Features of the Focuser’s Left Side
Signature
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Signature Series
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TMB Optical
TMB-92
Thomas M. Back
Congratulations on your purchase of this TMB Optical TMB-
92 Signature Series air-spaced apochromatic triplet refractor.
This exceptional 92mm f/5.5 scope uses an element of costly
FPL-53 ED (Extra-low Dispersion) glass to achieve true apo-
chromatic visual and photographic performance, free from chro-
matic aberration and spurious color.
At only 13.25” in length, the TMB-92 is a premium quality
ultra-compact refractor that we believe elegantly and more than
capably fills the void left by the long-discontinued 90mm Astro-
Physics Stowaway (a legendary refractor that sells for up to
$6000 on the used market).
The fully-collimatable TMB-92’s truly outstanding optical
and mechanical quality will provide you with many years of
highly portable visual observing and imaging enjoyment.
PHOTO/VISUAL CONFIGURATIONS
The TMB-92 consists of an optical tube assembly with lens
and dew shield, a Feather Touch focuser that slips into the op-
tical tube and is locked firmly in place by three brass knobs that
allow 360° rotation of the focuser, and
a 2.25” extension tube that can be in-
stalled between the optical tube and the
focuser for visual use.
When the optical tube and focuser
are assembled without the extension
tube, the resulting physically short tele-
scope configuration provides the long
back focus needed by those primarily
interested in serious CCD imaging us-
ing long equipment trains: camera, filter wheel, adaptive optics
system, etc. This arrangement also allows binoviewer use with-
out the need for a Barlow or optical compensator lens.
For occasional visual work, a simple optional compression
ring extension tube (from Astro-Tech, TeleVue, and others) can
be slipped into the scope’s 2” accessory holder to provide the
necessary extra physical length to reach focus with a star di-
agonal and eyepiece.
For those primarily interested in visual observing and only
casual imaging with a 35mm camera, webcam, or Deep Sky
Imager-type CCD camera, a 2.25" extension tube (shown above)
is supplied to provide the proper back focus for comfortable
visual use and casual astrophotography. This extension installs
between the optical tube and focuser. A groove in the extension
tube engages the large brass lock knobs in the optical tube (that
normally control focuser rotation) to lock the extension tube
firmly to the optical tube. Three nylon-tipped metal lock knobs
at the opposite end
of the extension tube
now lock the focuser
in place and allow
focuser rotation, just
as the large brass
lock knobs in the op-
tical tube do when
the extension tube is
not used. A Teflon
insert in the focuser
end of the extension
tube assures smooth
focuser rotation.
The supplied carrying case is precut to hold the TMB-92 in
either short body imaging or long body visual configurations.
FOCUSER
Your TMB-92 features an ultra-precision Feather Touch
Model 3025 dual-speed rack-and-pinion focuser from Starlight
Instruments. This exceptionally smooth focuser has a 3” diam-
eter drawtube for non-vignetted astrophotography with medium
format film cameras and very large chip CCD cameras. The
drawtube ends in a 2” accessory adapter with a non-marring
compression ring eyepiece/accessory holder that will not dam-
age the barrels of your 2” accessories. Three hand-tighten lock
knobs on the adapter assure the compression ring’s solid grip
on heavy accessory loads. This adapter allows visual use with
2” star diagonals, plus imaging with 35mm film cameras and
2” barrel large format CCD cameras.
The 2” accessory adapter is secured to the focuser drawtube
by three recessed hex-head screws. Undoing these allows you
to remove the supplied 2” adapter and replace it with a custom-
made Feather Touch adapter for medium format (Pentax 6 x 7)
film photography using A-P photographic accessories. Please
contact Starlight Instruments directly at (847) 985-9595 for any
specialized adapters you may need in this regard.
The supplied 2” to 1.25” accessory adapter is designed for
visual use with 1.25” star diagonals, plus imaging with stan-
dard 1.25” format CCD, webcam, and 35mm adapters. This
Left coarse
focus knob
Focuser rotation/extension tube
lock knob (1 of 3)
2”
accessory
adapter
1.25” accessory adapter
Compression
ring
Retracted
lens shade
Compression
ring
1.25”
accessory
lock knob
2” accessory
lock knob
(1 of 3)
2.25” visual extension
tube installs here
Hex head
screw
attaching 2”
adapter to
drawtube
(2 of 3)
Supplied
Extension Tube
Extension Tube Installed
Extension
tube
Original focuser
rotation lock knob
(1 of 3)
New
focuser
rotation
lock knob
(1 of 3)
TMB Optical, Cleveland, OH 44131 E-mail: TMBoptical@aol.com
© 2008 by TMB Optical
camel’s hair brush with attached blower bulb, for example).
The use of canned or compressed air should be avoided, if pos-
sible, as the propellant in the can may spit out and leave diffi-
cult-to-remove deposits on your lens. Also, the expanding com-
pressed 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 stub-
born 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 de-
signed 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. Ex-
change the cotton ball and/or turn the cloth frequently so you
always have a clean portion of the cotton ball or cloth in con-
tact 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 condi-
tion. 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 be-
fore you start pushing a damp tissue around the lens surface.
Clean your optics only when absolutely necessary. If you take
proper care of your TMB-92, cleaning should rarely be needed.
THE STAR TEST
We strive to make the best apochromatic optical systems avail-
able. 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 continually changing as the see-
ing conditions in our living atmosphere vary from moment to
moment.
No optic is perfect, and every brand and model scope will
show some error in an 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 no better than 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 wave-
lengths change the star test pattern from technical perfection.
However, TMB objectives are corrected at the peak visual
wavelength centered around 560nm in the green-yellow por-
tion of the visual spectrum. The eye sees over 80% of the vi-
sual detail at this wavelength. It is the correction at this visual
peak that makes the difference between a merely good objec-
tive and a superb one. Our lenses are figured for the best pos-
sible wavefront at green-yellow wavelengths, for the sharpest
images and highest contrast.
While star tests are interesting and useful, they should not be
the only criterion for judging any telescope’s quality. Most ob-
servers spend their nights enjoying the views of extended and
detailed objects – like nebulae, galaxies, star clusters, and planets
– not simply examining a star test’s single out-of-focus point
of light. We feel that the true proof of optical excellence will be
found in your nightly observing, not just in star testing.
A FINAL WORD
Thank you for your purchase of our TMB-92. We believe
that this ultra-compact apochromatic refractor will outperform
any other telescope type, inch for inch, and is the most trouble-
and maintenance-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 life-
time. Use it to enjoy the wonders of the night sky!
BRIEF SPECIFICATIONS
Aperture ............................................................. 92mm (3.6”)
Focal Length ............................................................... 506mm
Focal Ratio ...................................................................... f/5.5
Objective Type ...................... air-spaced apochromatic triplet
with an FPL-53 ED glass element
Optical coatings .......................................... fully multicoated
Resolving Power (Dawes’ Limit) ................. 1.26 arc seconds
Visual Limiting Magnitude ............................................. 12.3
Light Grasp Versus the Eye ........................................... 173x
Focuser ............... Starlight Instruments Feather Touch #3025
3” dual speed rack and pinion, with 10:1 ratio fine focus;
2” and 1.25” compression ring eyepiece holders;
and 360° rotating camera angle adjuster
Focuser Travel .................................................. 2.56” (65mm)
Tube Diameter .......................................... 114mm (4.5”) o. d.
Minimum tube Length ................................ 13.25” (337mm)
(set up for imaging, lens shade and focuser retracted)
Tube Length ...................................................... 16” (406mm)
(set up for imaging, lens shade extended, focuser retracted)
Maximum Tube Length ............................... 20.75” (522mm)
(set up for visual use, lens shade and focuser extended)
Optical Tube Weight ................................... 8.5 lbs. (3.86 kg)
Case Dimensions ................................... 22.25” x 12.75” x 8”
