TMB Optical
Thomas M. Back Signature Series
Congratulations on your purchase of our 80mm f/6.3 TMB-80
apochromatic FPL-53 ED triplet refractor. Its truly outstanding
optical and mechanical quality will provide you with many years
of highly portable observing and imaging enjoyment.
Your new TMB-80 is usable with many different types of
telescope mounts. A suitably sturdy altazimuth mount, such as the
Vixen Porta mount, is a good choice for general visual use. The
venerable German equatorial mount is also a popular choice, as it
can be used for both visual and imaging purposes.
The TMB-80 has a removable L-shaped combined tripod adapter
and dovetail mounting shoe. The dovetail is sized and shaped (with
slanted sides) to fit the dovetail slot on the head of many equatorial
and altazimuth mounts. It will mount without modification directly
onto Celestron Advanced Series; Meade LXD-75; and Vixen Great
Polaris, Porta, and Sphinx mounts, among others.
In addition, the mounting shoe has two 1/4”-20 thread mounting
holes that allow it to be installed on any suitably sturdy camera
tripod that has a standard 1/4”-20 thread mounting bolt.
The front hole balances the scope for 1.25” diagonal and eyepiece
use with the lens shade extended. The rear hole balances it when a
camera or 2” diagonal and eyepieces are mounted. The mounting
shoe can also be unbolted, rotated 180° on the scope body, and
reinstalled. This will move the balance point further back on the
scope body to help balance the TMB-80 if very heavy 2” accessories
are used. Cork pads on the underside of the mounting shoe help
keep the scope from swiveling when mounted on a tripod.
Your TMB-80’s backlash-free 2” dual-speed Crayford focuser
terminates in a 2” compression ring eyepiece holder. This allows
visual use with 2” accessories and imaging with large format CCD
cameras. The non-marring soft brass compression ring won’t scratch
your star diagonal barrel as an ordinary thumbscrew can.
There is also a 2” to 1.25” compression ring adapter for visual
use with 1.25” accessories and for imaging with standard format
CCD, webcam, and 35mm photo adapters. The barrel of this adapter
is threaded to accept standard 2” filters and has a slight taper at the
top that engages the compression ring of the 2” adapter. This
prevents the 1.25” adapter from slipping out of the focuser should
the 2” focuser’s thumbscrew accidentally loosen during use.
The precision-made focuser has dual-speed focusing. There are
two coarse focusing knobs. The right knob also has a smaller
concentric knob with a 11:1 ratio reduction gear for microfine
focusing. This provides exceptionally precise image control during
high power visual observing or critical film or CCD imaging. The
focus knobs have ribbed gripping surfaces so they are easy to
operate, even while wearing gloves or mittens in cold weather.
The focuser drawtube has a long 80mm (3.1”) travel. It has a
scale marked on the top in 1mm increments so you can note
individual focuser positions for easy return to the correct focus
when switching between visual use and photography. A knob under
the focuser lets you lock in your photographic focus.
A built-in camera angle adjuster (focuser rotation system) lets
you rotate the focuser a full 360°. This lets you rotate an attached
camera to line up in either a landscape or portrait orientation (or
any orientation in between) without losing focus. It also lets you
put your star diagonal and eyepiece into the most comfortable
observing position as you move from one area of the sky to another.
Turn the red ribbed ring at the front of the focuser to the left to
loosen the focuser, rotate the focuser to the desired orientation,
then turn the red ring back to the right to temporarily lock in the
focuser’s new orientation.
Camera angle
locking collar
Focusing scale
on drawtube
lock knob
lock knob
Soft brass
compression ring
1.25” accessory
2” accessory
adapter lock knob
Your TMB-80 is supplied with an attachment point for a quickrelease finder bracket on the upper left front side of the focuser. It
has a screwdriver-slotted insert that can be unscrewed to provide a
mounting point for any of a number of red dot type non-magnifying
finders. Contact your telescope dealer for an appropriate finder if
you do not already have one that is suitable.
For any optical system to give its best wavefront (for the sharpest
and highest contrast images), the optics must be at or very near the
temperature of the surrounding air. The “cool down” time needed
to reach ambient temperature varies considerably, as the temperature
of the scope must change from a typical 72° Fahrenheit indoor
temperature to an outdoor temperature that can range from a high
of over 100° down to 10° below zero or less. In small doublet
refractors, the cool down (or heat up) time is quick, usually less
than 30 minutes. In subfreezing temperatures, it may take a small
refractor twice that time or more to reach its best performance.
This is particularly true with triplet refractors, where the thermal
load of the center lens is isolated from the surrounding air by the
lenses on either side of it. This slows the transfer of the center lens
heat load to the outside air.
If you’d like to shorten the wait to reach thermal equilibrium,
placing the telescope in an unheated garage for an hour or two
before observing can speed up the cool down process considerably.
Another technique to shorten cool down time is to retract the
insulating dew shield to allow direct exposure of the cell and lens
to the night air so they can reach thermal equilibrium faster. Once
the lens has cooled, extend the dew shield again. This provides a
faster cool down time, and generally will still keep the lens from
dewing up. Only on the highest dew point nights will the objective
lens form dew on its front optical surface.
The best way to avoid dew forming on the lens after you bring
the telescope into the house is to take your closed scope case outside
when you observe, so it can also reach ambient temperature. When
you are finished observing, cap the telescope with its dust caps
and place it into the carrying case. Bring it into the house and let it
slowly warm back up to room temperature, then remove the dust
caps to allow any trace of dew to evaporate. Once the objective is
free from dew, replace the dust caps and store the scope away.
The best policy is not to let the lens get dirty and/or dusty in the
first place. The regular use of the dust caps is highly recommended.
However, no amount of preventative measures will keep your
objective from eventually collecting dust and airborne pollutants
on the first optical surface. We recommend that you do not clean
the objective too often, no matter how frequently the urge to do so
may hit you. A few specks of dust on the lens will not be visible in
your images, as they are not in the focal plane and don’t block
enough light to measure, let alone be seen.
Depending on how often you use your scope, and the amount of
pollutants in your air, you may have to clean your scope as often as
twice a year, but generally no more than that – and preferably no
more than once a year. If the front lens surface becomes dusty,
smeared, or shows fingerprints or any other surface build-up, and
you find it absolutely necessary to clean the lens, use the following
cleaning technique.
First, gently blow away any surface dust or particles with a clean
air blower (a child’s ear syringe or a photographer’s camel’s hair
brush with attached blower bulb, for example). The use of canned
or compressed air should be avoided, if possible, as the propellant
in the can may spit out and leave difficult-to-remove deposits on
your lens. Also, the expanding compressed air drops in temperature
as it leaves the can. The cold air coming out of the tiny tube that
most compressed air cans use to direct the air flow has been known
to chill a lens to the point of spalling chips off the lens if pointed at
the same spot on the glass for too long.
If you want, or need, to use compressed air to remove stubborn
particles, use a high quality compressed air duster (of the R-134
propellant type). ChemTronics sells a high-quality unit. Do not tip
or shake the can. Blow any loose particles off the lens surface using
short blasts at an angle to the glass, without getting too close to the
lens surface or aiming directly at it.
Next, moisten a ball of USP grade pure cotton with a few drops
of a photographic-quality optical cleaning solution designed for
multicoated camera and binocular lenses. You can use Formula
MC (available from many telescope dealers) or your own mixture
of distilled water and a drop or two of mild soap. A well-worn
100% cotton handkerchief also works well and Zeiss and Kodak
both make suitable cleaning fluids. Blot the entire surface with the
dampened cotton ball or cloth to pick up any stubborn particles
and to clean the surface. Exchange the cotton ball and/or turn the
cloth frequently so you always have a clean portion of the cotton
ball or cloth in contact with the lens.
Use a very small amount of liquid – not so much that the fluid
could be wicked between the lenses by capillary action. Do not
drip the cleaning fluid directly on the lens. Do not, at any stage,
apply hard pressure. Using a fresh piece of cotton or a lint-free
white facial tissue, carefully clean the surface of the lens by wiping
across in a radial direction. Repeat the process with denatured
alcohol, using a blower brush to clean off any dust that may fall on
the lens as you are cleaning it.
If you want to take the ultimate step in cleaning, a final rinse
with high-grade acetone will clean the surface to new condition.
You may notice a few faint streaks from the dried solvent. They
will not affect performance, but they can be removed with light
pressure and a Q-Tip slightly moistened with a small amount of
alcohol or acetone. Finally, a clean air blower bulb can be used to
remove any remaining dust.
Avoid overcleaning your scope. The multicoatings on the lens
are quite hard and durable. However, frequent overzealous cleaning
can scratch the coatings if all the dust particles (which are often
tiny flecks of windborne rock) are not removed before you start
pushing a damp tissue around the lens surface. Clean your optics
only when absolutely necessary. If you take proper care of your
scope, cleaning should rarely be needed.
We strive to make the best apochromatic optical systems
available. You may find your lens may not test “perfect” during an
extended and stringently-graded “star test,” particularly if the scope
has not fully reached thermal equilibrium before the test. This is
not an indication of a poor optic, but is rather due to the test star’s
complex wavefront of light being changed constantly by our living
atmosphere as seeing conditions vary from moment to moment.
No optic is perfect, and each will show some error under a detailed
and extended star test. The sensitivity of the star test under perfect
conditions is 1/20th wave P-V on the wavefront for third order
aberrations, and 1/60th wave for sharp (fifth order) aberrations. It
is highly unlikely that even the most ardent observer can see errors
of this small a magnitude on an extended object, even under very
good seeing conditions (which is when the atmosphere typically
presents a 1/4th wave P-V wavefront to the instrument).
The refractor also presents another factor: the change in spherical
aberration with a change in wavelength. This “sphero-chromatism”
is found in all refractors. As a lens is tested in the longer (red)
wavelengths, the lens becomes “under-corrected.” Tested in the
shorter wavelengths (blue), the lens becomes “over-corrected.”
These overlapping corrections at different wavelengths change the
star test pattern from perfection.
However, TMB objectives are corrected at the peak visual wavelength centered around 560nm in the green-yellow portion of the
visual spectrum. The eye sees over 80% of the visual detail at this
wavelength. It is the correction at this visual peak that makes the
difference between a merely good objective and a superb one. Our
lenses are figured for the best possible wavefront at green-yellow
wavelengths, for the sharpest images and highest contrast.
While star tests are interesting and useful, most observers spend
their nights enjoying extended and detailed objects, not just
examining sharply focused points of light. We feel that the proof
of optical excellence is in the observing, not just in the testing.
Thank you for your purchase of our TMB-80. We believe this
apochromatic refractor will outperform any other telescope type,
inch for inch, and is the most trouble-free telescope that you can
buy. Its versatility for visual, photographic, and CCD work is
unmatched. With a little care, this fine apochromatic refractor will
last you a lifetime. Use it to enjoy the wonders of the night sky!
Tom Back
Aperture ................................................................... 80mm (3.1”)
Focal Length ..................................................................... 504mm
Focal Ratio ............................................................................. f/6.3
Objective Type ........... triplet apochromatic, FPL-53 ED element
Optical coatings ................................................. fully multicoated
Resolving Power (Dawes’ Limit) ...................... 1.45 arc seconds
Visual Limiting Magnitude .................................................. 12.0
Light Grasp Versus the Eye ................................................ 131x
Focuser .............. dual-speed Crayford with 11:1 ratio fine focus;
2” and 1.25” compression ring eyepiece holders;
and 360° rotating camera angle adjuster
Focuser Travel ......................................................... 3.1” (80mm)
Tube Diameter ................................................. 90mm (3.5”) o. d.
Tube Length (lens shade retracted) ................... 15.25” (387mm)
Tube Length (lens shade extended) ................... 17.75” (450mm)
Optical Tube Weight ........................................ 6.5 lbs. (2.95 kg)
Case Dimensions ........................................... 22.5” x 12.75” x 8”
TMB Optical, Cleveland, OH 44131
© 2007 by TMB Optical