16
Understanding the R.A. and Dec. Clutch Knobs
We suggest that you read this before assembling the remainder of your system.
1. What do they do?
The four R.A. and four Dec. clutch knobs depicted in Diagram 2 have the function of connecting the R.A. and Dec.
axes to their respective drive worm wheel gears. Their function is progressive, from no tension (axes free to move
- as required during correct balancing of the telescope) to a completely "locked up" state.
2. How can you find out what they really do?
As shipped, all 1200 mounts have all four R.A. and Dec. clutch knobs firmly hand tightened. This will give you a
good idea of the maximum tightness (clutch action) that can be achieved by hand effort alone. At this point, you
must bear in mind that for optimum performance all four clutch knobs on each axis (R.A. or Dec.) should be
tightened evenly with the same tension i.e. all four half tight, all four fully tight, etc.
In order to feel the effect of the clutch knobs, you may wish to partially assemble your mount. Fit together the R.A.
and Dec. assemblies plus mounting plate and counterweight shaft. Do not put scope and counterweights on at
this stage. With the above assembly (with the clutch knobs firmly hand tightened - "as shipped"), you can feel the
amount of force needed to move each axis by hand. Grab each end of the telescope mounting plate and move it
with a backward and forward movement of the Dec. axis. You will feel considerable resistance to this motion.
Perform the same operation on the R.A. axis by moving the counterweight shaft backward and forward. With a
well-balanced telescope, the above tightness of the clutch knobs will be sufficient for all normal conditions of use.
Now, if you proceed to mount up and balance your telescope, you can "feel" what this resistance in R.A. and Dec.
(movement backwards and forwards) is like when you make these motions from the eyepiece end of your
telescope as you would during normal use when slewing (pushing) by hand to acquire an astronomical object
within the field of view of your finder or scope.
3. How tight can the clutch be and can you do any damage by pushing against them?
The maximum tightness of this clutch system is 1/3 turn (with a 5/32 allen key) further in than the tension you can
achieve with the knobs by hand. You will see that each clutch knob has a 5/32 hex socket for tightening with an
allen key. With this extra 1/3 turn on each clutch knob, the axis (axes) will be considered completely "locked up"
and you should not attempt to push your scope by hand against this "locked up" resistance, or undue stress will
be placed on the worm wheel/worm and bearings.
However, if you are undertaking a very long astrophoto exposure, it is advisable to increase the pressure on each
clutch knob (with the 5/32 key) by about 1/8 turn on Dec. and 1/8th turn on R.A. You may safely slew the scope by
hand with this tension, however you will notice considerably more effort is required to achieve movement. This is
the absolute maximum tension that can be used for hand slewing. As a general rule, if you have a big scope (7" or
8" refractor) with all the accessories, you will need more clutch tension than a 5" or 6" scope.
Resist the urge to overtighten the clutch knobs with the hex wrench. This will only cause them to deform and lock
into position. If you find that you are no longer able to adjust the tension and the knobs are locked firmly in place
so that the axis will not move, contact Astro-Physics for technical assistance.
Attach Counterweight Shaft and Counterweights
IMPORTANT:
•
Always attach the counterweights before mounting the telescope to the cradle plate to prevent sudden
movement of an unbalanced tube assembly, which may cause damage or injury.
•
Remember counterweights are heavy and will hurt if they fall on your foot.
1. Thread counterweight shaft onto the Dec. axis.
2. Remove the hand knob and washer from the end of the counterweight shaft. Add sufficient counterweights (10 or
18 lb. counterweights are available) to the shaft to balance the telescope you intend to use. Loosen the
counterweight knob and hold the counterweight with the knob pointing downward so that the brass pin will move
from the center opening allowing the counterweight to slide into position. Always use two hands to attach or move
them on the shaft.
3. Reattach the hand knob and washer to the end of the counterweight shaft. This will help to prevent injury if
someone accidentally loosens the counterweight knob.
A firm tightening of the counterweight knob will not damage the surface of the counterweight shaft. The pin that tightens
against the stainless counterweight shaft is constructed of brass. Likewise, the bronze sleeve that has been press fit into
the center of the counterweight will prevent marring of the shaft as you move the counterweights.
Summary of Contents for GTOCP3
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