1 Purpose 2 Preparation
ASTR 230 ASTRONOMICAL LABORATORY - SPRING 2016
LAB#1
INTRODUCTION TO TELESCOPES AND OBSERVING TECHNIQUES
Due: Tuesday, February 23, 2016 in class
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Purpose
The purpose of this project is to introduce you to the basic characteristics of telescopes and
familiarize you with observing techniques that you will need for your projects. In the process
of doing this lab you will learn to identify constellations and enjoy observing several celestial
objects. Initially the lab assistant or instructor will be present to assist you, explain things,
fix things that do not seem to work right, etc., − but in the end you must do the actual
observing and writing of the lab report. Students will observe together in small groups
(typically two people), but each person should make and report their own observations.
Watching others is not the way to learn how to observe (though there are some benefits in
working with an experienced observer).
This laboratory will take several outside observing periods to complete. An observing period
is about 3 hours or more under good (less than 50% cloudy) weather conditions. You are
encouraged to observe as long as you can if you have a clear sky and low humidity (rare
in Houston!). While all of the observing for the first lab can be done on campus, some of
the exercises such as identifying constellations are much easier to do at a dark site such
as George Observatory in Brazos Bend State Park. You are encouraged to make a trip to
George for one of their public viewing nights held on the weekends. Alternatively, we will
have at least one night where the class may come to the prof’s house in Manvel to observe
with the Meade 10”. These sessions where someone finds the objects for you do not count
for the Deep Sky part of the lab, but they are a fun way to see a lot of different objects in
one observing session.
Before you begin, you will need to read the material assigned in class, and work with the
computer program Stellarium (get a copy on-line, current vesion seems to be 0.13.1).
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Preparation
Good preparation is essential for success in this and any observing project. An observational
astronomer usually has to spend much more time preparing for an observing run than actual
observing. Before you touch a telescope, you should:
• Carefully read this entire writeup before you go out.
• Do the portions of the lab that don’t require a telescope – working with Stellarium, and
constellation identification.
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• Study the star charts in your reference book of choice to find out which stars and planets
are visible by going outside your home or college if weather permits to check out “what is
up there” – before observing with the telescopes.
• Buy yourself a flashlight and lab record book and be sure someone in your group has a
watch/phone with a working timer (or analog equivalent).
• Your time at the telescope will be spent more efficiently if you know the basics of telescopes
(types, magnification, f-ratio, resolution, etc.), the equatorial coordinate system (right ascension (RA), declination (DEC), sidereal time (ST), universal time (UT), meridian, celestial
equator, zenith, etc.), the astronomical stellar magnitude system (apparent magnitude, color
index, etc.), and the nature of the object (the Moon, planets, star clusters, nebulae, galaxies,
etc.). Most of this will be explained in our weekly lectures, but you may have to do some
outside reading on your own.
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Lab Assignment
The assignment consists of several activities that don’t require a telescope which are meant
to introduce you to how celestial objects move in the sky and what types of objects are
currently visible. The observing program consists of exercises that first familiarize you with
the performance of the C8, and then develop your observing skills by identifying various
objects. Section 3.1 can be done without a telescope, and you should do this first. Sections
3.2 & 3.3 introduce you to the operation of the telescope and can be completed your first
night. The remaining portions of the lab can be done on the second or third nights of
observing, while observing that requires dark skies may be done at George Observatory or
in Manvel south of Houston.
You will need to write up the answers to the questions in each section in a lab report, so
take good notes about what you are doing.
3.1
Stellarium (telescope not required)
Stellarium is a very useful program for studying the sky and identifying objects for telescopic
study. It saves hours of work in preparing finder charts and obtaining current coordinates
for both amateur and professional observing. It’s also fun to play with, so feel free to explore
the night sky via computer simulation beyond what you are directly instructed to do. You
can get plots of the night sky at any place and at any time of the year (including thousands
of years in the past or the future). It can be used to find many objects by name, show where
they are in the sky, make finding charts of the fields where they are located, show pictures
of numerous objects, animate the motions of the sun-moon-planets, etc.
You must do this part before you observe. You can work with the computer software at your
convenience, but do this before beginning your first outdoor session. You will need several
hours to do this, so don’t wait until the last minute and think you are going to do this while
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observing. Using the programs is good “cloudy night” astronomy fun...
The program is generally self-explanatory, and you simply need to spend some time with it
so you know where to look for the feature you want. When you first open the program it
will launch a full-screen window that depicts what it thinks the sky should look like at the
time and location it inferred from your computer. You may need to set these as described
below. First though, familiarize yourself with the two menu bars. The bottom menu can be
found by sliding your cursor to the bottom-left of the window. A horizontal bar will appear
there detailing about a dozen options. Hover your cursor over the icons. Briefly, these are
from left to right:
• Constellation Lines: Toggles lines on-and-off between stars to help you see constellation
patterns. I usually turn this on.
• Constellation Labels: Toggles the names of the constellations. There are 88 of them in all.
I leave this on as well usually.
• Constellation art: This is rather amusing, but allows you to see, for example, what Hercules
is supposed to look like. Hmmm. Somebody 3000 years ago had a good imagination. I find
this distracting and leave it off, but it is a fun thing to toggle.
• Equatorial grid: This is the key coordinate system used to identify objects in the sky. It
is what we will spend time learning, and it essential to know when at the telescope in order
to determine what is up in the sky at any given time. The visibility of any object depends
on the time, date, year, and location on the Earth. It looks a lot like longitude and latitude.
I always leave this on.
• Azimuthal grid: Another spherical coordinate system. This one determines how far above
the horizon an object is situated, and its azimuth (e.g., north, south, northeast). Either on
or off.
• Ground: The horizon of the Earth. Remove to get view from space or if you want to see
where something is located when it is not up.
• Cardinal Points: Labels N, S, E, and W. Up to you.
• Atmosphere: A haze layer simulating the atmosphere near the horizon. Usually leave this
on. Makes the sky blue during the day. Shut it off to see the stars during the day.
• Deep Sky Objects: Labels bright clusters and nebulae in small orange symbols. These
are all candidates for you to discover this semester! Click on them to learn more about the
object.
• Planet Labels: Does what you’d think.
• Switch between equatorial and azimuthal mount: Silly thing related to telescope orientation. Ignore for now.
• Center on selected object: You should be able to select an object by clicking on it. Hitting
space-bar or clicking this button will rotate the field to center the object.
• Night mode: Turns everything red. Good when observing so your eyes stay dark-adapted.
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• Full screen: Toggles this mode.
• Time controls: To set current time, speed things up so you can watch objects rise and set
(key J), slow them down (key L), or return rate to normal. Note how the clock changes as
you speed up and slow down.
• Quit: Ends program.
• Navigation: On a Mac, the arrow keys allow you to move around the sky. A click-drag does
the same. Select an object with a click, and deselect with a right-click (side of keypad). To
zoom in and out use command-arrow-up or command-arrow-down. Don’t use the two-finger
scroll to zoom in and out - at least on my laptop that freezes the click and I have restart it.
Similarly, the side menu is accessed by sliding your cursor to the left. Here you will find
menus for location, time, search, and various configuration menus as to how bright stars
appear, how many to display, and so on. Go to the Sky and Viewing Options window and
choose the Markings tab. Turn on the Meridian (a green line between north and south) and
the Ecliptic (plane of the solar system and path of the Sun, marked as a red line).
If you click on one of the brighter stars (try the Sun if the screen comes up displaying a
daylight view), the star’s (or other type of object) name, its coordinates, spectral type,
distance, and other information should appear on the left. The coordinates are listed as the
RA and Declination of the object, and these are the coordinates you will need to use with
the telescope when trying to find things.
Use the arrow keys to rotate the display to face south, and use command-arrow-down to
zoom out a bit so the horizon looks curved and you can almost see both east and west. Use
the up-arrow keys to look up until the ground is near the bottom of the display. Turn off
the atmosphere, and click ’L’ until the stars start to move. Watch as objects rise and set
and note the time and date in the lower right corner of the bottom menu bar. Stop the fast
rotation with ’K’ or reverse it with ’J’. Return to the current time and normal speed using
the bottom menu bar.
Zoom out a bit, be sure the the ecliptic is visible, and set the time to be around noon so the
Sun is up. Right-click to remove any star information you may be currently highlighting.
We want to move forward and backward one day at a time. Go forward with the ’=’ key,
and backward with the ’-’ key. Note what’s moving and what’s not, and what’s moving the
fastest (why?)...what’s that big grayish thing zooming through and changing phases? Do
you see planets that circle the Sun? Do they all do that? Do all the planets move relative
to the stars? Isn’t this neat? You have a celestial time machine here, or more properly, a
computer planetarium.
Back to business; reset the date and time to some time tonight, say 9:00 pm Is the Moon
out? If not, try command-= to move ahead one hour at a time until it rises? What phase is
it? Try clicking on the Moon, and then type / to zoom in (and to zoom back out). This is
what you are going to see if you go observing tonight!
Let’s get coordinates for the objects in Appendix I... Find “Algol” using the search menu
on the left. You will notice there are two coordinate systems, one for 2000.0 and another
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for 2015.0. They are very similar to one-another, and you simply need to be consistent with
what you use. Usually we work with 2000.0 coordinates for convenience. We will learn about
the difference in the lectures. Write down the coordinates and other information to use when
you try to observe it. Do the same thing for all of those other named stars in the Appendix
I.
OK, what about those deep-sky “M” objects (from the catalog of Messier – a comet hunter
in the 18th century who cataloged 110 nebulous objects that fooled other comet hunters)?
Follow the same procedure as you did with Algol. Once you find M42 you can zoom in and
out to get an idea of where it lies relative to bright stars you can see with your unaided eye.
While you are at it, if you’re looking for an (otional) entertaining Houston activity, take
an hour to attend a planetarium show at the Burke Baker Planetarium at the Museum of
Natural Science. The show you should sign up for is Starry Night Express or whatever they
are calling it these days. There may be a nominal fee for the planetarium show. It is a fun
thing to do and helps you to visualize the celestial sphere.
What to turn in with your lab report:
1. A statement that you have gone through all the steps in the previous paragraphs. Include
the coordinates of the bright stars in Appendix I. Describe motions of moving objects when
you animated the display by changing the time.
2. A list of the constellations in which each of the planets, Sun, and Moon appeared in on
the date and time of your birth, and a description as to where/if each planet was visible in
the night sky. You should turn on the constellation boundaries for this part.
3. When is the next time that Jupiter and Saturn will lie close together in the sky as seen
from Houston? Can you identify an evening when Jupiter, Saturn and the Moon form a
nearly perfect isosceles triangle, with separations less than 7 degrees?
4. You are on a quest to observe Mercury. You are willing to travel anywhere on Earth.
Where and when would you go in 2016? You’ll want Mercury as high as possible in the
sky as the Sun sets or rises. Explain why some times are better than others, and why some
locations are better than others.
After pursuing this exercise, I hope you now see just how much there is to study “up there”
and how convenient it is to prepare for telescope observations using computer software and
databases. Now comes the hard (but enjoyable) part: going into the cold dark night and
observing the real stuff!
3.2
Constellations (telescope not required)
Use the starmaps in your book or just Stellarium, find at least one dozen constellations and
the brightest dozen or so stars. Try to find these stars and constellations in the sky. If you
were not able to find a particular star or constellation, was it because it never rises from
Houston, because it was up at a later or earlier time, or some other reason? Note the date,
time, location (campus, George Observatory etc.) the sky conditions including: the % of
the sky covered by clouds, any haze, moonlight and interfering street lights. Now locate
the faintest star you can see with the naked eye (no telescope). Find it on your star chart
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and record its magnitude. This will be the limiting magnitude and should be mentioned in
your lab writeup. Later, when you go to a dark site outside the city, record and compare
the limiting magnitude at this site with the limiting magnitude in Houston. Review the
constellations and stars during subsequent observing evenings until you know many of them
well. The patterns will not change over your lifetime - once you learn the constellations
they will become your permanent companions. Their locations in the sky tell you directions,
herald the dawn and the coming and going of seasons, and even give your location on the
Earth.
What to turn in with your report:
1. Your list of constellations and stars, and any specifics related to your observations as
described above.
2. The limiting magnitude.
3.3
Basic Observing Setup
We will start using the telescopes in small groups the second week of class. Most of the
telescopic work in this first lab will be done with one of our two 8-inch Celestron (C-8)
telescopes. No more than two people will be working with one of these at a time, so we can
accomodate a total of four at a time for the first part of the lab where you are learning to
set up the telescope and start observing. The professor will watch the weather conditions
and send out an email announcement when the sky looks promising. The first four people to
respond will then be able to observe that night. The goal is to get you comfortable observing
with the C-8s so that later in the semester, you can use them on your own. It will almost
certainly be necessary to schedule more than one night to complete all parts of the lab, and
nights may well be lost due to weather (a plague for professionals too). Do not leave this to
the last week – schedule and observe early.
3.3.1
Setting up the Telescope
The telescopes and other observing equipment are kept in a storage room (room 401) on
the 4th floor of Brockman Hall. You will need to take what you need from this room out
onto the 4th floor observing deck (room 400) of Brockman Hall to do your observations.
There are power outlets on the deck to plug in extension cords for the telescopes. We will
try to keep the equipment near the front of the storage room, but through the course of the
semester, things often get moved around. Your professor and lab assistant will help you find
the equipment you need:
Essentials: Two Celestron-8 telescopes and essential accessories that you should normally
need to observe – eyepieces, diagonals, telescope bolts and wrenches, a power cords for the
telescope, and extension cords.
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Heaters: A blow dryer in case it is humid and your lenses fog up (try using the large black
dewcap over the end of the telescope), and a couple of heater coils if the temperature drops
to near or below freezing and the gears no longer move smoothly.
Power Cables and Drives: Extra power cables and extension cords.
Large Tools & Small Tools: Various tools.
The most important items for this part of the lab activity are the C-8 telescopes. This
storage room also contains a larger, Celestron-11 telescope, and a Meade 8-inch telescope
which is not currently mounted on an equatorial mount. A solar telescope is also in this
room. You will not be using these for the first lab, but they are available for work later in
the semester if needed.
C-8 Telescope (2): The two C-8s are both mounted on tripods and use dual-axis equatorial
mounts. These telescopes have fully computer driven mounts; however, the point of this first
lab is to get you aquainted with how the sky and telescopes move, so you will be doing most
of the work manually. Lift carefully from the tripod in order to move the telescope. Do
not lift using the telescope tube, the finders (small telescopes on the side) or the eyepieces.
Avoid all contact with the optical surfaces of the telescope (lenses, mirrors, eyepieces). Do
not put your hands or fingers on the clear glass corrective plate at the end of the telescope.
If dew forms on this surface, do not wipe it off (you could have used our dew caps!). Instead,
if you anticipate a high humidity night, use the “hair dryer” whenever the corrector plat on
the end of the tube needs to be cleared.
Note on moving (slewing) the C-8 optical tube (the main scope): The C-8 has RA and DEC
clamps such that the telescope should not be “slewed” (moved large distances by hand) in
RA or DEC with these clamps tightened. The clamps are located near each axis of rotation.
The appropriate clamp must be loosened when slewing the telescope and tightened when you
find the object in the finder. These mounts do not have slow motion knobs. You will need
to use the arrows on the hand paddle to make fine adjustments, so you will need to power
up the telescope and go through some menu items to be able to do this. Note that when
using the paddle controls to move the telescope, the clamps must be tightened to properly
engage the motors.
Powering Up the Telescope for Movement and Tracking: There is a power adapter
(like a laptop power supply) for each telescope. Use and extenstion cord to plug this and
then plug it into the base of the telescope mount at the top of the tripod. Then switch
the mount on. The hand paddle will start displaying a number of messages, asking you to
verify positions and set the date, time, and time zone. None of this matters for what you
are doing in this lab, so simply hit the “enter” key each time it prompts you for something
until you get to the menu for “Select Alignment.” At this point, use the up and down arrow
keys (also marked “scroll” which are the # 6 and 9 keys) to select ”Last Alignment” and
then press “enter.” This will get the mount and hand paddle all ready to use. You should
see the heading ”Advanced GT” on the paddle. You can now use the 4 direction arrows to
move the telescope. By default, you will be in a relatively slow speed, but you can change
this by hitting the “Motor Speed” button and then pressing a number (9 is fastest) to select
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a speed.
3.3.2
Finder Alignment (can be done at dusk)
Before you power up the C-8 it is wise to check to see that the finder scope and the main
scope are aligned. Put a low magnification eyepiece in the C-8 (25 – 50mm focal length)
and find some distant terrestrial light (a street light or the illuminated corner of a building),
being careful to loosen the RA and DEC clamps before slewing. First put the object in the
center of the finder. Then look through the eyepiece, adjusting the objective’s focusing knob
if the object is out of focus. Is the object centered in the eyepiece field of view? If yes,
then the finder and main scope are aligned. Most of the time they will not be aligned, so
while viewing through the main scope’s eyepiece, move the telescope around until you can
center your object of interest in the main telescope’s field of view. Now look back into the
finder. The cross hairs should be on the center of the object if the two scopes are aligned.
If not, adjust the position of the finder scope until the cross hairs are on the object which
is at the center of the field of view in the main telescope eyepiece. Remember if you tighten
one screw you must loosen one of the others at the same time to move the finder without
damaging it. This may take some effort and you might have the lab assistant help the first
time. This alignment is something you must check at the beginning of the night or if you
bump the finder scope during the night.
Note that once you start looking at celestial objects you may have to change the focus of the
telescope. It is easy to focus on a bright star; you simply adjust the knob next to the center
eyepiece tube until the star appears as small as possible. DO NOT FORCE THIS KNOB...
if you find resistance, turn it in the other direction!
3.3.3
Pole Alignment
Now you have to align the polar axis of the C-8 to the NCP (North Celestial Pole). This is
painful at first, but gets easier the more you do it. Find the pseudo-North Star, Polaris (the
Pole Star), in the sky. The mount has a very handy system for aligning the telescope. First,
check to make sure the tripod is setup so that the telescope mount is level. There is a bubble
level built into the base of the mount. You may need to adjust the tripod legs in order to get
the bubble centered properly. The mount has a small telescope mounted inside it looking
through the RA axis of the mount. There is a small dust cover on the front. Remove this,
and physically adjust the mount (moving the tripod or adjusting the tilt of the mount itself
using its set screws) until you have Polaris properly placed while looking through the small
telescope. When looking through this, you should see a small crosshair in the center, with a
ring around that and a small circle in the ring as pictured below.
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Outside this ring, you will also see a few constellations not to scale. You should rotate
the small telescope in the mount so that the constellations you see through it are properly
oriented as they appear in the sky. This is required because Polaris is not exactly at the
North Celestial Pole and this ensures you get the offset in the correct direction. You can
now adjust the mount until Polaris is in the little circle inside the ring that appears around
the crosshair. When done, the mount’s RA axis will now be aligned to the pole. You can
check this by now rotating the telescope in DEC so that you can see Polaris in the finder.
If you place Polaris close to center of the finder, you can rotate the telescope around in RA
and Polaris should move in a tight circle around the center of the finder. If this does not
happen, try moving the telescope to place Polaris a little closer or further from the center
of the finder (or on the other side of the crosshairs) until you can rotate the telescope in
RA and see Polaris move around the center of the finder (this of course will only work if the
finder and the telescope are aligned). If you are unable to get this to work, double check
the alignment of your mount. Once this is done, everything is now aligned. You can then
check the DEC cordinate on the telescope. It should read 90◦ . If it does not, you can move
the DEC setting circle until it does, or, if it is close, just remember the offset from 90◦ and
apply that to the DEC coordinates for each object.
Once the finder and C-8 optical axis are aligned, you can power the internal drive that moves
the telescope to compensate for the Earth’s rotation. To do this, press the “Menu” (# 7)
key on the hand paddle. By default, you should see “Tracking” appear on the paddle display
unless you have previously been in some other submenu. If you see “Tracking,” press enter.
If you do not see this, use the scroll arrows to select “Tracking” and then press enter. You
should now see “Mode” displayed. Press enter again. At this point there are 3 options. You
want to select “EQ-North.” If this is displayed, press enter, otherwise use the scroll buttons
to bring this selection up and then press enter. When you need to turn tracking off, go
through the same menus and select “off” instead of “EQ-North.”
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3.3.4
Setting Circles
Once you think that the C-8 is satisfactorily aligned to the pole, it is time to set and check
the setting circles, the dial readouts in RA and DEC. To do this you will have to find a bright
star you can locate with your eye and for which you can get known coordinates. See the
suggested list of bright stars in Appendix I and get the coordinates of these stars ahead of
time from the Peterson’s guide or from Stellarium, center one of them in the C-8, check the
DEC circle reading and if it is off, adjust or (easier) write down the error and compensate
other future settings. Note the DEC circles on the two sides of the fork won’t necessarily
read exactly the same. Now set the RA circle (move it by hand) to the star’s RA. Next
carefully move the telescope (blindly) to the DEC and RA coordinates of another bright star
about 90 degrees away in the sky (adjusting for the DEC circle error, if necessary). Is it in
the finder? Is it in the telescope field (low magnification)? If it is not, you goofed! First try
another star pair. If that fails, then you will have to go back to the NCP alignment process
(or check to see if the circles are slipping). After you are successful, record the names of the
stars you used and mention them in your writeup along with a general discussion of how
easy or difficult it was to accomplish the polar alignment.
Once you have confidence in the circles, almost anything that can be seen can be found (in
a low magnification eyepiece) by setting the circles to the coordinates of a star (centered in
the C-8) near an object, then moving to the coordinates of the object. Care and practice
is all it takes! Note that once you have the telescope polar aligned DO NOT move or bump
the wedge while completing the rest of your observations. If you do move the wedge you will
have to realign it. At this point, you may continue on if you have time and begin observing
the objects listed in each of the following sections. When you finish observing, make sure
everything goes back exactly where you found it, nothing is left, and all doors are locked.
It is always a good idea to record in your lab book where you are located, the beginning
and ending time of your observing session as well as the conditions of the sky. Estimate the
amount (%) of cloud cover, point out the presence or absence of man-made light sources,
the temperature (if you know it), humidity (low, medium or high based on the dew on the
telescope!) and whether or not the wind was strong enough to rock the telescope. If sky
conditions change significantly during the observing session also note that.
What to turn in with your report:
1. Descriptions of your procedure, success/failure with the tasks described in the preceding
four subsections.
3.4
Local Sidereal Time
Before you go to the telescope, synchronize your watch with Universal Time or Central Time
by using the National Institute of Standard’s clock available under the ‘assignments’ link on
the course home page (http://www.ruf.rice.edu/∼cmj/astr230/astr230.html).
The local sidereal time (LST) is defined to be the RA of stars crossing the meridian at
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your location. It varies with location and date (Why?). A rough estimate of the sidereal
time can be made by locating a star which is both high up in the sky and which can be
identified by you so that its RA and DEC are known from Peterson’s book or some other
source (Stellarium). Center the star in the eyepiece of the C-8. Adjust the RA circle so that
it reads the RA of the star (use 2014 or 2000 coordinates depending on what is available
in your reference). Now slew the telescope in the Right Ascension axis only (do not change
Declination) until the long rod holding the counter weight is parallel with the ground (use
the small blue level to determine this). Record the sidereal time that you found from the
RA circle and and the time (CDT, CST or UT) from your watch. Estimate the accuracy
with which you were able to determine the local sidereal time based on the accuracy with
which you can read the RA dial (± how many min, sec).
When it is time to write up your lab report you will use the UT and local time to calculate
the LST, and compare it to the value you read off of the C-8 RA dial.
What to turn in with your report: 1. Description as to how you measured LST experimentally.
2. Your calculations of LST, and comparison with your observed value.
3.5
Telescope Parameters (magnification vs. light gathering power)
The magnification of a telescope objective-eyepiece combination is equal to the ratio of the
focal length of the telescope divided by the focal length of the eyepiece, where they are both
in the same units so their ratio is a dimensionless quantity. The focal length of the C8 is 2032
mm. Note that the focal length of eyepieces are always written on them in mm. Calculate
and record in your lab book the magnification of each of your eyepieces when used with the
telescope. Include these magnification values in your lab report.
Now you are to determine the size of the field of view (in arc seconds) of your highest and
lowest magnification eyepiece. Put your lowest magnification eyepiece in the C8 and find a
star relatively high above the horizon and as close as possible to the celestial equator where
DEC=0◦ (check The Sky or another reference ahead of time). Now measure the field of view
of your lowest magnification eyepiece by letting the star “trail” through the widest diameter
of the eyepiece field. Do this by centering the star in the eyepiece field of view, turn off the
drive and see how it moves. With the drive still off then use the RA slow motion knob to
move the star so it is just barely outside the eastern edge of the eyepiece field. Time how
long it takes to move all the way across the middle of the field of view to the western edge.
You will need a watch with a second hand to do this. Now insert the highest magnification
eyepiece and repeat the steps, recording how long it takes to move across the field.
For your writeup you will want to calculate the size of the field of view for each of these two
eyepieces. You can do this as follows: For a star on the celestial equator, the conversions
from time units (units of RA) to angular units (seconds of arc) are 1 hour = 15 degrees; 1
min = 15 arcmin, 1 sec = 15 arc sec. For a star off the equator, you will have to make a
declination correction to the above relations (see below). You can quickly do the calculations
now or wait to do it during your writeup. In your writeup compare the size of the fields of
view of these two eyepieces to their magnifications you calculated above.
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If your telescope drive has been off for more than a few minutes, your RA dial will no longer
be accurate. Reset the RA dial to the RA of the star you are observing before moving the
telescope. The DEC dial should not need adjusting.
Finally, put the low magnification eyepiece in the telescope and go to a star whose coordinates
you know and is near declination 60◦ or as close to that as you can get. Once you have
centered the star turn off the drive and repeat the timing procedure for this new star. You
only need to do this for one of the eyepieces, either your low or high power ones from above.
You should find that the time it takes for the star to drift across the field is noticeably
longer. In fact, for stars off the equator with declination (δ), the relation between RA time
units and angular units changes (Why?), such that the angular units in the above equatorial
relations have to be multiplied by cos(δ). In your writeup show this is true by comparing
your observations for the star at the equator with the star at a declination of 60◦, keeping
in mind that your field of view in the eyepiece did not change, so the conversion from time
to angular units is different.
Once again, if your drive has been off for more than a few minutes, recalibrate your RA dial
to the coordinates of the new star and turn on the drive.
Find an emission nebula, such as the Orion Nebula (M 42) (use circles and coordinates
if necessary) using a low magnification eyepiece (the 32 mm Plossel is suggested). Use a
sheet of paper or notebook to slowly cover the objective (have a friend hold it) and see how
the surface brightness of the nebula decreases. For a fixed focal-length or magnification,
the surface brightness should vary with the telescope’s light gathering area. Record your
observations in your lab book.
Now put in a higher magnification eyepiece (e.g., ∼9mm or so) and observe the nebula;
notice how much fainter its surface brightness gets and how the stars look (larger? blurrier?).
Record your observations. What does this tell you about the relative importance of telescope
light gathering power compared with magnification “power” for observing extended deep-sky
objects? Remember this if you ever buy a telescope. Next insert the “nebular” filter, the
green eyepiece filter. Compare the appearance of the nebula to what you saw earlier. Is the
contrast better? Why? Note that a nebular filter blocks out man-made sodium and mercury
emission lines, but lets in light between those wavelengths.
Another important characteristic of a telescope when it comes to viewing or photographing
extended objects is the f/ratio = focal length of telescope divided by the objective diameter.
From the information you have about the C8 (or C11) what is its f/ratio? Suppose the
f/ratio of the C8 was f/5 (but it still had an 8-inch objective); would the Ring Nebula and
other extended deep-sky objects appear brighter or fainter through a given (say 32 mm)
eyepiece? If you were a deep-sky fanatic (that is you were interested in mostly observing
faint extended objects such as nebulae and galaxies) would you buy an f/5 or f/10 telescope
of a given objective size for observing them? Note that we also have a “focal reducing lens”
which makes the f/10 C8 a f/5.6 telescope. Attach this (carefully) to the back of the C8, put
in the same eyepiece as above (32mm or so) and reobserve the nebula. Does it look smaller
compared to direct viewing with the same eyepiece? Does it look brighter? Record your
comments.
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What to turn in with your report:
1. Your field of view calculation for both low and high power eyepieces. Calculations of the
power for each.
2. Your timings of stars on the celestial equator and at δ = 60◦ . Appropriate comments.
3. Description as to what happens to the nebula as you a) cover the objective, b) change
the magnification, c) use the nebular filter, and d) use the focal reducer.
4. Answer the focal ratio question regarding deep sky objects.
3.6
Resolving Power and Astronomical Seeing
The atmosphere limits the effective resolving power of ground-based telescopes. The theoretical resolving power (RP) of a telescope in radians can be computed by RP = 1.22λ/D. The
wavelength of the observed light, λ, and D, the telescope objective diameter, must be in the
same units. If λ = 5500 Å (green light) and D is the telescope objective diameter in inches,
the resolving power of a telescope in seconds of arc (1 radian = 206,265 arc sec) becomes
RP(arcsec) = 5.6/D(inches). Derive this result in your writeup. What is the theoretical
resolving power (in green light) of the C8 in arc seconds?
Now let us see how close we can get to this theoretical resolving power when limited by the
Earth’s atmosphere.
When light passes through the atmosphere it is refracted and scattered, such that images
are degraded with blue light being affected more than red. To see this, first find a bright
star near the horizon and observe it through a high magnification eyepiece (∼9mm or so).
See the colors separate (refraction) and how it is blurry and wiggles (scattering). This is
why stars “twinkle” (find Sirius or Canopus for the most spectacular examples). Look at
the horizon star through blue and red filters attached to the high magnification eyepiece. If
the star is bright, it should be definitely larger and more blurry through the blue filter. Is
it? Record your observations.
Seeing is defined as the apparent diameter of a stellar disk as viewed through the telescope.
Because stars are so far away, they should appear as points of light without size. However, the
atmosphere distorts and spreads out the incoming light such that the stars have a diameter
called the “seeing disk” or simply “seeing”, which is measured in angular seconds of arc (′′ ).
The better the seeing, the smaller this value. Typical values in the Houston area might be
2-5 arcseconds but you may find the seeing to be better or worse than this depending on the
weather.
Quantitative estimates of actual seeing using the eye are very subjective. The best way to
determine seeing is to observe a double star of known separation (using their separation as
a “ruler” to compare the stellar diameters). Excellent examples that are up in the winter/spring skies are listed at the end of this handout. Observe with a range of eyepieces to
see which one works best at separating the close pairs. Can you see the individual stars?
What eyepiece magnification gives the best view? What would you estimate the “seeing”
(defined as the apparent stellar image size) to be? Try the red and blue filters again and see
if the separation is better seen through the red filter. Try observing two or three of the other
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double stars from your list. Calculate the seeing from each one of them. Do you notice a
degradation in seeing for stars lower in the sky? Why would you expect this to happen?
What to turn in with your report:
1.
2.
3.
4.
5.
The
The
The
The
The
3.7
derivation described above.
seeing estimate and how you measured it.
best choice of eyepieces for the seeing estimate.
effect of seeing as the objects set.
effect of color on seeing.
Lunar and Solar System Observing
You should observe the Moon with the C8 at your earliest opportunity. Look in the Peterson
Field Guide before beginning your lunar observations. The best time to observe details on
the Moon is near 1st and 3rd quarter, but do not wait too late in the semester to complete
this part of the lab.
Observe the Moon with both low power and high power eyepieces. It might help if you use
one of the eyepiece filters, especially the red filter, for improving the seeing and reducing the
surface brightness of the Moon. Sketch the features you see through the low power eyepiece
and at least one region through the high power eyepiece. Record any comments about the
features as you observe. Discuss their color, shading and sharpness. In your lab writeup you
should write a paragraph or two about your lunar observations based on these comments.
When you pick out features to observe on the Moon, you may be interested in looking for
those that have some geologic significance. For example, large faults called grabens often
occur along the edges of the Mare, and are caused when heavy lava in the Mare subsides and
the edges of the basin are pulled apart. The Moon also has many rilles, which are lava tubes
that may have once fed basaltic lunar basins. Several interesting craters also exist where the
impact occurred at a grazing angle and left a highly elliptical scar. Notice how some craters
sit on top of rays from other craters, indicating that the rayed crater impacted first. Mare
also have ‘wrinkle ridges’ that form as the lava subsides. You might try magnifying one of
the shadows from the lunar mountains to see what the shape of the mountain looks like.
Over the course of a few hours you can watch as the shadow gradually changes its length.
You are witnessing the sunrise on another world.
Planets usually do not twinkle like the stars do. Locate and observe one of the naked-eye
planets. Does it twinkle? Why do you think the planets do not twinkle? If you can not
figure this out, think about how a planet appears in a telescope compared to a star. Even
distant planets like Uranus and Neptune do not twinkle through a telescope ...why (are they
visible now?)?
Currently, among the planets, Jupiter is very bright and rises around 10pm (earlier as the
semester progresses). Mars is an early morning object, but will rise around midnight by
March. Saturn and Venus are visible now right before dawn. Mercury is always moving
around quickly from dawn to dusk skies. We will discuss why all these things happen in the
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lectures.
Of the four brightest asteroids (Ceres, Vesta, Pallas, and Juno), two are visible this semester
(which ones?). For a challenge, try to find it and watch it move over the course of a few
nights. Check out what Saturn and the others should look like in the Peterson Guide, then
sketch at least two of them in your lab book. You should be able to see phases on Venus,
and determine if the planet appears to get larger or change phase over the weeks. Look at
Jupiter and see if you can see cloud bands, and identify some of the bright satellites. How
prominent are the rings of Saturn? Do you see any of its moons? What about Uranus and
Neptune (either one visible)? If you draw a picture of how the solar system looks right now
as viewed from the north ecliptic pole, it will show why certain planets are visible in the
morning and others in the evening.
What to turn in with your report:
1. Sketch of your lunar observations, description of what you found using the different powers.
2. Identification of some of the features you observe.
3. Similar observations of the planets you found.
4. Answer the twinkle question.
5. Draw the solar system picture.
3.8
Deep Sky Observing
The deep sky observing can be done with the C8, but is far more exciting to do with the
computerized Meade 10-inch (especially at a dark site) or with the Campus Observatory
16-inch.
At the end of this writeup in Appendix I is a list of several “deep sky” objects (star clusters,
nebulae, and galaxies). Locate at least five of these on your own and note their appearance
including a sketch in your notebook. Observe them with a variety of eyepieces and note
which one seems to give the best view of each object. For nebulae, try using the nebula filter
to see if it really does give better contrast between the object and the sky background.
For a deep sky object to ‘count’ for the lab, you (with your partner) must be the ones who
find it. Objects found for you by the professor or lab assistant for your enjoyment don’t
count.
What to include in your report:
1. A paragraph or two on each Deep Sky object as to what it is, how far away, the size, age,
etc.
2. Sketch the object and compare its appearance with the Peterson’s picture.
3. Specifics of the observation.
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3.9
Exoplanets
Exoplanets are all the rage these days. Hundreds of stars have been found that exhibit
periodic radial velocity variations indicative of a planetary companion. While you will not
be able to see any of these planets, you can find their host stars and imagine the planetary
systems there.
Go to exoplanets.org and identify three stars with exoplanets that you can find with the C-8.
Here is where knowledge of star names comes in handy. You’ll want bright stars to make
your job easier! Find the star in the eyepiece using either setting circles or by star-hopping.
Describe how it is that you know that you have the correct star and not some imposter
nearby (or far away if you messed up the pointing somehow).
What to include in your report:
1. A brief description of the planetary system(s) around your star of choice.
2. Visual magnitude, spectral type and distance to the stars, date and time of observation,
eyepiece used, appearance of the star.
3. Some rationale as to why you think each of the three stars you found were the right ones.
4
Lab Report
Writing good project reports, just like publishing papers for real scientists, is a necessary
part of life in order to obtain proper recognition and credit for study or research done.
Therefore, don’t hurt yourself by turning in poorly-written reports. If you wish, talk with
your instructor or lab assistant about pointers on how to write a good report. Furthermore,
you are given specific suggestions below about the format and what to include.
Every experiment or research project is different, and so must be the report. Some are very
quantitative, others are qualitative and descriptive. Whatever the case may be, the best
general advice is to outline what you did and then describe the results that you obtained
(including failures), by writing a smoothly flowing narrative. The aim of this narrative should
be to provide a complete but concise description of what you did such that a new student
who reads it would get an accurate idea of what the lab is about. Do not short-change
yourself by doing all the work in the lab and then not spending the time and effort writing
up a good report. Remember also that while you should answer all questions and include
all relevant material in your writeup, the length of your report is not always an indication
of how well it is written.
A suggested report outline for this follows.
1. GOALS
– State in your own words what do you consider to be the purposes of this project. Be rather
specific regarding the various goals and experiments. A well-written paragraph suffices here.
2. EQUIPMENT
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– Make a table of the dates, times that you observed and where you were located, with some
notes about weather conditions each night (clouds, haze, humidity, etc.).
– Provide a list of the equipment used.
– Describe in detail the basic setup of the telescope(s) using a diagram or drawing if appropriate.
– Note any problems encountered with the equipment.
3. RESULTS & ANALYSIS
– Answer the questions posed above.
4. CONCLUSIONS
– Try to draw conclusions regarding your experiences in using the telescopes and performing
the various tasks. Specifically, what worked well and what did not?
– What did you learn and/or discover? Did you fail at anything and why?
5. REFERENCES
– Include a list of references at the end of your narrative of written material and/or “quotes”
from your helpful staff.
6. EVALUATION
– Projects like this one evolve and hopefully improve. Any suggestions on improving the
project and/or the writeup are solicited. These do not affect your grade, so feel free to be
frank in your evaluation of the project and suggestions.
Finally, we urge everyone to word-process and print out the basic text of their report, with
drawings, equations, etc. written in as appropriate. The original observational notes and
data can be photocopied and attached to the end of the report as an appendix.
APPENDIX I – Suggested Objects for January-February Observing in the Houston area
Below are suggested objects for the various parts of the observing laboratory appropriate for
January and February. Use The Sky to get further information on them (e.g., coordinates,
magnitudes, identifications, pictures). You can substitute any other bright stars, binaries,
etc., for those listed below.
Bright stars for calibrating setting circles:
– Aldebaran, Algol, Alpherat, Betelguese, Capella, Procyon, Regulus, and Sirius (pick two
widely separated in sky)
Double Stars for seeing determination:
θ Ori (in M42; a quadruple with all stars ∼ 6th mag, closest separation is 8.7”)
λ Ori (3.7, 5.7, 4.4”)
γ Leo (2.4, 3.6, 4.4”)
α Gem (1.9, 2.9, 3.9”)
ζ Ori (1.9, 5.5, 2.6”)
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η Ori (3.7, 4.8, 1.4”)
–
Beautifully colored double: γ And (2.3, 5.1, 10.0”)
Moon:
• First Quarter: Jan 16th, Feb 14th, Mar 15th, Apr 13th, May 13th Full: Jan 23rd, Feb
22nd, Mar 23rd, Apr 22nd Third Quarter: Jan 31st, Mar 1st, Mar 31st, Apr 29th New: Jan
9th, Feb 8th, Mar 8th, Apr 7th, May 6th.
Comets: These are fun to watch from night to night as they move among the stars. It is
much easier to spot comets from a dark site where the surface brightness of the sky is lower
so don’t be disappointed if you don’t find them right away under the city lights of Houston.
Planets:
• Venus: Bright in the morning sky now, but gradually makes her way into the twilight by
semester’s end.
• Saturn: Visible in the east in the pre-dawn hours. See the rings!
• Jupiter: Rising around 10pm now, and generally well-placed all semester.
• Mars: Currently a morning object, but slowing moving to the evening sky. Gets brighter
later in the semester as he gets closer.
• Uranus, Neptune: ...find out where using Stellarium!
• Mercury: – the “catch me if you can” planet! (why? where is it?)
Deep Sky Objects (clusters, nebulae, galaxies, asteroids):
• Our favorites are:
→ M42 (HII region, the Orion Nebula),
→ M44 (old open cluster; The Beehive),
→ Pleiades (“Seven Sisters”, a young open cluster)
→ h and χ Persei (Young double open cluster)
→ M35, 36, 37, & 38 (nice open clusters in Aur and Gem),
→ NGC 2392 (the “Eskimo” planetary nebula, faint, has visible central star)
→ M31 (The Andromeda Galaxy).
→ M82 (Starburst galaxy in UMa, a late evening object).
→ Many other excellent objects available ... explore with the Field Guide!
→ see if you can find an asteroid or comet
→ or for a real challenge... Pluto!
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