablypositionedand
relaxed.Usingagoodobservingchaircansatisfytheneed
betterthananobservingstancealone.Manyobserversuse
firmchairsandevenstepstoolsforsittingwhileobserving.
Specially designed observing chairs can be more readily
adapted to the changing height needs while viewing.
Regardless, any sort of seating can help observers avoid
neck,back,foot,andlegstrainwhileobserving.
Seek the darkest observing location possible. Seek
outthedarkestlocationyoucanifyouhopetoseethings
at their best. Perhaps the most expensive lesson I ever
learned was purchasing an 18” telescope. When viewing
side-by-sidewithmy11”telescope,Ifoundthattheviews
were not really all that different. Yes, a larger telescope
collectsmorestarlightandshowsmoredetails,butitalso
collectsmoreatmosphericlight.Largetelescopesusedfor
visualobservingperformbestwhenusedundertrulydark
sky conditions. Unfortunately, central Illinois rarely has
truly dark observing locations due to light pollution
emanating from nearby cities. Still, the darker the
observing location the better because the contrast
betweencelestialobjectsandthebackgroundskyismuch
better.Highercontrastmeansbetterviews.Notethatpoor
contrast between celestial objects and the bright
background is the very reason why we don’t see stars
duringthedaytime.
Use the best eyepieces you can afford. Most
telescopescomewitheyepiecesthattypicallyareoflower
quality.Dependingonthemanufacturer,youwillnormally
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receivetolerablygoodeyepieces,buttheycannotprovide
thestunningviewsthatmoreexpensiveeyepiecescan.The
apparent fields of view of inexpensive eyepieces might
only subtend 40° or 50°. Your more expensive eyepieces
(for instance, of Nagler design with a number of
manufactures such as TeleVue and Orion) can provide
fields of view on the order of 82°, 100°, or even 120°!
These ultra-wide apparent fields of view give larger true
fields of view and the impression of “falling through the
eyepiece” as the edges of the field of view often extend
intotheregionsofperipheralvisionwheretheyarebarely
noticed.
Consider using eyepiece filters.Eyepiecefilterscome
in quite a variety. Some of the more common are the
Skyglow and UHC (ultra high contrast) multi-band pass
light pollution filters, the neutral density moon filter,
broadband filters such as red, green, blue, orange, and
yellow, and narrowband filters such as OIII, SII, and Hα.
The broadband filters are good for heightening contrast
when observing planets, whereas the narrowband filters
are good for enhancing contrast between the sky and
certainnebulas.Therearenumerousothertypesoffilters
besides. Broadband filters often come in sets and are
ratherinexpensive.Narrowbandfiltersaresoldindividually
duetotheirvaryingappealandthefactthattheyarequite
expensive.
Maintain the cleanliness of your telescope and
eyepieces.Cleanlinessisoneofthemostimportantthings
to consider when maintaining your telescope and
eyepieces. The need for cleanliness should be obvious. A
lens or mirror coated with dirt, dust, and oil from one’s
handswillnotbeagoodreflectoroflight.Cleaninglenses
and mirrors is beyond the scope of this handout, but you
canconsiderusingagoodqualitylenscleanerandclothto
cleanupyourlenssurfaces.DoNOTspraylenscleaneron
youroptics;rather,sprayitonthelensclothsoexcessfluid
doesn’tleakthroughandgetbetweenlenssurfaces.Once
that happens, you’ll have no choice but to disassemble
youreyepiece.Thisisnotsomethingrecommendedforthe
novice.
Maintaingoodtelescopecollimation.Establishingthe
proper optical alignment of lenses and mirrors in your
telescope system is critical to proper views. Poor
collimation results in stars appearing out of round, and
produces scattered light. More of a problem with
reflectors than refractors, collimation should not be done
the first time without the advice of an expert. Novices
oftenmakethingsworseratherthanbetterwhentryingto
collimate optics when they have little or no experience
doingso.Apoorlycollimatedtelescopecannotbeproperly
focused because the image plane is no longer
perpendiculartothetelescope’sopticalaxis.
Be certain you have the sharpest focus. Focus is
generally at its best when stellar images are as small as
possible. By turning your telescope’s focusing knob back
and forth, you can obtain the best focus. Still, you might
want to consider using a
Bahtinov mask for focusing
telescope to achieve the best
possible focus. This screen –
placed over the objective end
of the telescope – produces
diffraction patterns as shown
in the pair of star images
shown here. In the leftmost
starimage,thediffractionlines
donotcrossatasingle
point; the telescope is
notinfocus.Intheright
starimage,thelinesdo
intersect at one point;
the telescope is in
focus.
Watch out for dewing and frost. When the air
temperaturedropstothepointofcondensation(a.k.a.the
dewpoint),eitherdew(ifT>32°)orfrost(ifT≤32°)will
begin forming on your observing equipment. This is a
significant problem with SCTs and refracting telescopes
whose corrector plate or objective, or a reflector’s mirror
usedwithanunshieldedtrusssystem,isnotprotectedby
anysortofashieldsuchasascreenortube.It’samazing
howquicklycondensationofsomesortcanoccuronyour
equipment. Check for it regularly, but especially on
evenings when the temperature is dropping quickly and
the wind is calm. There two approaches to dealing with
this condensation: (1) either prevent it, or (2) treat it. In
the former case, consider using a dew shield on exposed
opticsorgetaheaterforyourtelescope.Inthelattercase,
you can purchase a heated electrical strip that is placed
around the telescope’s optics to keep them warmer than
thenightairorusealow-wattagehairdryertoevaporate
the condensation. If you don’t have heating equipment –
aimyourtelescopetowardthegroundandleaveitinthat
positionforawhile.Infraredradiationfromthegroundwill
slowly evaporate the condensation (though frost will take
longerthandewtoevaporate).Whenallelsefailsandyou
just can’t keep up with the rate of condensation, it’s best
justtopackupandgohome.
Ensure thermal equilibrium of your equipment.
Telescopesyieldthebestviewsiftheiropticalsystemsare
inthermalequilibriumwiththeair.Asthetemperatureof
optical and mechanical components rises and falls, these
items will expand or contract respectively. Different parts
of a lens or mirror will cool at different rates due to
variations in thickness, so while temperature change is
takingplacethephysicalfeaturesoftheopticalshapesand
surfaceschange.Suchchangesreducethequalityofviews
provided by these telescopes. If you keep your telescope
shadedandoutside,yougenerallywon’thavetowaitforit
to come to thermal equilibrium with the air. If, however,
you keep you telescope in a colder house or warmer
vehicle,youmightneedtosetitupandwaitforatleastan
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33
hourforittoreachthermalequilibriumwiththeair.Some
of the more expensive telescopes come with built in fans
toincreasetherateofthermalexchangewiththeair.
Avoid unnecessary air currents. Choose your
observingsitecarefully.Therearemanyfactorstokeepin
mind when choosing an observing site. If you set up on a
hard surface such as concrete or asphalt, you won’t have
to worry much about your feet getting wet due to dew
formation. However, you will have to worry about air
currentscausedbythermalconvection.Asphaltduetoits
dark color tends to absorb heat from the sun during the
day and give it off at night. This produces convection
currentstoformovertheobservingspot.Yourbestviews
are obtained when the atmosphere is stable. Convection
currents cause images to shimmer thereby reducing the
qualityoftheview.Concreteismuchthebetterchoicefor
viewing on a hard surface. It is lighter in color and
therefore does not absorb and give off as much heat as
asphalt. Viewing from a grassy spot is even better, but
there is a problem with dew. In a similar vein, avoid
viewing through an open window or viewing objects
locatednotfaraboverooftops.Thermalcurrentsformdue
toconvectioninbothsituations.
Choose your observing location carefully. Sky
darkness is only one factor in choosing an observing
location. Observatories built in the flatlands of the
Midwest are frequently built near lakes and reservoirs.
Have you ever wondered why? This is so because water
has much higher thermal inertia (it takes a lot of heat to
changethetemperatureofwatersubstantially)thanland,
and the water temperature changes very little over the
course of day and night. Less heat being emitted from a
lake at night means that air over it is less likely to
experience the thermal effects commonly seen over soil
whose temperature can range widely over the course of
dayandnight.
27.OptimizingObservationsofDeepSpaceObjects
A number of factors determine the quality of ones
telescopicviews.Thestabilityoftheatmosphere(seeing),
the transparency and darkness of the sky, filter use, dark
adaptation, the size and quality of ones telescope
(including the mount), and even the powers of ones
telescopecanadverselyaffectonesviewoftheheavens.If
oneistooptimizetelescopicobservations,thenoneneeds
to understand how these factors interact to produce the
best(andworst)telescopicobservations.
Telescopes have three “powers” – light-gathering,
resolving, and magnifying. Bigger objectives, if well made,
produce brighter and sharper images that can be viewed
withtheuseofaneyepiece.Thechoiceofaneyepiececan
becriticalinoptimizingtheview.
Perhaps the least understood of the powers of the
telescopeismagnifyingpower.I’vebeenreflectingonthis
aspect of telescopes for several months now, and have
resolved to cast some light on this particular power, and
providesomeimplicationsforeyepieceselection.
MagnifyingPower–Themagnificationofatelescope
–thesizeofanobjectseeninaneyepiececomparedtothe
sizeofthatsameobjectseenintheskywithanunaided
eyecanbedeterminedwithasimpleexpression:
MagnifyingPower=EFLt÷FLe
Because the effective focal length of a typical
telescope (EFLt) remains fixed (unless, say, one inserts a
telecompressor or Barlow lens into the optical train to
changetheeffectivefocalratioofaninstrumentfromf/10
to f/6.3 or from f/8 to f/16 respectively), one varies the
magnificationbyusingeyepiecesofdifferentfocallengths
34
(FLe). My f/10 configured CPC 11” telescope has a focal
length of 2800mm. When used with an 18mm eyepiece, I
getamagnifyingpowerof2800mm÷18mm=156X;with
the use of a 32mm eyepiece, I get a magnifying power of
88X. The shorter the focal length of the eyepiece, the
higherthemagnifyingpoweritwillprovide.
DrawbacksofHighMagnifyingPower–Manypeople
misunderstand magnifying power. They think “the more
the better.” Not so. First and foremost increased
magnification reduces image brightness. A telescopic
imagemagnified50Xwillappear2,500times(502)dimmer
that the image obtained with the unaided eye. Granted,
this is offset somewhat by the light-gathering power of a
telescope, but telescopes rarely provide increased image
brightness. This is the province of some of the lower
powered binoculars with large objective lenses. Higher
magnifying powers also amplify the rate of motion of
celestial objects through a field of view, and reduce the
field of view making things harder to find. Higher powers
alsocannegativelyaffectimagequalityasperceivedbythe
eyeaswell.Ifatelescopemountiswobbly,anyvibrations
willbesimilarlymagnified.
Exit Pupil – Before moving on to lowest and highest
useful magnifications for a particular telescope-observer
combination,Ineedtomentionabitabouttheexitpupil.
The exit pupil is the diameter of the small disk of light
emanatingfromaneyepiece.Foroptimalviewingatlower
powers, an observer must place his or her eye at such a
position that the eyes pupil is coincident with the
eyepieces exit pupil. If the diameter of ones fully dilated
eye pupil is less than the telescopes exit pupil, the
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observer will see a vignetted image, wasting much of the
light-gathering power of the telescope. (This effectively
reducestheapertureofatelescope.)
The diameter of the exit pupil of the telescope is
dependent on the aperture of the objective and the
magnification, and they are related in the following
manner:
Eyepieceexitpupildiameter=Aperture÷Magnification
Astheequationshows,lowermagnificationsproduce
larger exit pupils, and higher magnifications produce
smaller exit pupils. In order to obtain the best low-power
views in a telescope, the exit pupil of the eyepiecetelescope combination must match the maximum pupil
diameteroftheobserver’seye.Now,thepupildiameterof
the typical adult human eye is mostly a function of age.
Young adults on the order of 20 years of age will have a
fully-dilatedpupildiameterofasmuchas7.5mm,whereas
someonewhois70yearsofagewillhaveadark-adapted
pupil diameter on the order of 3mm. A simple formula
relating average pupil diameter of the eye to the adult
observersage(≥20)isgivenasfollows:
Averagepupildiameter=(-0.09mm/yr)xAge+9.3mm
(Age≥20yr)
Hence, in my case (56 years old) selecting a low power
eyepiece-telescope combination that produces an exit
pupil of greater than 4.2mm probably would not be
advisable.
Lowest Useful Magnification – As a result of the exit
pupil considerations addressed last month, there actually
isalowestusefulmagnificationthatanobservercanuseto
achieve the brightest possible image for viewing with
direct vision – at least if that observer expects to use the
entireapertureofthetelescope.Itisconvenienttoexpress
the optimal lowest power eyepiece (OLPE) in terms of its
focal length, which happens to depend on a telescope’s
focalratioandthemaximumdiameterofthefullydilated
pupiloftheobserver’seye.Theexpressionis:
OLPEFocalLength=ExitPupilDiameterxFocalRatio
For example, in my case the OLPE focal length for
direct vision will be (4.2mm x 10) or 42mm. Using an
eyepiece in this range (say a 40mm) will provide me with
the brightest views of celestial objects given my
telescope’s characteristics and my observing eye’s
maximum dilation. The resulting magnification will allow
forthebestpossibledirect-visionviewsbecauseIamthen
dealing with the brightest possible image for a given
telescope-observer combination. My optimum low
magnification with a 40mm eyepiece in my CPC 11”
telescopewouldbe70X.
A Common Misconception – It is often said that
telescopesmakecelestialobjectsbrightersotheobserver
canseethem.Thisisacommonmisconception,andinthe
vast majority of cases patently false. Almost all
astronomical telescopes will dim celestial objects rather
thanmakethembrighter.Considerthatmy11”telescope
gathersabout3,500timesmorelightthanmyeye(taking
intoaccountthepresenceofthesecondarymirror,andthe
loss of light due to absorption and reflection). Using my
telescopeatamagnificationof70Xwillactuallyreducethe
brightnessoftheimagebysome4,900times(702).Hence,
when observed with this combination of telescope and
eyepiece, the image in the eyepiece is about 70%
(3,500/4,900) as bright as it would be seen with the
unaided eye. Only some binoculars with larger apertures
(e.g., 50mm) and lower powers (e.g., 7X) will actually
increasetheapparentbrightnessofanobject–assuming,
ofcourse,thattheexitpupilcriterionismet.Observerssee
more details in telescopes merely because extended
objects appear larger and more resolvable than when
observedwiththeunaidedeye.
Two Highest Useful Magnifications – As any
experiencedobserverknows,thebestwaytoviewfainter
objects is with the use of averted vision. Direct vision is
fine if an object is bright enough to stimulate the cone
receptorsinthefoveaoftheeye.Ifanobjectisverydim,it
is best viewed with the use of averted vision. In such
situations the observer views a dim object “out of the
corner of the eye.” This allows light to fall on the much
more sensitive rod receptors located outside the fovea of
theretina.
From a practical standpoint, there is a highest
magnificationonemightusewithavertedvisiontoseethe
maximum detail in an extended, non-stellar object.
Historically, a general rule of thumb has been given that
states that the highest useful magnification is about 50X
perinchofaperture.Thisruleisbasedontheabilityofan
observertovisuallyseparatebinarystarsincloseproximity
to one another, but it does not take into account other
limiting factors such as poor atmospheric steadiness,
inferioroptics,ashakymount,orgettinganeyepiecewith
adequateeyerelief(thedistancefromtheoutersurfaceof
theeyepieceandthefocalpointoftheimage).Inaddition,
this 50X rule is too “simplistic” to the extent that it does
not apply meaningfully to extended deep-space objects
suchasnebulas,supernovaremnants,andgalaxies.
ResearchconductedbyH.RichardBlackwell(Contrast
thresholdsofthehumaneye,JournaloftheOpticalSociety
ofAmerica,Vol.36,No.11,November1946)showedthat
therearebetterwaystomaximizethehumanabilitytosee
fainter objects using averted vision, and this is subject to
both illumination and image size. Work using Blackwell’s
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35
data, represented graphically by Roger N. Clark in Visual
AstronomyoftheDeepSky,1990,canbesummarizedwith
asimpleformulathattakesintoaccounttheuseofaverted
vision in relation to optimal highest power (OHPE). It is
givenbythefollowingformula:
OHPE=6.2xAperture+35(4”≤Aperture≤16”)
So,bythiscriteriontheoptimalhighestpowerformy
11” telescope will be approximately 103X (6.2 x 11 + 35).
Convertingthisintofocallengthoftheeyepieceusingthe
first equation in this article series, the OHPE focal length
for me would be approximately 27mm (2800mm/103X)
whenviewingextendedobjectsusingavertedvision.While
this is the highest power for seeing maximal detail using
avertedvision,itisnotnecessarilythehighestpowerone
might want to use. One may safely double this optimal
magnification with a minimal reduction in the averted
vision visibility index according to Blackwell’s work. The
increased magnification might dim the object, but the
trade-offisacceptable.Itwillmakeextendedobjectslarger
and more resolvable to the human eye as a result even
withthelossofbrightness.
WhenI’mobservingcertainplanetarynebulaeonthe
AL observing club list, I must push the magnification far
beyondtheOHPEconditionsothatIcanresolveanebula’s
near stellar image. Higher powers will allow me to
distinguishthenebulafromfieldstarsthatdonotgrowin
size with increasing magnification (unless the seeing is
poor). Because telescopes, observers, and observing
conditions vary so much, it’s really up to the observer to
decide when a certain magnifying power is too much.
Whenincreasingthemagnificationmakesanimageworse
ratherthanbetter,thenanobserverknowsthatheorshe
reallyhassurpassedoptimumhighestpower.
While telescope aperture and magnifying powers are
criticalcomponentsforoptimizingviewsofextendeddeep
space objects (dark, emission, reflection, and planetary
nebulas as well as galaxies), they are not the only
considerations.Anotherwaytoenhancevisibilityofthese
celestialobjectsistoincreasetheircontrastrelativetothe
background sky. This can be achieved in two different
ways:(1)observingcelestialobjectsfromalocationwitha
darker sky, and (2) using filters that transmit only certain
wavelengths of light while blocking others. Additional
considerationsalsoapplyandtheseinclude:(3)observing
only with dark-adapted eyes, (4) using averted vision
properly, (5) observing only when the sky is very
transparent,(6)maintainingyouroptics,and(7)observing
objectsonlywhentheyarehigherupinthesky.
Enhancing the contrast of extended celestial objects
relativetothebackgroundismosteasilyaccomplishedby
observing from remote dark-sky locations (e.g., mountain
tops, Chile, or in some years the Illinois Dark Sky Star
Party). Even viewing from sites not terribly far removed
fromcities(e.g.,SugarGroveNatureCenter)enhancesthe
36
views over those obtainable by observing under urban
skies. Also, observe when the moon is not present in the
skytoachievemaximumdarkness.Whenthenightskyisat
itsdarkest,thecelestialobjectsareviewedattheirbest.
Increasingthecontrastbetweenanextendedcelestial
objectandtheskyalsocanbeaccomplishedwiththeuse
of narrow-band filters such as the OIII (doubly ionized
oxygen), UHC (ultra high contrast), Skyglow, and so on.
Anyonewhohasobservedwithmerecentlyandseenthe
NorthAmerican,Veil,orHelixnebulasknowsthe“power”
of the OIII filter to improve visibility of these objects,
especiallyonnightswhenthecontrastbetweentheobject
andtheskyislow.Asexperiencehasshown,theseobjects
are essentially invisible from SGNC with my telescope
without the use of the OIII filter no matter what the
conditions.
Another way to get a good view of extended deep
spaceobjectsistoallowyoureyestoproperlyadapttothe
dark. Eyes will typically take about 30 minutes to reach
most of their dark adaptation, but observers will notice
additionaladaptationafterseveralhoursindarkness.Note
thatsubjectingyoureyestoverybrightdaylightcanaffect
yourabilitytodarkadaptforseveraldays.
Using a dark red-filtered flashlight of low intensity is
one way to maintain your dark adaptation. Red
wavelengths of light do not have sufficient energy to
destroy the chemical rhodopsin that is created by the
retinaasameansofadaptingtothedark(theothermeans
is to dilate the pupil). Deep red LED flashlights with
dimmersaretheideal.(IhavefoundtheOrionRedBeamII
LEDvariable–brightnessastroflashlighttobeideal,thanks
toarecommendationbyWilliamCarney.)Whenobserving,
don’t let nearby lights or passing headlights of cars ruin
your night vision. Close your eyes and look away when a
car is approaching an observing sight. While observing,
some observers will employ hoods that cover the
observer’s head and extend all the way the telescope
eyepiece.Failingthat,someobserverswillcuptheirhands
around the eyepiece providing for a bit darker situation.
Such approaches can perceptibly improve and preserve
one’snightvision.
Be certain to use averted vision to see additional
detail. The cones at the back of the eye are color
receptors, but don’t work very well under dim light
conditions(explainingwhywetendtoseethingsinshades
ofgrayatnight).Therodssurroundingthefovea’sconesat
thebackoftheeyearemoresensitivetosubtledifferences
in lighting. Look at extended space objects “out of the
corner of your eye” if you’d like to see more detail. This
method requires and improves with practice, as the eye’s
peripheralvisionrodsarenotattachedtothebraininthe
say way the direct vision cones are. Too little attention is
paid to this important observing technique and, frankly, I
was using improper technique for years. Don’t turn your
eyetowardyournosewhenusingavertedvisionduetothe
blind spot at the back of the eye. Directing light into this
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blind spot will reduce an object’s visibility rather than
enhanceit.
Projecting and maintaining your optics will lead to
improved visibility. Scattered light, dust, and dew can
destroy image quality, brightness, and contrast. If
observing with a truss-tube assembly, be certain to cover
theopenpartsoftheopticaltubeassemblywithashroud.
Also,becertainthatstraylightcannotstrikethesecondary
mirror. Keep you optics clean. Dust can scatter light
making for a more diffuse image. Watch out for dew, but
especially if you are using a refractor or SchmidtCassegrainwherethecorrectorplateisnotprotectedbya
tubeassembly.Onnightswhenwatervaporiscondensing
(orfreezing)onexposedoptics,becertaintoeitherusea
dew shield to prevent or a low-wattage hair dryer to
evaporatecondensation.
Dewshieldsprovideanaddedbenefitinthattheyreduce
thepresenceofscatteredlightintheopticaltubeassemble
andthatfollowingonasecondarymirror.
Heightened sky transparency will also increase the
visibility of extended deep space objects. The best views
occur on cold winter nights, and following the passage of
cold fronts at other times of year. Often associated with
these weather conditions is enhanced twinkling.
Fortunately, the twinkling phenomenon doesn’t tend to
strongly influence the quality of views of extended deep
spaceobjectsthataremostoftendiffuse.
Lastly, to get the best views of extended deep space
objects,becertaintoviewthemwhentheyarehigherup
in the sky. Personally, I rarely observe objects when they
arelessthan30degreesabovethehorizon.Whenlooking
closetothehorizon,oneispeeringthroughathickerlayer
ofatmospherethanwhenanobjectisviewedhigherupin
the sky. The light of objects close to the horizon travels
though as much as five times as much atmosphere as
objectsviewedoverhead.Togetthebestviewsofcelestial
objects, be certain to observe them when they are
transiting the meridian, crossing from east to west across
thenorth-southlineinthesky.
Two recent developments have more strongly
influencedmy“ability”toobservedeepspaceobjectsthan
anything else. They are the advent of “go-to” telescopes
andtheAstronomicalLeague’sobservingprograms.When
Ifirstheardaboutgo-totelescopesintheearlypartofthis
decade, I wasn’t quite sure what to expect. I shortly
thereafterobservedwithMichaelRogersasheusedhis8inch Meade go-to telescope and fell in love with the
conceptsof“autofinding”celestialobjects.Thanksinpart
to Michael, I moved to the next stage of amateur
astronomy.
I was tired of seemingly crawling around on the
groundonmyhandsandkneesinordertokeepseeingthe
sameobjects.Irarelytookthetimetoobserveanyobject
that required me to search using approaches such as
sweepingandstarhopping.Iespeciallyhatedbendingover
my telescope or contorting my body to use the straightthrough finder to locate object nearly overhead.
Astronomywasquicklygettingolderthanme,andliterally
quite a pain in the back. The prospect of finding celestial
objectsatthepushofabuttonheldgreatappeal.
After using the SGO’s 12-inch Meade LX200 go-to
telescope for the first time under the tutelage of William
Carney, I was hooked. A few weeks later, I was
immediately convinced of the good of my own go-to
telescope after finding 60 celestial objects with the SGO
telescope in just over one hour. In the summer of 2006 I
purchasedmyfirstgo-totelescope.ThatCelestronCPC11inch now makes finding deep space objects a breeze, and
has increased my viewing pleasure immensely. I just align
the telescope on two bright stars and start observing by
pushingafewbuttons.Nothingcouldbeeasier.
Using a go-to telescope has effectively increased the
visibility of celestial objects in a most impressive fashion.
Deepspaceobjectsofeverytypearenoweminentlymore
accessible. I now can spend much more time observing
deep space objects, and much less time searching the
heavensforthem.IhaveusedmyCPC11-inchtoglimpse(I
reallycan’tcallthisobserving!)morethan100galaxiesina
two-hourtimespan.Whilethecostofahighqualitygo-to
telescope can be in the thousands, trust me, it is well
worthit.
Equipped with a powerful go-to telescope, one can
really take a tour of the universe. Having an observing
program improves viewing almost immeasurably, but is
often NOT thought of as way to improve “visibility.” I
assureyou,itis.HadtheAstronomicalLeague’sobserving
clubsnotexisted,Iwouldneverhaveviewed100features
onthemoon,110Messierobjects,400Herschelobjects,
100 Urban objects, nearly 60 planetary nebulas and 50
globular clusters (to date)! Neither would I have found
curious individual objects such as comets, asteroids, and
deepredcarbonstars.
So, folks, there you have it, how to optimize
observations of deep space objects. I hope that you have
beenasinspiredasIhavebythefour-partseriesandwill
spend some time out under the stars this summer. Now,
let’sstarputtingthisknowledgetouse.
Note: Portions of this article are based on “The virtual
observer: A new breakthrough technology for the visual
observer” by Roger Blake appearing in Astronomy
TechnologyToday,Volume2,Issue9,September2008.
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37
28.THEUSEOFOBSERVINGFILTERS
While a telescope can enhance your observations in
comparisontotheunaidedeye,eyepiecefiltersandfilters
thatfitovertheobjectiveofatelescopecanenhancewhat
youseethroughtheeyepiece.Therearethreebasictypes
of eyepiece filters with which we will concern ourselves
hereandnow–broadbandfilters,narrowbandfilters,and
other. Broadband filters typically pass a wide range of
wavelengths; narrowband filters typically pass only a
limited range of telescopes; other filters can be used to
reduce the brightness of an image across all wavelengths
ortodetectpolarization.Filterscanbefurthersubdivided
intothefollowingclassifications:lunar,planetary,nebular,
cometary,andlightpollutionfilters.
Lunar filters utilize polarization to adjust the
brightness of images for better observing. They consist of
twopolarizingplanesmountedinarotatingcell.Theyvary
the light transmission from 3% to 40%. The light
transmission can be adjusted depending on the phase of
themoon.
Planetary filterscomeinaverywiderangeofcolors.
Each is designed to pass certain wavelengths and block
others. This tends to enhance the contrast of features on
the planetary bodies. Each of these colored filters makes
use of the visible light transmission scale. The lower the
VLTnumber,thedimmertheobjectobservedwillappear.
VTL numbers of 40% or lower are not typically used on
small aperture telescopes because the images they yield
are simply too dim. Consider the following common
Wratten filters and their uses as described by
telescopes.com:
• #8LightYellow(83%VLT)helpstoincreasethedetailin
the maria on Mars, enhance detail in the belts on
Jupiter, increase resolution of detail in large telescope
when viewing Neptune and Uranus, and enhance detail
onthemooninsmallerscopes.
• #11 Yellow Green (78% VLT) helps to bring out dark
surface detail on Jupiter and Saturn, darkens the maria
on Mars, and improves visual detail when viewing
NeptuneandUranusthroughlargetelescopes.
• #12 Yellow (74% VLT) help greatly in viewing Mars by
bringingoutthepolaricecaps,enhancingbluecloudsin
the atmosphere, increasing contrast, and brightening
desert regions. Yellow also enhances red and orange
features on Jupiter and Saturn and darkens the blue
festoonsnearJupiter'sequator.
• #21 Orange (46% VLT) helps increase contrast between
light and dark areas, penetrates clouds, and assists in
detecting dust storms on Mars. Orange also helps to
bring out the Great Red Spot and sharpen contrast on
Jupiter.
• #25A Red (14% VLT) provides maximum contrast of
surface features and enhances surface detail, polar ice
caps, and dust clouds on Mars. Red also reduces light
38
glare when looking at Venus. In large telescopes, a red
filter sharply defines differences between clouds and
surface features on Jupiter and adds definition to polar
capsandmariaonMars.
• #56 Light Green (53% VLT) enhances frost patches,
surface fogs, and polar projections on Mars, the ring
systemonSaturn,beltsonJupiterandworksasagreat
general-purposefilterwhenviewingtheMoon.
• #58 Dark Green (24% VLT) increases contrast on lighter
parts of Jupiter's surface, Venusian atmospheric
features,andpolaricecapsonMars.Darkgreenwillalso
help bring out the cloud belts and polar regions of
Saturn.
• #80A Blue (30% VLT) provides detail in atmospheric
cloudsonMars,increasescontrastonthemoon,brings
out detail in belts and polar features on Saturn,
enhances contrast on Jupiter's bright areas and cloud
boundaries.Abluefilterisalsousefulinhelpingtosplit
thebinarystarAntareswhenatmaximumseparation.
Nebulafilterscomeinawidevarietyoftypes,butthe
mostusefularetheOxygen3(OIII),Hydrogenalpha(Hα),
Sulfur 2 (S2), and Ultra High Contrast (UHC) filters. OIII in
particularhasabroadapplicabilityinthatitcanbeusedto
enhance the contrast between the various regions of an
emission nebula and help observers distinguish tiny
planetary nebulas from surrounding stars. OIII also
increases the contrast of these objects with the night sky
rendering them more visible to the observer. SII and Hα
filters can be use similarly if not as effectively by visual
observers. UHC filters are particularly useful for revealing
nebulosity. This is achieved by passing three nebula
emissionlines–twodoublyionizedoxygenlines(496and
501 nm) and the H-beta line (486 nm) – while blocking
light-pollutionandskyglow.
LightPollutionReduction(LPR)filtersaredesignedto
selectivelyreducethetransmissionofcertainwavelengths
of light, specifically those produced by artificial light. This
includes mercury and both high and low pressure sodium
vaporlights.Inaddition,theyblockunwantednaturallight
causedbyneutraloxygenemissioninouratmosphere.Asa
result,LPRfiltersdarkenthebackgroundsky,makingdeepskyobservationandphotographyofnebulae,starclusters
andgalaxiespossiblefromurbanareas.LPRfiltersandnot
used for lunar, planetary or terrestrial photography. The
SkyGlow filter enhances deep-sky observations in
moderately light-polluted skies. It is a broadband filter
blocksthemostcommonwavelengthsoflightpollutionfor
increased contrast and better views. The filter improves
viewsofnebulas,galaxiesaswellasopenandglobularstar
clusters.
Solarfiltersfallintotwobasictypes:opticalMylarand
coatedglass,oftenreferredtoasaTypeII.Thesolarimage
through Mylarisapaleblue;throughtheglassfiltersitis
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orange. Sunspots and faculae are readily visible. They can
either be purchased for the full aperture of the scope, or
smaller and used off-axis. The glass filters are more
expensive, but more durable. If scratches or pinholes
appear on these filters, you can simply cover them over
with a black waterproof felt-tip pen. This will in no way
diminish light transmission. Never use a solar filter that
screws into an eyepiece. The telescope’s objective will
focus an intense beam of heat and light on the dark
absorbinglenssometimescausingittoexplosivelyshatter.
Solarretinopathycanresultfromtheeyebeingsubjected
tofull,un-attenuatedlightofthetelescope.
29.RECORDINGYOUROBSERVATIONS
It’sgoodpracticetorecordyourobservations.Thiswill
provide a “look back” record should you wish to review
what it was that you observed on any particular night. In
addition, it will provide critical information necessary for
completing observing programs such as those available
throughtheAstronomicalLeague.
Observing records should contain at a minimum the
following information shown in this sample observing
record.
Object:
Date:
Instrument:
Seeing:
Transparency:
LimitingMag:
Time:
Eyepiece:
Power:
Location:
Filter:
Notes:
Acompletedrecordmightlooksomethinglikethis:
Object:
M42
Date:2/27/16
Instrument:CPC11
Seeing:3/5
Transparency:4/5
LimitingMag:5.5
Time:9:31pm
Eyepiece:Plössl
Power:87X
Location:SGNC
Filter:OIII
Notes:The nebula shows an incredible amount of
detail and is so bright I can even see a greenish
coloration throughout without a filter. The
Trapezium is clearly visible at the center of the
nebula. The 0III filter really brought out the
detail.
An observing record might also include room for
drawings. Each such record might include one or two
sketchtemplates(highversuslowpowerorfilteredversus
non-filtered views) for information about the true field of
view as well. Amateur astronomers can create their own
observing record templates using a word processor.
Running off multiple copies can aid with making a
professional-lookingrecord.
Somethingtokeepinmind,however,istheutilityofa
tabletapplicationlikeSkySafarithathavebuiltinobserving
records plus a tremendous amount of other information
availableattheobserver’sfingertips.Seriousobserverswill
definitely want to consider purchasing such a device and
application. Even an older cast off can be readily
employed. Cell phone versions of the application are
similarlyavailable,butwiththesmallscreenscanbehard
toworkespeciallyundercolderconditions.
30.MOONPHASESANDTHEIREFFECTONOBSERVING
Thecausesofthemoon’sphasesarewellunderstood,
though many people hold to the mistaken belief that the
phasesarecausedbyEarth’sshadowfallingonournearest
neighbor. Nothing could be farther from the truth. When
that occurs, we are experiencing lunar eclipse and such
eclipsescanoccuronlyduringfullmoonphase.
Themoontakes29.5days–andabout389°ofangular
motionasseenfromEarth–toexhibitthecompletesetof
phases. However, the period of time required for the
moontoorbitEarthoncewithrespecttothestarsonceis
only about 27.3 days – its sidereal period. The longer
period of time required to exhibit a complete set of the
moon’s phases – its synodic period – exceeds its sidereal
period by about 2.2 days. This results from the sun’s
0.9856°/day eastward motion along the ecliptic over the
course of time (360°/365.25 days). The moon takes 2.2
additionaldaystocatchupwiththesuninordertoexhibit
thenextnewphase.
The moon moves eastward among the stars and
constellations at a rate of about 13.2° per day due to its
orbitalmotionaroundEarth.Asaresult,themoontravels
about 97.2° to move from new to first quarter and from
first quarter to full phase, and so forth. Hence, the time
betweenphasesisabout7⅓daysonaverage.
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39
The moon’s phases can have a pronounced negative moon is only half as bright as the full moon a mere 2.4
effect on certain aspects of astronomical observing. days before or after full phase. Even though 95% of the
Moonlight is a naturally occurring form of light pollution. moon is illuminated at this time (and most casual
When the moon is present, the air scatters its light. This observers would think the moon was entirely full), its
brightens the sky, thereby reducing the contrast between brightnessisroughly0.7magnitudeslessthanatfullphase
the sky and celestial objects. This is generally not a makingitappearonlyhalfasbright.
problem when viewing planets or star clusters because
The moon also can vary in brightness due to the
these objects are much brighter than the moonlit sky; a changes in Earth-Moon and Earth-Moon-Sun separations.
problem does exist with viewing such things as comets, The full moon, for instance, appears brightest when it is
nebulas,andgalaxieswhenthemoonispresentinthesky. closest to Earth. Its brightness will increase even more if
One of the more surprising things about the moon is theEarth-Moonsystemisnearerthesun.Thisgivesriseto
thatitsbrightnessisnotdirectlyrelatedtotheamountof the so-called “supermoon” phenomenon that is readily
lunar surface illuminated. For instance, the moon’s perceptibleinbothbrightnessandsizetotheexperienced
apparent magnitude at full phase is −12.7 whereas at the observer.
two quarter phases it is less than 10 percent as bright. If
As a result of the moon’s non-uniform changing
themoon’ssurfacewereperfectlysmooth,themoonata brightness,thebesttimesforobservingwilloccurbetween
quarterphasewouldbehalfasbrightasitisatfullphase. waning and waxing crescents when the moon in near the
Thisdoesn’thappenthough,andthereasonhastodowith sunintheskyandrisesshortlybeforeorsetsshortlyafter
theruggedlunarlandscape–especiallyintheregionsnear the sun and in the twilight hours. Between first quarter
theday/nightedge(theterminator).Thelunarlandscapeis and waxing gibbous, and between waning gibbous and
riddled with holes and shadows cast by mountain, third quarter, the observer needs to carefully time
boulders,andeventinygrainsofthelunarregolith(soil).In observations to avoid the moon’s presence in the sky,
addition,themoon’sfacehasdark,splotchyregionsknow unless of course one wants to observe the moon. During
as lunar maria. The result of the interaction between the fullmoonthemoonrisesatsunset,setsatsunrise,andis
sunlight, the terminator, and these lunar features and in the sky all night long. The following table will help the
colors is that at quarter phases the moon appears much reader better understand the relationship between moon
dimmer than would be expected. Believe it or not, the phaseandoptimalviewingtimes.
Moon
Approximate
ApproximateSet
OptimalViewingTimes
Phase
RiseTime*
Time*
6AMStd
6PMStd
New
Moonnotvisible.Bestviewingfromaftertosunsetto
7AMDST
7PMDST
Moon
beforesunrise.
(sunrise)
(sunset)
Waxing
Crescent
First
Quarter
Waxing
Gibbous
FullMoon
Waning
Gibbous
Last
Quarter
Waning
Crescent
9AMStd
10AMDST
9PMStd
10PMDST
The dim moon is low in the west after sunset. Moon
doesn’tsignificantlyinterfere.
12PMStd
1PMDST
(noon)
12AMStd
1AMPDT
(midnight)
Themoonisinthesouthatsunsetandnowinterferes;
bestviewsaftermidnight.
3PMStd
4PMDST
3AMStd
4AMDST
The bright moon is in the southeast after sunset. Best
viewsbeforesunrise.
6PMStd
7PMDST
(sunset)
6AMStd
7AMDST
(sunrise)
The moon rises in the east at sunset and sets in the
westatsunrise.Nobesttimes.
9PMStd
10PMDST
9AMStd
10AMDST
The moon rises some time after sunset. Best viewing
duringtheearlyeveninghours.
12AMStd
1AMDST
(midnight)
12PMStd
1PMDST
(noon)
The moon rises around midnight. Best viewing
conditionsuptomidnighthours.
3AMStd
4AMDST
3PMStd
4PMDST
Thedimmoonrisesafewhoursbeforesunrise.Moon
doesn’tsignificantlyinterfere.
*“Std”referstostandardtime;DSTreferstoDaylightSavingTime
40
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
The times of rising and setting in the above table are
at best approximate as there is a significant seasonal
variation for a given location. For instance, near the
September equinox we experience the harvest moon
phenomenon. The full to near-full moonrises at this time
aredelayedbyonlyabout30minutesfromnighttonight
whereastheaveragedelayinmoonrisetimeaverages50.2
minutes per day over the course of a year. Moonrises
around the March equinox are delayed by as much as
about70minutesfromnighttonight.
To get precise moonrise and moonset times it is best
to use a computer/tablet/cellphone application. Also,
check out the following very useful and helpful URL:
http://aa.usno.navy.mil/data/docs/RS_OneYear.php
31.MOONPHASEVERSUSBRIGHTNESS
Have you ever wondered about the relationship
between the moon’s phase and its brightness? You can
find out by examining the chart below. This chart shows
themoon’sbrightnessasafunctionofdaynumberrelative
to the brightness of the full moon for June 2016. It is
plotted specifically for Central Illinois. In order to
determine the relationship between phases and
brightness, one merely needs to know that in June 2016
the first quarter phase occurs at approximately day
number7.22;thatis,7.22dayspastnewmoon.Fullphase
occurs around day number 15.33, and last quarter phase
around day number 22.64. New phase occurs on days 0
and29.5.
PercentMax.Brightness
%MAX.BRIGHTNESSVS.DAYNO.
120%
100%
80%
60%
40%
20%
0%
-20% 0.0
15.0
20.0
25.0
30.0
DayNumber
With the above chart we can see that on day 8 (just asmoothsurface;rather,itssurfaceislitteredwithcraters
past first quarter phase) the moon is only about 10% of ranging from hundreds of kilometers in diameter to
maximum brightness. On day 12 the moon is only 33% smaller than can been seen with the unaided eye from
maximum brightness; on day 13 it’s just under 50% of close up. Additionally, the lunar surface has many high
maximum brightness. Of course maximum brightness mountain ranges and cliffs such as the Straight Wall. The
occurs near but not always on the date of the full moon. lunar surface is also composed of different types of rock
Thismightseemcurious,butkeepinmindthatthemoon’s andregolith(soil),eachwithitsownreflectivity.Partofthe
orbitaroundtheEarthisnotcircular,andthatthemotion lunarsurfaceisdark(thelunarmariaforinstance)andpart
of the observer on the spinning Earth also changes the of the lunar surface is bright (the lunar highlands for
distancebetweenthetwo.Whenthemoonisclosertothe instance). These topographic differences are spread
observeritlooksbrighterandviceversa.
irregularlyoverthemoon’ssurface.
To further understand this curious light curve, we
When the moon is at full phase, all shadows on the
mustbecognizantofthefactthatthemoondoesnothave lunar surface disappear giving the surface its maximal
5.0
10.0
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41
brightness. When the moon is at any other phase, the
lunar surface has many shadows due to its irregular
surface and the lower elevation of the sun as seen from
themoon.Thesefactors,incombinationwiththechanging
lunar phases, give the moon strangely varying brightness
versesdaynumbercurve.
32.DEALINGWITHWEATHER
ThereisamyriadoflegitimateexcusesforNOTgettingouttoobserve,andamongthemistheweather.It’ssaidthat
nothingcanputadamperonobservinglikeanovercastsky.That’strue,andthereisnothingthatcanbedoneaboutit.
Theothercomplaintsarethatit’stoohotortoocold.Fortunately,wecandosomethingabouttheseifbuttoalimited
extent.Theadvantagethatwinterobservinghasoversummerobservingisthattherearemorethingsthatcanbedoneto
overcomethecoldthantheheat.Let’slookatafewguidelinesfordealingwiththecoldandheatwhileobserving.
COOLANDCOLDWEATHEROBSERVING
ó Dress in layers. Dressing so allows air to get trapped ó Don’tdelaydressingup.Dresswarmlybeforeyoustart
between layers of clothing where it serves as an
observing. If you wait to get cold before dressing
insulator.Itisbetterthereforetosuitupinseveralthin
properly, it will be very difficult for your body to catch
layers of loose fitting clothing than in one thick layer.
upwhensustainingconstantheatloss.Thetimetodress
The advantage of doing so is that you can add or
warmly is before you start feeling cold. When you step
subtractlayersofinsulationshouldyougettoowarm.
outofyourvehicletostartobservingonacoldnight,itis
ó Keep your core body temperature up.Thebestwayto
fine to be uncomfortably warm so long as you are not
keep your hands and feet from getting cold during
beginning to perspire. Perspiration will have an
observing is to maintain your core body temperature.
unwantedcoolingeffect.
Whenyourtorsoiswarm,extrabloodisshuntedtoward ó Wear gloves. If your body core temperature is
to extremities as a way of cooling off. If the core body
maintained,thebloodcoursingthroughyourhandswill
temperaturedrops,thebloodflowisreducedandhands
be maintained. If your core temperature is maintained,
and feet will rapidly become intolerably cold. To help
eventhingloveswillkeepyourhandswarm.Barehands
keep your hands and feet warm, keep your core body
cangetcoldwhentouchingmetalsthatcanmorerapidly
temperatureup.Properattireandhotrefreshmentscan
conduct heat away from your hands than can still cold
helpwardoffthatchill.
air.Asimilarsituationoccurswithblowingwindandthe
ó Avoid alcoholic drinks. While consuming an alcoholic
accompanyingwindchill.Yourbodymightnotbeableto
beveragemightseemtobeagoodwaytokeepwarm,it
keep up with excessive heat loss even with a wellis a double-edged sword. Shortly after imbibing,
maintained core body temperature. If you are using
alcohol’sbyproductssometimescauseaflushingeffect.
electronicequipmentwhileviewingsuchasacellphone
This can make the skin feel warmer – at least
or tablet, consider getting a set of touch screen gloves
temporarily. This is achieved when capillaries open and
that are electrically conductive and permit you to use
warm blood moves to the skin where it then rapidly
thesedeviceswhileyourhandsarecovered.
cools. Cool blood then returns to the body’s center ó Cover your head. More heat is lost from the head and
whereitdropsthecoretemperaturethatistheopposite
aroundtheneckthanfromanyotherpartsofthehuman
oftheeffectweintendtoachieve.
body.Lotsofbloodflowsclosetothesurfacehere,and
ó Start with a high calorie snack. Eating a healthy, high
thefacetendstogetdirectlyexposedtotheairwhereas
calorie snack prior to observing can fortify an observer
other parts of the body are frequently covered. A hat
against the heat loss that is to come. When one loses
with ear covers and good insulating properties can
heat, the body’s cells will produce it by burning energy
dramaticallyreducethisheatloss.
supplies.
ó Wear a hood, hoodie, scarf, cowl, or balaclava.
ó Take periodic warming breaks.There’snothingtotake
Wrapping or otherwise protecting the neck and face
the chill off an observer like periodic warming breaks.
regionisveryimportantwhenobservingoncoldnights.
Sittingdowninawarmlocation(yourrunningvehicleor
The best material for doing so is micro fleece fabric.
a building) can help you maintain your core body
Microfleeceisathinpolyfleecefabricthatisalightbut
temperatureandfighttheeffectsofheatloss.Itisbest
highlyeffectiveinsulatorthatwicksmoistureawayfrom
totakewarmingbreaksbeforeyougetsocoldthatyou
theskin.Thefabricissoftagainsttheskin,notscratchy
startpickingupa“chill”andbeginshivering.Whenthis
likewool.Becertaintoavoidtrappingbreath(aswitha
occurs, the battle against the cold is lost. It’s time to
neck gaiter) as this results in condensation around the
packupandgohome.
lowerportionofthefaceandcanleadtodiscomfortand
evaporativechilling.Thebesthoods,hoodies,andhood
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ó
ó
ó
ó
scarvesarethosewithflexibledrawcordsthatcanhold ó Consideraugmentedheating.Thereareactuallyelectric
thefabricclosetoyourfaceandreducecontactofyour
glovesandsocksthatanobservermightconsider.Also,
skinwiththecoldnightair.
chemical-basedhandwarmerskeptineachoftwocoat
Consider a Micro Fleece vest or jacket. Micro fleece is
pocketscanprovideanaddedboostifelectricglovesare
soeffectiveinkeepingtheobserverwarmthatitkeeps
outofthequestion.
making its appearance among these recommendations. ó Stayoutofthewind.Windchillcanhaveadevastating
Itcanbeoneofyourmultiplelayers.
effect upon the observer. When a cold air passes over
Consideracoatwithadrawcordatthebottom.Draw
human flesh it conducts away heat faster than if there
cords are elastic bands that can be used to close off
were no wind at all. This produces a wind chill factor.
areas of undesirable wind flow such as around the
Thewindchilltemperatureistheeffectivetemperature
wrists,waist,andneckregions.
the exposed skin experiences. Staying out of the wind
Wear thick pants.Avoidwearingthinpantsduringcold
willallbuteliminatethewindchillfactor.
weatherobserving.Bluejeansprovidebetterprotection. ó Consider remotely controlling your telescope. One of
You might also want to consider the use of thermal
the most difficult tasks when viewing under cold
underwear.
weather conditions is spending large amounts of time
Wear insulating sock and shoes. Avoid thin socks and
searchingforthingsusingfinderscopesorstarhopping.
dressshoes.Theyprovidelittledefenseagainstheatloss
Consider acquiring a “goto” telescope that can find
to the cold ground. Better are thick-soled work boots
objectsforyouwithafewtapsofakeypad.
worn with thicker cotton socks. Avoid, however, tight
socksasthesecanrestrictbloodflowtothefeet.
WarmandHotWeatherObserving
ó Remain fully hydrated.Observingonhot,muggynights
can be exhausting if not downright debilitating. Be
certaintoremainhydratedbecauseyoucanlosealotof
water through evaporative cooling (sweating). Consider
drinking cold, non-alcoholic beverages that can help
reduceyourcoretemperatureandpreventdehydration.
ó Dresslightly.Clothingisaformofinsulationthatcauses
the body to retain heat. During warmer months, wear
lightweight,loosefittingclothingpreferablyofwhiteor
offwhitecolor.Insectslikemosquitoesarelesslikelyto
be attracted to light-colored clothing as animal sources
of“nutrition”areusuallydarkerincolor.
ó Use insect repellents.Becausehumansproducecarbon
dioxide,theywillalwaysattractmosquitoesatnightno
matter how they dress. Insects like mosquitoes are
attracted by the CO2 in human breath. Because of the
growingnumberofdiseasescarriedbymosquitoes(Zika,
WestNile,Denguefever,etc.),itisimportanttodefend
yourself with the use of an effective insect repellent.
ManypeoplesuggestapplicationsincludingDEET.
ó Defend yourself from ticks. Becautiousintick-infested
areas.DogticksarecommoninIllinois,andthesmaller
deer tick is becoming so. Deer ticks are of a greater
concern because they can carry Lyme disease. You are
lesslikelytoencounterticksifyouobservefrom,say,a
parking lot. If, however, you are walking through
vegetation to reach your “safe” observing site, the
chances of picking up a tick increases. You need not
actuallybrushagainstaplanttopickupatick.Tickswill
literally jump from their resting location to any nearby
movingobject–includinglegsandarms.Checkyourself
ifyouexperiencea“crawling”feelingonyourskin.Also,
carefullycheckyourselfuponreturninghome.
ó Avoidchiggers.Whilesomeobserversliketolayontheir
backs at night just to soak up starlight, avoid doing it
directlyonthegroundnomatterhowenticingthegrass
mightappear.Thegrassisoftenfullofchiggers.Chiggers
aremitesthatlatchontopeoplethesamewayticksdo.
Chiggersaremostnumerousinearlysummer.Afterthey
attach to humans, they feed on the skin, often causing
itchingandotherformsofirritations.Theserelativesof
ticksarenearlymicroscopic,measuring0.4mmandhave
ametallicorangecolor.
ó Choose your observing site wisely. Avoid setting up
your observing site near still, stagnant water. Ponds,
discarded tires, watering tanks, dump sites, and
roadside and agricultural ditches can become infested
withmosquitolarvae.
ó Consider a slightly breezy location. Sometimes
amateurs will set up their telescopes in out of the way
places to avoid a breeze. If you are not doing
astrophotography, then choosing a slightly breezy
locationcanactuallypaydividends.Abreezecanhavea
coolingeffect.Inaddition,abreezecankeepopticsfrom
dewingandmosquitoesatbaywhentheyarepresent.
ó Worktopreventdewing.Whentherelativehumidityof
air is high, objects that cool faster than the air will
experience the accumulation of dew. When the
temperature of moist air decreases, relative humidity
increases and condensation occurs. Consider a glass of
ice water on a warm day. Note how quickly the cold
glass causes condensation to occur on the exterior.
Telescopeopticscanradiateheatmorerapidlythanthe
airandaresubjectedtothiscondensation.Becertainto
use a dew shield or an electrical warmer on lenses and
mirrorstokeepthiscondensationfromoccurring.
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
43
ó Take cooling breaks. If you have access to a cooler ó Protectyourfeetfromthedew.Ifyouhavethehabitof
location(airconditionedvehicleorbuilding),youmight
viewingfromagrassylocation,considerplacingatarpor
want to consider taking a cooling break from time to
someothersortofmatonthegroundpriortosettingup
time. Short cooling breaks can be refreshing on a hot,
your telescope. Grass will quickly gather dew as the
humidnight.
temperature drops and your feet will soon get soaked.
Thiscanledtoaveryuncomfortableviewingcondition.
33.CELESTIALOBJECTSTOOBSERVE
Now that you have access to a telescope and know
howtouseit,whatwillyouobserve?Thereisconsiderable
variety among celestial objects worth viewing and many
observingprogramstohelpyoufindtheminasystematic
fashion.Considerthetypesofobjectsthatyoucanobserve
withatelescope:
Solar System Objects: Thesunandobjectswithinitslocal
environment. Each is most commonly known by a proper
name.
• Sun–VisualandnarrowbandviewssuchasHα.Requires
special filters. Caution: The sun can be extremely
dangerous to observe due to the intense sunlight. Seek
outexpertadvicebeforeattemptingtoviewthesun.
• Moon–Oftenoverlookedbecauseitisanaturalformof
light pollution, the moon shows more detail than any
othercelestialobject.
• Planets and their moons – Mercury, Venus, Mars,
Jupiter,Saturn,Uranus,andNeptune.Mostoftheouter
planets(JupiterthroughNeptune)havemoonsthatcan
beobservedwithagoodsizeamateurtelescope.
• Dwarf planets – Don’t forget Pluto and Ceres. Both are
easilyobservedwithsuitabletelescopes.Cereswasonce
considered to be the largest of asteroids, but was
reclassifiedaboutthetimethePlutowasdemotedfrom
planetstatus.
• Asteroids–Thereareliterallyhundredsofasteroidsthat
canbeobservedoverthecourseofseveralyearsasthey
range widely in brightness over time. They look like
slow-movingstarswheremotioncaneasilybeobserved
fromnighttonight.
• Comets – These are visible much of the time, but most
are faint are require telescopes to view. They, like
asteroids,tendtohavereadilyobservedmotions.Some
comets pass near Earth and their motion can be noted
afteronlyafewminutes.
DeepSpaceObjects:Objectsbeyondthesolarsystemoften
identified by Messier (M), New General Catalog (NGC), or
IndexCatalog(IC)numbers.
• Open clusters – Loose gatherings of stars of tens or
hundreds of stars “recently” formed from dusk and gas
cloudsinspace.Morethan10,000areknown.
• Globular clusters – Tightly packed groups of stars
numberinginthetensorhundredsofthousands.These
are among the oldest stars in the Milky Way galaxy.
About 140 are known to cluster around the Milky Way
galaxy.
• Novas – When the least massive stars end their lives,
theydiein“quiescent”explosionsinwhichtheycastoff
theirouterlayersofatmosphere.Moreofa“puff”than
an explosion, these stars brighten enough to be seen
withthenakedeye.MostareobservedwithinourMilky
Waygalaxy.
• Planetary nebulae – The remnants of low mass stars
thathaveejectedtheirouterenvelopeofgas.Morethan
100 are readily visible in modest-sized amateur
telescopes.
• Supernovas – When the most massive stars end their
lives, they die in titanic explosions that can allow them
to become nearly as bright as a galaxy of stars. Most
frequentlyseenfromEarthindistantgalaxies.
• Super nova remnants – The remains of massive stars
thathaveexplodedleavingneutronstarsorblackholes
intheirwake.Afewarevisibleinamateurtelescopes.
• Emissionnebulae–Cloudsofgasthatareforcedtoglow
as a result of hot stars in their vicinity. These are
abundant, but not all are bright. Perhaps a couple of
dozenarereadilyviewedwithamateurtelescopes.
• Reflectionnebulae–Cloudsofdustandgasthatmerely
reflect starlight from cooler stars in their vicinity. Their
number and visibility is much like that of the emission
nebulae.
• Dark nebulae – Clouds of dust that obscure stars and
bright nebulae located behind them. Sometimes they
aresodarkthattheyappearlike“holesintheheavens”.
• Galaxies–Islandsinthedepthsofdeepspaceconsisting
ofwhirlingmassesofhundredsofbillionsofstars.Many
varietiessuchasspiral,barredspiral,elliptical,spherical,
and irregular. Some – the Seyfert galaxies – have
intensely bright interiors and are some of the most
remote.
• Quasars – The most remote objects you will ever see
with your telescope. Only a handful are visible through
moderatesizedtelescopes.Thoughttobeprotogalaxies
forming during the dawn of time powered by massive
blackholes.
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34.AstronomicalLeagueObservingPrograms
Observers are encouraged to begin regular observing
programs, specifically Astronomical League observing
programs.AsnotedontheAstronomicalLeaguewebsite…
(seehttps://www.astroleague.org/observing.html)
The Astronomical League provides many different
observing programs. These programs are designed to
provideadirectionforyourobservationsandtoprovidea
goal.Theprogramshaveawardsandpinstorecognizethe
observers’ accomplishments and for demonstrating their
observingskillswithavarietyofinstrumentsandobjects.
As a quick reference, you can compare the programs
intheselists:
• AlphabeticalListingwithimagesofthepins.
• Listingoftherequirementsforeachprogramina
gridformat.
• Listing of programs showing observer level
(beginner,intermediate,advanced).
• Listing of programs showing equipment needed
(naked-eye,binocular,telescope).
Programs offer a certificate based upon achieving
certainobservinggoalsandcompletionisrecognizedwith
a beautiful award pin. You are required to observe a
specificnumberofobjectsfromalistorofaspecifictype
(meteors, comets, etc.)with a specific type of instrument
(eyes, binoculars, telescope). Some programs have
multiplelevelsofaccomplishmentwithintheprogram,and
some permit observations of different types (manual vs.
go-to,visualvs.imaging)andnotethisonyourcertificate.
There is no time limit for completing the required
observing (except for the Planetary Transit Special
Awards),butgoodrecordkeepingisrequired.
The programs are designed to be individual effort.
Each observer must perform all the requirements of each
programthemselvesandnotrelyonotherpeopletolocate
the objects. This is called "piggy-backing" and is not
acceptableforloggingobjectsforanyoftheprograms.You
are allowed to look through another observer’s telescope
to see what the object looks like, but you still need to
locateandobservetheobjectonyourown.
Whenyoureachtherequisitenumberofobjects,your
observing logs are examined by an appropriate authority
andyouwillreceiveacertificateandpintoproclaimtoall
thatyouhavereachedyourgoal.Manylocalastronomical
societiesevenpostlistsofthosewhohaveobtainedtheir
certificatesasdoestheAstronomicalLeague.
Whenyoucompleteaprogrambyyourself,youshould
feel a sense of pride and great accomplishment for what
you have just completed. Each program is designed not
only to show you a variety of objects in the sky and to
learn some science related to those objects, but to also
familiarize you with your telescope and how to use it,
night-sky navigation (the ability to find the objects in the
vastnessofspace)andtolearnsomeobservingtechniques
that will enhance your viewing of the objectsin the
programs.
ADVANCEDTOPICS:
35.CLEANINGYOUROPTICS
Whether a telescope sits in a closet for years or gets
usedunderthenightskyonafairlyregularbasis,thereis
one thing for certain – the optical surfaces of your
telescope and eyepieces eventually will have to be
cleaned.Evenwiththejudicioususeofdustcaps,mirrors
andlenseswillgetdirty.Considerforinstanceeyepieces.
With repeated use eyepieces will collect dust and
pollen carried on the wind and oils from contact with
eyelashesandotherbodyparts.Thehandsoftheamateur
astronomer will sometimes inadvertently touch eyepiece
lenses and this results in fingerprints. At other times
observers – generally novices – viewing through the
telescope in the dark will actually touch an eyepiece’s
optical surface with their noses leaving an oily smear.
Despiteourbesteffortstoprotectoureyepieces,theywill
get soiled. Objective lenses, mirrors, and corrector plates
are similarly subject to becoming soiled through a variety
ofsimilarmeans.
How should one clean such optical surfaces? The
answer to this question is important, for if an improper
procedure is used, the optical surface can be scratched
and/or left with residues that will negatively affect
performance.
Thechoiceofacleaningfluidisimportant.Whilemost
optical surfaces that have anti-reflective coatings (lenses)
or silicon monoxide over coatings (mirrors) are pretty
durable, no liquid short of a corroding agent is going to
damage them. Nonetheless, the incorrect cleaning fluid
can leave a film that will deteriorate the performance of
theopticalcomponents.
Some observers prefer Windex or Glass Plus as a
cleaningfluid.Othersliketouseisopropylalcoholorpure
methanol. (The author prefers “No-Glare Lens Cleaner”
available through the Wal-Mart’s vision center.) Once a
suitable cleaning fluid has been obtained, one must be
verycarefulwithitsuse.Alwaysapplythecleaningfluidto
the cleaning tissue (Microfiber works well – avoid using
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
45
facial tissue that often contain softeners such as aloe)
rather than to the optical surface, but especially with
eyepieces.Ifthecleaningfluidisappliedtothesurfaceof
an eyepiece, it will often seep around edges where it can
becomelodgedbetweenopticalsurfaces.)Methodologyis
moreimportantthanalmostanythingelse.
Beforeapplyingacleaningtissueorclothtoanoptical
surface,itisbesttouseacamel’s-hairbrush,compressed
air, or a bulb-type puffer to blow away any particles
adhering to the surface. Gently does it. Particles, if not
removed with a gentle touch, can abrade the optical
surface when a cleaning rag is used that applies more
pressure to the optical surface. If a brush or air does not
remove surface particles, gently blot the surface with the
cleaning cloth without rubbing. After a few minutes, try
the brush or air once again. That should dislodge any
abradingagent.
Oncetheopticalsurfacehasbeenclearedofpotential
abradingagents,thesurfacecanthenbecleanedwiththe
cleaning tissue. Remember to apply the cleaning agent to
the tissue or cloth sparingly – perhaps using a mister. Do
notsoakthelensortissuebecauseifyoudo,excessfluid
willbreakawayfromthecleaningclothleavingspotswhen
itdries.Sweepgently(donotrub)fromthecentertothe
edgeoftheopticalsurface.Useanewtissueorportionof
cleaning cloth with each pass. This will prevent any
contamination removed during an earlier sweep from
reappearingontheopticalsurface.UseadryQ-tiptoclean
areas near the edge of an eyepiece’s surface lens. If you
use a wetted Q-tip, capillary action could draw the liquid
intothegapmakingitnearlyimpossibletoreach.
Keep in mind that the cleaning of a telescope’s
objectivesometimesmeanstakingatelescopeapart.Ifyou
shouldeverdothis,youwillhavetocarefullyre-collimate
yourtelescopebeforeitwillworkproperlyonceagain.
Amateur astronomers should be vigilant about
maintaining clean optics. Proper storage using objective
covers and eyepiece cases would seem to been a
reasonablefirststep.
N.B.Ifafteryouhavecleaned your telescope’soptics and
stillseemtogeta“greasy”lookingview,checkyourglasses
ifyouwerethem.Theyoftenbecomesmudgedwithbody
oilsandthiscancauseimagestolookastheyaresmeared
across the field of view. If you do not wear glasses,
consider getting checked for cataracts. Cataracts can
diffuselightpassingthoroughatelescopejustaseasilyas
oil on bifocal lenses. If you are still uncertain about the
cleanlinessofyouropticalsystem,lookthroughyour“nonobserving”eye.Thecanspeakvolumes.
36.CHECKINGTHECOLLIMATIONOFYOURTELESCOPE
Collimating a telescope can be one of the most
dauntingtasksanamateurastronomercanencounter.The
collimation process itself depends heavily on the type of
telescopeandtheapproachused,andisbeyondthescope
of this publication. Rather than attempting to explain this
complicatedprocess,itisadvisablethatthenovicesatisfy
himselforherselfwithassessingthecollimationofhis/her
telescope.Consultingwithanexpertaboutthecollimation
processwouldbebestatthisstageofthegame.
Testing the collimation of a telescope can be done
using a variety of means. There exists one general
approach, however, that can be used to assess the
collimationofanytelescope.Itistheeyepiecetest.
Whenatelescope
is
perfectly
collimated, stars will
appear
in
the
eyepiece as point
sources and extended
objects such as planets will be as clear as atmospheric
seeingandthediffractionoflightpermits.Ifatelescopeis
un-collimated, stars will appear unfocused even though
their image size has been reduced to a minimum. The
image above shows what to expect with a telescope
unobstructedbyasecondarymirror.
Tocheckthecollimationofyourtelescope,itisbestto
chooseabrightstarnearthezenithandviewitwithahigh46
magnificationeyepieceplacedslightlyoutoffocus–either
inside or outside of best focus. When this is done, the
star’sdiffractionpatternismademorereadilydiscernable.
In a properly collimated telescope with a
secondary mirror, the star
will appear as a tightly
spacedseriesofconcentric
bandsoflightasshownto
the left in the accompany
image.Thebandingiscausedbydiffractionassociatedwith
interference of wave fronts as they pass through the
telescope. When a telescope is not collimated properly,
these bands will not be uniformly distributed and the
shadow of the secondary mirror will be offset from the
center.
Don’t confuse the
consequences of poor
collimation with bad seeing.
Whenbadseeingoccurs,the
air will from time to time
holdsteadyenoughforclear
imagesofstarsorplanetsto
be seen. When image quality is degraded as a result of
impropercollimation,thentheimagewillnevercomeinto
clearfocus.
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37.CHECKINGYOUROBJECTIVE’SOPTICS
Not all telescopes are equal. While most telescopes
aresupposedly“figuredto¼waveorbetter”(requiredto
perform on a “diffraction-limited basis”), sometimes
objective mirrors and lenses have a defects that reduce
their ability to perform up to expectations. This is more
commonly true with toy telescopes than more advanced
instruments made for experienced amateur astronomers.
Still, if your telescope is not operating up to expectations
after it has been properly collimated, one needs to start
lookingforevidenceofproblemswiththeobjective.
The image to the right shows how various defects in
anobjectiveaffectinthediffractionpatternsofbrightstars
viewed at higher magnification. The images here are for
unobstructed telescopes such as an ordinary refractor or
Schiefspiegler (a reflector with no secondary mirror). The
central column shows what is expected under various
defects when a star is at best possible focus. The left
columnshowswhattoexpectwhentheeyepieceisracked
insidefocus.Therightcolumnshowswhattoexpectwhen
theeyepieceisrackedoutsideoffocus.
Thefollowingerrorsarethusillustrated:
Row 1: Astigmatism. The images are not radially
symmetric. The focal length is different along different
axes.
Row 2: Coma. More of a problem with short focal ratio
telescopes and stars near the edge of the field. Most
commonlyobservedatverylowpowerwherethefieldof
viewislarger.
Row3:Sphericalaberration:Theresultsfromthefailureto
completely parabolize a mirror or lens. Mirrors are
produced with a spherical form and then parabolized.
Sometimestheprocessisincomplete.
Row 4: Turned down edge. Resulting from a failure to
properly parabolize a mirror. The focal length near the
edge of the objective is longer than zones nearer to the
center.
Row5:Zones.Thisisavariationoftheturneddownedge.
Again, the focal lengths of various concentric zones on a
mirror or lens are not the same causing light to be
defocused at certain points in the image resulting in
dimmingand,incomparison,brighteninginotherzones.
Row 6: Perfect optics. Here we see an in-focus diffraction
pattern with 84% of the light in the central peak and just
under 16% in the first diffraction ring. Not be confused
with the consequences of the turned-down edge. In
perfect optics, the diffraction images are the same inside
and outside of best focus. With turned-down edge, the
asymmetry of inside and outside focus images is quite
noticeable.
Unlessyouareatelescopemaker,ifyouhaveapoorly
madeobjective,youwillhavetolivewiththeseproblems.
Thebestwaytoavoidhavingtolivewithsuchproblemsis
to make certain that your telescope is performing up to
expectationswhenyoupurchaseit.Ifitdoesnot,thenthe
bestthingtodowillbetoreturnthetelescopewhileunder
warranty to the distributor and ask for a different
telescope.
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47
38.BALANCINGYOURTELESCOPE
No matterthe typeoftelescope and mount you have, there isone criticalconcernfor
viewers – that the telescope be balanced. This is of particular concern if you wish your
telescopeisofa“pushto”kindwithun-lockablefrictionpadsandyouwantittoremainin
placeafteryou’vedirectedittoaparticularlocationinthesky.Thereisnothingworsethan
looking for a faint object, finding it, and then rapidly losing it as an unbalanced telescope
swings out of place after one’s hand is removed. The problem is readily resolved by
balancingyourtelescope.
German Equatorial Mount (GEM).Mosthighqualitytelescopeswillarriveprettymuch
inbalance–atleastwhenthecounterweightsaresetintoproperposition.However,when
observersmovethingsaboutormodifyatelescopebyswitchingtoaheavierclassofeyepiecesorputtinganewfinder
intoplace,thetelescopebecomesquicklyunbalanced.Everyamateurshouldthereforeknowhowtobalanceatelescope
regardlessofthetype.
Tobeproperlybalanced,aGEM’sbalancemustbeensuredalongtworotationaxes–thepolar(orrightascension
axis)andthedeclinationaxis–ifthetelescopeistoremaininplaceafteritismovedtoposition.Thiswillalsoreducethe
amountofloadthemotorswillhavetomoveinordertoslewortrackthestars.
StepsforBalancingaTelescopeonaGermanEquatorialMount
Itisimportanttobalancethedeclinationaxisfirst!Acommonerroristoreversethesesteps.
1) Beginbylooseningthepolaraxisandrotatingtheunituntilthedeclinationaxis
connecting the telescope and the counterweight bar is parallel to the ground
as shown to the right.Tighten the clutch mechanism on the polar axis so the
declination axis remains horizontal. Carefully loosen the clutch of the
declinationaxisandseeifthetelescopeisfront-heavyorrear-heavy.Pushthe
frontofthetelescopeupanddowngentlytoseeifitwillstayinplacewhenit
comestoastop.Ifthefrontorbackcontinuestofallorrise,thenthetelescope
isetherfront-heavyorback-heavyrespectively.Thisimbalancenormallycanbe
correctedbymovingthetelescopeforwardorbackwardinitscarriage.
2) Loosentheclutchonthepolaraxis.Movethetelescopearoundthisaxisuntilit
is directly over the mount and aimed north. Slide the telescope forward or
backward in its carriage to achieve proper balance. If the telescope is frontheavy,thenslidethetubebackwardonitsmount.Ifthescopeisrear-heavy,
then slide the tube forward on its mount. Return the telescope to its former
test position and retest. Continue to make small adjustments as necessary. If
movingthetelescopebackwardandforwardinitscarriageisnotpossible,then
addacounterweighttotheoppositeoftheheavyside.
3) Returnthetelescopetothehorizontalposition
once it has been balanced in its declination
axis. Slowly and carefully release the clutch of
thepolaraxistoseeifthesetupistelescope-or
counterweight-heavy. Be careful not to let the
telescopegointo“freefall.”Ifthearrangement
is telescope heavy, the telescope could crash
intothepier.Ifthearrangementprovesheavy
toward the counterweight end, slide the
counterweights up the shaft toward the
telescope.Ifthetelescopeisheavytowardthe
telescope end, slide the counterweights down
theshaftawayfromthetelescope.
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BalancingaTelescopeonanAltazimuthMount
The procedure for balancing a telescope on an altazimuth mount is simpler than that of a GEM. When the altazimuth
mountedtelescopemovesarounditsverticalaxis(inthedirectionsoftheredcircleinthefigureshownbelow)itcannot
beoutofbalance.Whenimbalanceoccurs,italwaysoccursinthehorizontalaxisthatallowsthetelescopetobeaimed
eitherhigherorlowerinthesky.Ifthetelescopeisfrontheavy,eitherweightmustberemovedfromthefrontofadded
totheback.Ifthetelescopeisbackheavy,eitherweightmustbeaddedtothefrontorremovedfromtheback.
Because friction plays an important role in maintaining the balance of a telescope in the
Dobsontypemountshownhere,itisimportanttoaddresstheproblemoffriction.Ideally,when
atelescopeispushedtoagivenposition,itwillstayexactlywhereitwasdirected.Thiswillnot
always be the case with a Dobson mount as stresses build up under friction and distort the
wooden support box like a spring. Once this stress – caused by moving the telescope – is
released, the telescope springs backward driving the telescope off target. It is therefore very
importantinthistypeofmountthatthesurfacesonwhichthetelescopeglidesareofverylow
friction.Thislowfrictionsituationwillthenrequiregoodbalancingintheverticaldirection.
39.POLARALIGNINGANEQUATORIALMOUNT
Polaralignmentistheprocesswherebythepolaraxis
ofamountismadeparallelwithEarth’srotationaxis.Itis
necessaryifoneusesaGermanequatorialmount(GEM)or
aforkmountonawedgeandintendstotrackthemotion
ofthestarswithrotationaboutasingleaxisofthatmount.
WhenEarth’sandamount’srotationaxesareparallel,itis
possibleforatelescopetotrackthemotionofthecelestial
objects by turning about only the polar axis. If an
equatorialmountisnotproperlyaligned,thenadjustments
mustbeperiodicallymadeinboththerightascensionand
declination axes to keep an object centered in the
eyepiece’s field of view. The greater the misalignment of
these axes, the greater the amount of adjustment
required.
Ifoneiscasuallyobservingandobservationsarenotof
longduration,thenamarginallyalignedequatorialmount
will keep the object in the eyepiece’s field view for a
reasonable period of time before it drifts out of view. If
oneisusingthetelescope’sabilitytofindobjectsoneafter
another,thenthemorepreciselyalignedthetwoaxesare
theeasieritwillbetofindsubsequentobjects.
Ifphotographicobservationsarethegoal,thenproper
polar alignment is crucial. If the telescope’s polar axis is
non-parallelwithEarth’srotationaxis,notonlywillobjects
driftoutofthefieldofviewwiththepassagetime,butthe
field of view will also appear to rotate. Trying to take a
timeexposureofanobjectmovingsowillresultincurved
star trails rather than the pinpoint images for which one
mightbehoping.
Anequatorialmountshouldnotbeusedunlessitisat
least roughly polar aligned. If it is not, it will be next to
impossibletotrackobjectsinthesky.Anequatorialmount
canberoughlypolaralignedasfollows:
1. Thetelescopepierissetuponeitheralevelsurfaceor
itslegsaresoadjustedsothattheshaftofthepieris
2.
oriented vertically, pointing straight up toward the
zenith.Pierlevelingisassessedwiththeuseofatool
suchasabull’seyespiritlevel(best)oratubularspirit
levelusedalongtwoaxes90°fromeachother.
Withtheinclinationofthepolaraxisissetequaltothe
observer’slatitude,themountisplaceduponthepier
with the top end of the polar axis aimed toward
geographic (not magnetic) north and the general
vicinityoftheNorthStar.
Thistwo-stepprocessisadequateforvisualobserving
whereeachobjectisfoundbyhandoneaftertheother.If,
however,asuccessionofobjectsistobeviewedusingthe
hand screws or motor drives of the telescope after
“syncing”onastar,thenpolaralignmentmustbeensured
iffindingnewobjectsistobeexpected.
Thereareseveralwaystopolaralignatelescopewith
varying degrees of precision. More precise methods are
usedwithtelescopesthatarepermanentlymounted.Two
ofthesemoremethodsaredescribedhere:firstamodern
methodandseconda“traditional”method.Computerized
goto telescopes have their own methods to compensate
for misalignment and are not within the scope of this
publication.
The first of the more precise
methods involves the use of a polar
sighting telescope. These telescopes
can be used to either peer through
thecenterofahollowpolaraxisofa
telescope or are precisely aligned
with the polar axis in some fashion.
These
telescopes
have
an
illuminated reticle, which must be
rotated to match the current time,
date, and geographic location. This
rotation must be determined from
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49
an external source. Smartphone applications are a
convenientwaytoknowhowtomakethisadjustment,as
theytypicallyprovideallthreepiecesofinformation.Polar
alignment is then achieved by maneuvering the mount in
such a way that the North Star appears in the proper
positioninthesightingtelescope.Ifyouhaveoneofthese
sightingtelescopes,seethemanufacturer’sinstructionsfor
use.
Anevenmorepreciseapproach–thedeclinationdrift
method–allowsanobservertogetamoreaccuratepolar
alignment still. This method requires that the observer
monitor the drift of selected stars. The drift of each star
helps assess in which direction a telescope’s polar axis is
pointing away from the celestial north pole. Although the
method is simple and straight forward, it requires a time
and patience to complete. The declination drift method
should be done only after any one of the previously
mentionedmethodshasbeencompleted.
To use the declination drift approach, you need to
choosetwobrightstars.Oneshouldbeintheeasternsky
andoneduesouthnearthemeridian.Bothstarsshouldbe
near the celestial equator (i.e., 0° declination). You will
monitor the drift of each star – one at a time – in
declinationonly.Whilemonitoringastaronthemeridian,
any misalignment in the east-west direction of the
telescope’s polar axis is revealed. While monitoring a star
in the eastern sky, any misalignment in the north-south
directionisrevealed.Itisimperativetohaveanilluminated
reticle(crosshair)eyepiecetohelprecognizeanydrift.For
very close alignment, a Barlow lens is also recommended
forusewiththeilluminatedreticlebecauseitincreasesthe
magnificationandrevealsanydriftmoreclearly.
With the telescope level and facing due south, insert
the diagonal so the eyepiece points straight up. Do NOT
rotatethediagonalinrelationtotheopticaltubeassembly
at any time during or between the following steps. (The
reason will become clear momentarily.) Insert the
crosshair eyepiece and rotate the eyepiece until one
crosshairisparalleltothedeclinationaxisandtheotheris
parallel to the right ascension axis. Sighting on any bright
star, move your telescope manually in R.A. and DEC to
check parallelism. When the telescope is moved in each
direction,thestarshouldstayonormoveparalleltoeach
ofthecrosshairlines.
Next, choose a star near where the celestial equator
andmeridianmeet.Astarmap,cellphoneapp,oragood
planisphere will be helpful in making this selection. The
star need not be very bright, but should be within 7½° of
the meridian (|hour angle| < 30 minutes of right
ascension) and within 5° of the celestial equator
(|declination| < 5°). Center the star in the center of the
eyepiece’s cross hairs and monitor the motion after
turning off the motor drive if one is being used. Look for
anydriftindeclinationasthestarmovesoutofthefieldof
view.
50
• Ifthestardriftssouthrelativetotheleft-rightcrosshair,
thepolaraxisistoofareast.
• Ifthestardriftsnorthrelativetotheleft-rightcrosshair,
thepolaraxisistoofarwest.
Now,keepinmindthatinatelescopetheprimefocus
image produced by the optical system (either lens or
primary and secondary mirrors in combination) will be
inverted(flippedtoptobottomandfromsidetoside).Also
keep in mind that this orientation will change when a
diagonal is introduced. See in the accompanying diagram
what happens when an extra mirror is introduced.) A
telescope with a diagonal mirror will produce a reversed
imagewherenorthisupandsouthisdown.
Togetstraightinyourmindwhichwayiswhichinyour
eyepiece, move the telescope northward and then
southward in declination and note the direction the star
appears to move in the eyepiece. Take notes or draw a
sketch.It’sveryeasytogetconfusedonthis.
Maketheappropriateadjustmenttoyourtelescope’s
polar axis to eliminate drift. If you make a correction and
thedriftgetsworsebutonthesamesideofthecrosshair,
the chances are very good you corrected in the wrong
direction. If the drift switches sides of the crosshair, then
you’ve probably over-corrected. Repeat the process until
youaresureyouhaveeliminatedthedeclinationdrift.
Once you have eliminated all the drift, aim the
telescope to the star in the east-southeastern sky. Again,
do NOT move the diagonal in any way relative to the
opticaltubeassembly.Thestarshouldbeabout30°above
the horizon and within 5° of the celestial equator. (A star
nearertheeasthorizonstillispreferred,butrefractioncan
alsocauseproblems.)NOTE:Iftheeasternskyisblocked,
you may choose a star in the western sky, but you must
reversethepolarhigh/lowerrordirectionsgivenbelow.
• Ifthestardriftssouthrelativetotheup-downcrosshair,
thepolaraxisistoolow.
• If the star drifts north relative to the up-day crosshair,
thepolaraxisistoohigh.
Again,maketheappropriateadjustmentstothepolar
axis to eliminate any drift. Repeat as necessary.
Unfortunately, the latter adjustments interact with the
prior adjustments ever so slightly. So, repeat the process
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again to improve the accuracy checking both axes for
minimal drift. Once all drift has been eliminated, the
telescopeisaccuratelyaligned.
Careful drift alignment is capable of achieving
excellent polar alignment – within tens of arc seconds of
thecelestialpole–providedadjustmentsonthemountare
capableofthatkindofprecisemovement.Allthiscomesat
thecostoftime.Itcantakeahalfhourormoretoachieve
precisealignment.
Bear in mind that other factors affect your polar
alignment.Forinstance,settlingofthetripodintoagrassy
lawn or a careless bump in the dark can adversely affect
your ever so carefully aligned polar axis. Most polar
alignmentscopesarecapableofgoodalignmenttowithin
a few arc minutes. This is almost always good enough for
visual observation, and generally good enough for most
astrophotography applications. There’s nothing more
frustrating than spending an hour minutes getting a great
polaralignment,andthentrippingoverthemount’spower
cord.Don’tletpolaralignmentspoilthenight.Perfectionis
not necessary. Generally speaking, good enough often is
goodenough.
Cautionarynote:Thismethodworkswellforobjectsin
either the eastern or western parts of the sky. When a
German equatorial mount is “rolled over” the meridian,
the telescope can be out of alignment once again due to
orthogonality error. (The telescope’s optical axis and the
mount’s right ascension axis are not parallel.) This error
typically requires shimming of the telescope reducing the
misalignmentinrightascensionbyhalfandthenrealigning
thepolaraxis.
SUGARGROVEOBSERVATORY:
ClubmembersworkingwithvolunteersconstructedSugarGroveObservatoryin2000.Itisa3-storydomedstructure
thattodayhousesseveraltelescopesonthehighestlevel,aworkspaceonthemiddlelevel,anda“welcomecenter”on
thegroundlevel.TCAAmemberswhoqualifyfortheuseoftheobservatory(normallybycompletingthisIntroductionto
Amateur Astronomy course) will be provided with a sub master key (a key holder fee applies) that will allow for open
accesstoSGO.Membersmayuseallthreefloorsofthisstructureintheirpursuitofthehobbyofamateurastronomy.
40.Operatingthe10’AshDome
SGOiscappedwitha10’-diameterrotatingdomewith
a shutter that can be opened and shut. The shutter rolls
backacrossthedomewhenopened.Theshuttercoverhas
two sections, a short lower section and a long upper
section.Becausethedomemustallowaccesstothezenith,
the shutter must be considerably more than 90° of arc.
When observing objects near the zenith, therefore, the
lowerpartoftheshuttershouldnotberaised.Ifitisraised
alongwiththeupperpart,theshuttercoverwillnotallow
access to the zenith. To separate the lower and upper
sections, pull on the white cord attached to the latch
between the upper and lower sections and put the loop
overtheboltontherightsideoftheslotwhileopeningthe
dome.
To Open: Obtain the crank shaft that looks a bit like a
shepherd’s hook hanging in the stairwell just before
entering the last flight of steps into the dome. Use the
crankshafttoopenthedomeshutter.Besuretokeepthe
crankshaftisroughlyinlinewiththerotationalaxisofthe
eyeonthegearboxwhileturningtominimizewear.Don’t
standdirectlyunderneaththeeyetocranktheslitopenas
this produces wear on both the hook and the eye (not
easilyreplaced).Returnthecrankshafttostorageafteruse.
ToRotate:Thedomecanberotatedtoanydirectionfrom
within.Turningthedomecanbeachallengedespitegood
lubrication,buttheeffortismadeeasierbypushingonthe
triangular handhold to the right of the bottom of the
shutter.
To Close: Closetheshutterjusttheoppositeofthewayit
was opened – keeping the crank shaft parallel to the
rotational axis of the gearbox eye to reduce the wear on
the hook and eye mechanism. When finished, return the
crankshafttoitsstoragelocation.
To Park: Ash Enterprises suggests that when the dome is
notinusetheshuttershouldbeturnedintotheprevailing
wind. This practice minimizes the possibility of blowing
dust,finesnow,ordrivingrainfromenteringthedome.It
is best therefore to direct the dome shutter toward a
westerly direction when in parked position. Southwest
might be best during the summer and northwest during
thewinter,andwestduringspringandautumn.
41.OperatingtheAstro-Physics1100Mount
ThereisnosubstitutionforthoroughlystudyingtheAstro-PhysicsGTOKeypadmanual.It’smanualis119pageslong,
thoughmostofitscontentdoesnotapplytothecasualuserofSGO.However,withoutbeingfamiliarwithallitsfeatures,
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
51
an observer simply will not know how to “recover” should anything untoward happen. A printed copy of the guide
is/will/should be found in the SGO proper for reference purposes. It should never be removed from the observatory.
Whatfollowsaresomesimplestepsthatobserverswillusetocasuallyobservethenightsky.
Tostartupthemount:
1. Turnonthemountbyusingtherockerswitchonthegraypowersupplysittingontheshelf
abovethesecondfloorworkstation.
2. Thekeypadusedtocontrolthetelescope(showntotheright)willturnonaskingforlocation.
Press“1”followedby“GoTo”
3. Thekeypadwillthenaskforstartupinstructions.Press“3”to“resumefromlastposition.”
4. Thiswillgetyoutothemainmenu.
Topointthetelescope:
Ifyou’renotonthemainmenu,pressthemenubuttonseveraltimesuntilthemainmenu
appears.Themainmenuistheonewith“1Object”intheupperleft.
Checkyourslewrate(therateatwhichthetelescopemovesfromoneobjecttoanother),thenumbernexttooption5on
themainmenu.Itgenerallyshouldbe600xunlessyouwantthetelescopetomovemorequicklywhenmovingtoanew
object.Tochangeit,justpressthe“5”keyrepeatedlyuntiltheslewrateyouwantappears.Lownumbersarelowspeeds;
highnumbersarehighspeeds.600x,900x,and1200xareavailablespeeds.Itisrecommendthatyouuseslowerratesin
coldweather.
Check your button rate, the number next to option 6 on the main menu. This determines the rate that the mount will
movewhentheN-S-E-Wdirectionalbuttonsonthekeypadarepushed.64xisaveryworkablerateforvisualobserving.
To change the button rate, press “6” on your keypad. Available options are .25x, .5x, 1x, 12x, 64x, 600x, or 1200x. The
selectionchangeseachtimeyoupressthebutton.
To slew to an object, press “1” on the keypad. The object screen will appear. Press the
number for the catalog or object type you want to view. If it’s a catalog, enter the catalog
numberandpress“GoTo.”Ifit’sanobject,likesolarsystem,presstheappropriate“4”and
press“GoTo.”Selectyoursolarsystemobjectfromthefollowingmenubynumberandpress
“GoTo.”
Caution:Alwayswatch the telescope as itslews.If itbehaves unexpectedly or the weightsgointoanabovehorizontal
position,hitthered“STOP”keyinthecenterofthe“NSEW”motioncontrolkeys.TheSTOPbuttonwillcancelaslewing
commandandstopthemovementofthetelescopeimmediately.Themountwillknowwhereitis,soyoucanproceedto
your next command. If you ever move the telescope by hand (and you should not), you must follow the recalibration
procedure.
Ifatanytimeyoulosepowertothemount,eitherbyapowerinterruptionorbyaccidentallyturningoffthepowersupply
switchbeforetelescopeismovedtoparkposition,themountwillhavetobesyncedonceagainwithaknownstar.You
canstillmanuallymovethemountusingthemotioncontrolkeys.Moveittoitsparkposition.Then,turnitoffandletthe
PropertyManagerknowwhathappened.
TipsforUsingtheButtonsandMenus:
• Correctingdataentries–Whenenteringcatalognumbersintothekeypad,youcanuse<PREVbuttontodelete
lastdigitentered.Entercorrectdigit.
• Scrolling object lists – Some of the object selection screens will display “<“ and/or “>“, usually in the corners.
Thesesymbolsindicatethatyoucanscrollalistofobjectsusingthe<PREVorNEXT>buttons.Ifyouholdthese
buttons,theobjectnameswillscrollquicklyinsomescreens(e.g.stars).
• N-S-E-W directional buttons – The N-S-E-W directional buttons can be used when you are in the Main Menu,
ObjectMenu,PhotographicTimer,mostSetupMenus,andwhentheobjectdatascreensaredisplayed.Theyare
notactivewhenyouareinobjectselectionscreens,i.e.whenenteringaMessiernumber.
52
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
•
•
•
•
•
•
•
•
Reviewobjectdataofcatalogitemspriortoslew–Enterthenumberoftheobjectthatyouareconsidering,then
press >NEXT. The magnitude, object type (galaxy, globular cluster, etc.) and constellation will display. You can
pressGOTOinthisscreentoinitiateaslew,ifyouwish,orpressMENUtoexittomakeanalternateselection.
Displaycurrentobject–ReturntoObjectMenuandpress<PREVbutton.Theobjectdataforthelastobjectthat
wasselectedforaslew(orthatyousyncedon)willdisplay.
Displaycoordinatesofcatalogobjectafteraslew–Press<PREVbutton.ThecatalogRAandDeccoordinateswill
display.Notethatthecoordinatesofthesolarsystemobjectsdisplayontheselectionscreenbeforeyouslew.
Actual current RA/Dec coordinates – When you are in Objects Menu, press NEXT>. The RA/Dec coordinates of
theexactpositionofyourmountwilldisplay.Thesemaybeslightlydifferentfromtheabovedisplaysincethe
calculations that compensate for atmospheric refraction and precession will change the coordinates slightly.
Also,ifyoumovethemountwiththeN-S-E-WdirectionalbuttonsorwithasoftwareprogramsuchasTheSky,
thisdisplayscreenwillbeupdated.
ActualcurrentAlt/Azcoordinates–WhenyouareinObjectsMenu,pressNEXT>.TheAlt/Azcoordinatesofthe
exactpositionofyourmountwilldisplay.Also,ifyoumovethemountwiththeN-S-E-Wdirectionalbuttonsor
withasoftwareprogramsuchasTheSky,thisdisplayscreenwillbeupdatedandthedisplaybycurrentobject
screenwillnot.
Re-calibration–Youcanre-calibrateoncurrentobjectatanytimetofine-tuneyourcalibration.Simplypressthe
RA/DECREVbuttononthekeypad,centertheobjectinyoureyepiecewiththeN-S-E-Wdirectionalbuttonsand
select 9=Re-calibrate. Please read section of the manual that discusses the uses of the recalibrate and sync
functions.
Sync–Youcansynconacurrentobjectafteraslewbypressingthe>NEXTbuttonandselecting1.Pleaseread
sectionofthemanualthatdiscussestheusesofthere-calibrateandsyncfunctions.
Cancelslewingatanytime–PressSTOPtocancelaslewingoperation.Yourtelescopewillstopimmediately.The
mount will know where it is, so proceed to the next object using the N-S-E-W buttons or catalog requests
describedbelow.Donotmovethetelescopebyhandoryouwilllosecalibration.
Toparkandshutdownthetelescope:
Gettothemainmenu.Press“2Setup.”Onthenextscreenpress“4Park.”Onthenextscreenpress“2Parkposition2.”
Park position 2 is weights vertical, telescope pointed at the east horizon. This position minimizes the amount of space
usedbythetelescopeinsidethedome,andallowforeasycoveringwiththebluetarp.Waitforthetelescopetohavenot
movedfor10or15seconds.Coverthetelescopesandmountwiththetarp.Turnoffthesystembyflippingtherocker
switchonthepowersupply.
42.OperatingSGO’sVisualandHαTelescopes
This section assumes that the observer is
knowledgeableabouttheuseoftelescopesingeneraland
the control the Astro-Physics 1100 mount in particular. It
further assumes that the observer knows that a telescope
should never be aimed at the sun unless it has a certified
safesolarfiltersecurelyfittedovertheobjectiveendofthe
telescope or otherwise using a telescope specifically
designedforsolarobserving.
To prepare one or both telescopes, remove the blue
tarp that protects the telescopes and the rest of the
equipment from dust, rain, and snow. Starting with the
telescope in its horizontal park position, remove the dust
cover(s)thatprotectstheopticsofwhichevertelescope(s)
youwishtouse.Storethedustcoverswheretheywillnot
bebrokenorlost–donotplacethemonthefloor.
Visual Telescope: The visual telescope (Celestron 11”
HD or Meade 12” ACF) is designed for viewing the sky
usingthecompletespectrumoflight–red,orange,yellow,
green,blue,indigo,andviolet.Itmayalsobeusedwitha
broadband solar filter that rejects some 99.99% of the
sun’swhitelightmakingitpossibletosafelyviewit.When
viewingthesun,alwaysfirmlyaffixtheprovidedsolarfilter
with the three hand screws that are part of the filter
holder.Alwaysputthesolarfilterintoplacebeforeturning
the telescope to the sun. Be certain to keep the cover on
thebluefinderscopewhenobservingthesun.
Viewing nighttime objects with the Celestron 11”
telescope is no more difficult that properly aiming the
telescope and looking through the eyepiece. The image
canbefocusedwiththeuseofasinglecircularknobonthe
back end of the telescope. There are two other
“triangular” knobs there that constitute a mirror lock.
When the mirror is locked in place, the focuser will not
work.Neverforcethefocusingknobfordoingsocanbreak
the focusing mechanism within the telescope. Keep the
mirrorlocksloosewhilevisuallyobserving.
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53
If the telescope does not point directly at an object
due to a polar misalignment or an “orthogonality error”,
the telescope will be aimed in the general vicinity of the
objectyouseektoobserve.Usingthefinderscopeandthe
keypad’s N-S-E-W keys, center to object in the finder. It
shouldthenbeinthefieldofviewofthemaintelescope.
***Duringsolarobservations,ifthe11”isnotbeing
usedwithawhite-lightfilter,itshouldremaincapped.***
Hα Solar Telescope:TheCoronado90mmSolarMaxII
telescope is for solar observing only. It is equipped with
two hydrogen-alpha interference filters. Their 0.5A
bandpass allows observers to see and photograph solar
details not visible in white light such as prominences,
filaments, plages, and spicules. There is a filter on the
frontandoneinthemiddle.Botharetunabletogivethe
bestpossiblecontrast.Thefrontfilteristunedbyrotating
the front black knurled ring. The center filter is tuned by
sliding the knurled knob sticking out of the side of the
telescope tube. This knob can be screwed into several
holes into the filter ring to allow greater adjustment.
Thereisalsoasmallbrasswheelonthefrontfilterwhich
turning allows the filter to be tilted to eliminate
reflections.
This telescope can also be configured for a “singlestack”bycompletelyunscrewingthefrontfilterelement.If
this is done, please place the filter element in a safe
location as it is the most expensive part of the telescope,
and replace it on the telescope when finished with the
single-stackobserving.
This telescope should only be used with the gold
Coronado diagonal in place as it contains a blocking
elementthatfurtherdimsthesolarenergybeforereaching
the eyepiece. Using any other diagonal in its place will
resultinirreparabledamagetotheeye.Anyeyepiecesthe
observerwishestouseareperfectlysafe.
43.FindingObjectswiththeSGOTelescope
Thousands of objects can be observed with the SGO
telescope. The mount’s keypad controller provides
numerous options to observing celestial objects using
simple identifier numbers: 1) Messier, 2) New General
Catalog,3)IndexCatalog,4)SolarSystemObjects,5)Stars,
6) More, 7) Right Ascension and Declination, and 8) Tour.
Option9referstoRecalibrate.
2.
3.
1.
M (an abbreviation for Messier) objectsare a set
ofastronomical objects first listed by French
astronomer Charles Messier in 1771.Messier was a
comet hunter, and was frustrated by objects that
resembledbutwerenotcomets,sohecompiledalist
of them, in collaboration with his assistant Pierre
Méchain,toavoidwastingtimeonthem.Thenumber
ofobjectsinthelistshepublishedreached103,buta
few more thought to have been observed by Messier
have been added by other astronomers over the
years. These are the “showcase objects of the
heavens.” This list comprises nearly all the most
spectacular examples of the five types of deepsky
objects – diffuse nebulae, planetary nebulae, open
clusters, globular clusters, and galaxies – visible from
midnorthernlatitudes.
4.
5.
6.
7.
8.
9.
NGC (an abbreviation forNew General Catalogue of
Nebulae and Clusters of Stars) is a well-known
catalogueofdeepskyobjectscompiledbyJohnDreyer
in 1888 as a new version of John Herschel’s General
Catalog of Nebulae and Clusters of Stars.The NGC
contains7,840objects,knownastheNGCobjects.Itis
one of the largest comprehensive catalogues, as it
includesalltypesofdeepspaceobjects.
IC(anabbreviationforIndexCatalog)isasupplement
totheNGC.Dreyerpublishedtwosupplementstothe
NGC in 1895 and 1908 a further 5,386 astronomical
objects.
Sol (an abbreviation for Solar System Objects) is a
listingofsun,moon,andplanets…
Strs (an abbreviation of Stars) allows the observer to
selectstarsbynameorcatalognumber.
More will lead the observers to listing of listings of
other celestial objects by type. Reference the AstroPhysicsmountreferencefordetails.
R/D refers to Right Ascension and Declination. The
observer can find objects by their coordinates. This
feature should be used to find objects that are not
partofthekeypad’sdatabase.
Tour is a feature that will take the observer to the
“best” celestial objects for viewing during the
observingsessions.
RcalstandsforRecalibrate.
44.IfSomethingGoesWrongwiththeA-P1100
Startup of the 12" in the SGO is quite simple. Power
on.Punch1thenGoTo.Punch3.Ifyoudon'tgetthisright,
54
simply shut it all off and turn it back on again (provided
youhaven'tyetslewedthemount).Ifyouhaveslewedthe
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
mount without executing the startup procedure as
specified,thenturnitoffbecausethetelescopehaslostits
synchronization with the sky. It will be offline until the
property manager or another authorized observer can
reset the mount. If you don't end up on the main menu,
punchtheMenubuttonafewtimes.
Shutdownissimpleaswell.Toparkthetelescope,get
to the main menu, punch 3, then 4, then 2. Once the
telescope has stopped moving, shut it all off. (Note that
parkpositionis2.)
Ifyou'regoingtoleavethetelescopeunattendedfora
while,gettothemainmenuandpunch8,thenselectthe
option for "Stop". This will prevent the telescope from
moving and inadvertently striking the pier while you're
away.Whenyoureturn,punch8,thentherateoptionfor
"Sidereal",andyou'llbeallsettoresume.
If at any point the mount slews in an unexpected or
dangerousmanner(lookslikethetelescopewillstrikethe
pier,orthetelescopepointsbelowthehorizon),punchthe
"X"inthemiddleofthedirectionkeystostopthemount’s
motion.Thensimplyshutitoff(don'tattempttoparkit).It
willbeofflineatthatpointuntil thepropertymanageror
anotherauthorizedobservercanresetthemount.
45.Personal,Observatory,andSGNCSafety
Since the beginning of the TCAA’s observations at
SGNC in the late 1990s, no one has ever had a serious
personalrun-inwithsafetyconcerns.Therehavebeenno
thefts or assaults, and the only perceived threats have
been skunks, opossums, raccoons, deer, and coyotes.
TherehavebeenreportsofSasquatchinFunksGrove,but
we have never seen them. No UFOs have dropped down
out of the sky to abduct an observer as of yet. There is
always a first time, so it doesn’t hurt to give personal,
observatory,andSGNCsafetysomethought.
To increase your personal security when using SGO,
feel free to keep the observatory door locked when you
are inside. When leaving the observatory unattended at
night,pleaseshutthedoor.Ifunattendedformorethana
few minutes, lock the door when you exit. Uninvited
"guests" have been seen entering the SGO after midnight
whenthedoorwasleftunlockedafteranobserverstepped
outside. It was a couple who had had a bit too much to
drinkbuthadnomaliciousintent.Therecanbequiteabit
of traffic at SGNC sometimes, even early in the morning.
It's almost always harmless, but there's a first time for
everything.
McLean Sheriff’s deputies who have noted our cars
have stopped to make sure we were authorized to be at
SGNCatnight.Thedeputieshaveletusknowthatweare
always free to phone them on their non-emergency
number(309-888-5030)ifwebecomeuncomfortablewith
somethinggoingonoutthere.Ifthereisanemergency,of
course,911isthecorrectactiontotake.
Note that beginning in 2015 and continuing to the
present day we have an infrared surveillance camera
working 24/7. While this will not project observers in any
direct sense, it does provide a record of who was in and
around the observatories should anything untoward
happen.
Ifyouhaveopenedthenaturecenterbuildingforany
reason, it is your responsibility to lock it again. Do not
assume someone else will do so. The SGNC graciously
allowsususeoftheirbuildingasneedarisesatnight,but
that permission can be withdrawn. Be sure the lights are
off, doors locked, and any trash you may have generated
has been properly disposed of before leaving. Few things
generate an email from the director quicker than them
finding the door unlocked or trash in their building when
they come in the morning after a clear night. They know
whereitcamefrom.
OnoccasionyoumayfindtheSGNCisunlockedforno
apparent reason. If none of the SGNC staff is in the
building,youshouldturnoffthelightsandlockthedoors
assuming you have access to a building key. (Your
observatory key will not work in the nature center lock.)
Not only does this help keep the building secure, but it
avoidsthesituationofthemfindingthebuildingunlocked
inthemorningandthinkingitwasourdoing.
When leaving the SGO for the night, check the
following:
1. Allequipmentturnedoff
2. Domeslotisclosed
3. Interiorlightsareoff
4. Cleanupafteryourself
5. Doorknobanddeadboltlocksarelocked
6. Duringthecoldmonths,anyspaceheatersshould
bephysicallyunplugged
The SGNC staff has let me know several times how
comfortingitistoknowthatwe'reoftenoutthereinthe
evening and at night. They're aware of the amount of
traffic there, and to have us pulling night watchman duty
helpsthemnotworrysomuch.
IntroductiontoAmateurAstronomyCopyright©2016TwinCityAmateurAstronomers,Inc.AllRightsReserved
55
46.ReservingSGOforanObservingSession
TherecurrentlyisnoformalwaytoreserveSGOforan
observing session. An online approach has been tried in
thepast,butitresultedinfailure.Asaresult,useofSGOis
pretty much “catch as catch can” and use is on a firstcome,first-servedbasis.
However, in order to reasonably ensure its use, it is
besttoplaceanoticeof“intenttouse”SGOontheTCAA
Yahoolistserv.Seeanyrecentnewsletterfordetailsabout
thelistserv.Weencouragesharingobservingtimeiftwoor
moreobserversarriveonsiteatthesametime.
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