Meade Instruments TMB-92 - Manual (Page 2)

adapter also uses a non-marring compression ring to hold your
1.25” star diagonals and accessories in place. The barrel of this
adapter is threaded to accept standard 2” filters and has a slight
inward taper at the top. The taper engages the compression ring
of the 2” adapter to prevent the 1.25” adapter from slipping out
of the focuser, should the 2” adapter’s thumbscrew acciden-
tally loosen during use.
Your Feather Touch focuser has dual-speed focusing. There
are two coarse focusing knobs. The right knob also has a smaller
concentric knob with a 10:1 ratio reduction gear for microfine
focusing. This provides exceptionally precise image control
during high power visual observing and critical film or CCD
imaging. The large focus knobs have ribbed gripping surfaces
so they are easy to operate, even while wearing gloves or mit-
tens in cold weather.
Your focuser drawtube has a 65mm (2.56”) travel. It has two
scales (one on either side) that are laser-engraved in 1mm in-
crements. These let you note individual focuser positions for
easy return to the correct focus when switching between visual
use and photography.
Because your Feather Touch focuser is so smooth in opera-
tion 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 not horizontal and has a significant
vertical component. The focuser therefore contains an adjust-
able-tension internal brake system to allow astrophotography
and visual use with heavy eyepieces without the possibility of
focus shift.
The brake consists of a lever arrangement within the focuser’s
pinion block. Adjusting the thumbscrew underneath the pinion
block causes the lever to press a friction pad onto the pinion.
This increases the torque needed to turn the focus knobs. If the
thumbscrew is turned all the way in (only 1 to 1½ turns), the
pinion shaft and drawtube are essentially locked into position.
Partial tightening of the internal brake offers a virtually infinite
range of braking force to accommodate various eyepiece and
accessory weights.
A separate large hand-tighten drawtube lock knob on the top
of the focuser body locks the drawtube securely in place at any
desired focus point.
A built-in camera angle adjuster (focuser rotation system)
lets you rotate the focuser a full 360° without losing focus.
This lets you rotate the focuser to line up a camera in either a
landscape or portrait orientation (or any orientation in between).
It also lets you put your star diagonal and eyepiece in the most
comfortable observing position as you move from one area of
the sky to another.
Three hand-tighten focuser rotation lock knobs around the
front of the focuser body let you adjust the focuser rotation
angle. Loosen the lock knobs by turning them counterclock-
wise to unlock the focuser. Rotate the focuser to the desired
new orientation. Turn all three lock knobs clockwise to lock
the focuser firmly in place at the new angle.
Your focuser drawtube glides on three stainless steel straps
that mate with Teflon bearing surfaces to provide very tight yet
extremely smooth motion. This design eliminates the need for
grease between the housing and drawtube. A greaseless draw-
tube prevents the collection of dirt that may eventually cause
problems between these surfaces, so no routine focuser main-
tenance is required except for an occasional wipe down of the
drawtube with a soft cloth.
TUBE RINGS
No tube mounting rings are supplied with your TMB refrac-
tor. However, precision-machined 114mm TMB hinged split
rings are available from your telescope dealer.
FINDERSCOPE MOUNTING
No finderscope is provided with your TMB-92. However,
there are two holes for a finder bracket drilled and tapped on
1.25” centers at the 2 o’clock position on the focuser body.
These holes will accept any of a number of finderscope mount-
ing rings. Contact your telescope dealer for an appropriate finder
if you do not already have one that is suitable.
COOL DOWN TIMES
For any optical system to give its best wavefront (for the sharp-
est 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 tem-
peratures, it may take a small refractor twice that time or more
to reach its best performance. This is particularly true with trip-
let refractors, such as your TMB-92, 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 equilib-
rium, 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 the 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 gener-
ally will still keep the lens from dewing up. Only on the highest
Features of the Focuser’s Right Side
Right
coarse
focus
knob
Fine
focus
knob
Focusing scale
on drawtube
(1 on each side)
Combined brake/drawtube
tension knob under focuser
Screws sealing the
mounting holes for an
optional finder bracket
Focuser rotation
lock knob (1 of 3)
Retracted
lens shade
2” accessory
lock knob
(1 of 3)
Drawtube
lock knob
Focuser
unthreads here to
install 2” visual
extension tube
center circles to form a single round circle. The illustration be-
low shows a close-up of one of the collimation screw pairs.
The taller screw is the actual collimation screw, while the
shorter of the pair is the lock screw that holds the proper colli-
mation after adjustment. Be sure to adjust only one set of colli-
mation screws at a time. Loosen the lock screws before attempt-
ing to turn the collimating screws. Make only small changes at
a time (turning the collimating screw only a fraction of a turn
and checking to see the results before turning further).
When you finish adjusting the collimating screws, be sure to
snug down all lock screws to hold the correct collimation. Do
not over-tighten the lock screws, but make sure they are firmly
in place. Once collimation is complete, your objective lens will
be on the same optical axis as the eyepiece, camera, or CCD,
and you will obtain the best possible optical performance.
The other method of collimation is to examine a first or sec-
ond magnitude star image at high power at night after the scope
has reached thermal equilibrium. This collimation method re-
quires an equatorially-mounted scope and a motor drive to keep
the star image centered while you adjust the collimation screws.
It is generally a more difficult and time-consuming collimation
method than using a Cheshire eyepiece, as you are working
with a small and droppable Allen wrench in the dark and are at
the mercy of changing seeing conditions.
Be sure not to choose a closely-spaced binary star as your
test star. Do not use a star diagonal when collimating, as you
may be attempting to correct a problem in the diagonal, rather
than in the telescope. Adjust the push-pull collimation screws
until the star images are tight and round, and the first and sec-
ond diffraction rings are uniform in their intensity all the way
around the Airy disk.
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 recom-
mended. However, no amount of preventative measures will
keep your objective from eventually collecting dust and air-
borne 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
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 scope case
outside when you observe, so it can also reach ambient tem-
perature. 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 tempera-
ture, 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.
COLLIMATION
Your telescope was carefully collimated prior to shipment to
your dealer. With normal care the optics will keep that align-
ment permanently. However, if the telescope takes a very strong
blow in shipping or while transporting it to your observing site,
there is the small possibility that you may have to recollimate
the lens. The lens cell has three pairs of push-pull collimating
screws that will allow you to collimate it using a metric Allen
wrench.
To gain access to the collimation screws, remove the black
trim ring/knife-edge baffle from the front of the dew shield by
unthreading it in a counter-clockwise direction. Set it aside and
push the dew shield back on the optical tube until the front of
the dew shield clears the lens cell and exposes the collimation
screws in the flange at the rear of the lens cell, as shown in the
illustration below.
We recommend either of two collimation methods. In the first,
a “Cheshire” eyepiece (available from Tectron or your telescope
dealer) is inserted directly into the eyepiece holder (without
using a star diagonal). The Cheshire method can be used in-
doors or in the field, day or night.
The dust cover is placed over the lens cell to seal the objec-
tive end of the scope and keep light from entering the optical
tube. A bright light source is aimed at the opening in the side of
the Cheshire eyepiece. You can use the light from the sky out-
doors during the day. At night or indoors, aim the light of a
flashlight directly at the opening in the side of the Cheshire.
Look through the hole in the end of the Cheshire. You will
see a series of faint circles of reflected light against a black
background. If the circles of light are all concentric, your scope
is in collimation. If the circles of light are not concentric, ad-
just the push-pull collimation screws until you line up the off-
The collimation screws
(one pair of three)
The collimation screw
(taller)
The lock screw
(shorter)
The lens cell
Lens
Dew shield
fully
retracted
Collimation screws
(1 pair of 3)
Lens cell
Trim ring /knife
edge baffle
removed from
dew shield
How to access the
collimation screws