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TMB Optical
TMB-80CF
Thomas M. Back
Congratulations on your purchase of the TMB-80CF carbon fi-
ber body FPL-53 ED apochromatic triplet refractor. Its exceptional
optical and mechanical quality will provide you with many years
of highly portable observing and imaging enjoyment.
MOUNTING POSSIBILITIES
Your TMB-80CF is usable with many different telescope mounts.
It comes with two 90mm felt-lined hinged split mounting rings
that have five 1/4”-20 thread holes drilled and tapped into the top
and bottom of each ring. You can use these to install the scope
directly on many German equatorial mounts, or on a telescope
mount dovetail plate such as those from Losmandy or Vixen. They
also let you mount an accessory plate on top of your scope.
A quality altazimuth mount, such as the Astro-Tech Voyager or
Vixen Porta mount, would also be a good choice for grab-and-go
visual use. Either mount can be used with the TMB-80CF simply
by adding a Vixen-style dovetail plate to the tube rings.
FOCUSER
Your focuser is a backlash-free dual-speed 2” Feather Touch
Crayford from Starlight Instruments. Its drawtube terminates in a
2” compression ring accessory holder to allow visual use with 2”
accessories and imaging with large format CCD cameras. The non-
marring soft brass compression ring won’t scratch your 2” 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. Its barrel is threaded to
accept standard 2” filters and has a slight taper at the top that en-
gages the compression ring of the 2” adapter. This prevents the
1.25” adapter from slipping out of the focuser should the two 2”
accessory holder thumbscrews accidentally loosen during use.
Your focuser has two coarse focusing knobs. The right knob has
a smaller concentric knob with a 10:1 reduction gear microfine
focusing ratio. This provides precise focusing during high magni-
fication visual observing and critical 35mm or CCD imaging. The
focus knobs have ribbed gripping surfaces so they are easy to op-
erate, even while wearing gloves or mittens in cold weather.
Because the focuser is so smooth in operation and moves so
freely, it can only hold a limited amount of weight (generally about
1 lb.) without drifting out of focus when the focuser drawtube is
tilted vertically. The focuser therefore contains an adjustable ten-
sion internal brake system to allow astrophotography (and visual
use with heavy eyepieces) without the possibility of focus shift.
A knob under the focuser lets you adjust the tension on the draw-
tube to accommodate differing equipment payloads. If the thumb-
screw is turned all the way in (only 1 to 1½ turns), the drawtube is
locked in position for long exposure astrophotography. Partial tight-
ening of the knob offers a virtually infinite range of braking force
to accommodate various eyepiece weights for visual use.
FINDERSCOPE MOUNTING
Your scope’s carbon fiber body does not have any attachment
points for a finderscope bracket. However, the 1/4”-20 holes in the
top of the tube mounting rings offer several possible mounting
locations for an optional finderscope. However, it is worth noting
that the scope’s short focal length, combined with the wide fields
of many eyepieces, makes the TMB-80CF its own best finderscope.
For example, a 2” 40mm TMB Paragon eyepiece on the TMB-
80CF yields a wide 5.4° field at 12.6x, thereby converting the scope
into the optically finest 80mm superfinder ever made.
COOL DOWN TIMES
For any optical system to perform at its best, the optics must be
at or very near the air temperature. The “cool down” time needed
to reach this ambient temperature varies considerably, as the scope
temperature must change from a typical indoor temperature of 72°
Fahrenheit to an outdoor temperature that can range from a high of
over 100° down to 10° below zero or less. In 80mm refractors, the
cool down (or heat up) time in most situations is often quick, usu-
ally less than 30 minutes. In subfreezing temperatures, though, it
may take an 80mm refractor twice that time or more to reach peak
performance. This is particularly true with a triplet like the TMB-
80CF, where the thermal load of the center lens is isolated from
the open air by the lenses on either side of it. This slows the trans-
fer 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 shorten the cool down process considerably.
Another technique is to retract the dew shield to allow direct expo-
sure of the lens to the night air so it 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 dew shield is oversized, with a 124mm o.d. compared to the
80mm lens diameter and 90mm tube diameter. This assures that
tube currents, which typically follow the walls of telescope tubes
and dewshields, will be out of the 80mm light path for the most
part and will have only a minimal effect on your images as your
scope cools down to ambient temperature. You can begin serious
observing sooner with your TMB-80CF than you can with an alu-
minum tube 80mm triplet with a conventional dew shield design.
A lock knob built into the dew shield lets you lock the dew shield
firmly in place so it can’t slip down while aiming at the zenith.
The best way to avoid dew forming on the lens after you bring
the scope 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.
CLEANING
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 ob-
Coarse
focus
knob
1.25” accessory
adapter
Fine
focus
knob
Coarse
focus
knob
1.25”
accessory
lock knob
2” accessory
lock knob
(1 of 2)
2” accessory
lock knob
(2 of 2)
Drawtube
lock and brake
adjustment knob
under focuser
Focuser Features
2” accessory
holder
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