Monday, May 15, 2017

Adjusting DEC Tension (tightness/looseness) on a Celestron OMNI XLT CG-4 German Equatorial Mount (GEM)

The mount I received from Celestron was overly tight in both DEC (declination) and RA (right ascension).

The problem with a tight mount is that you cannot properly balance the OTA (optical telescope assembly aka the telescope) and the result is that it places too much torque on the RA motor (for example) and will make it harder for the motor to track properly the motion of the stars across the night sky.

Finding precious little into on the web, I decided to put together a step-by-step instructional blog post to help my fellow astronomers who are having either a tightness problem or a looseness problem.

WARNING: You will likely VOID your warranty with Celestron if you attempt this fix. Just saying...
As for me, there is precious little to worry about. But proceed at your own risk.

*****First of all, remove the telescope from the mount so that you will not have it swinging around and potentially damaging both it and the mount needlessly.

The following instructions are for loosening or tightening the DEC portion of the mount.
(RA adjustments can be found HERE)

1. Tighten both the DEC knob and the RA knob and REMOVE all the counterweights but one (preferably leaving the lightest one on the long rod)
         WARNING: if you have the Celestron motor kit already attached to the mount
                      be sure to disconnect the DEC gears from the motor (see your instruction manual)
                      If you do not do this you will likely ruin your DEC motor while making this
                      adjustment. It may be possible to loosen the slip clutch but better careful than sorry.

2. Remove the black plastic cover on the mount (see pics below)

3. Locate the nut in the center of the portion of the mount that holds the OTA (telescope)
You will be loosening/tightening the DEC axis with this nut with a 5/8" socket wrench

4. Locate the nut inside the DEC housing...
where you will use another 5/8" wrench to HOLD this nut while you loosen/tighten the other nut on top with the socket wrench.
Note: if you cannot get the 5/8" wrench inside the housing, then remove the counterweight
still attached and loosen or remove the counterweight bar until you can get the wrench inside
to hold that nut.

6. Loosen the DEC axis lock knob. Test the present tension by swinging the counterweight carefully to see if it is too tight or too loose.

7. Looking down on the top nut 
....* turning it clock-wise will tighten the DEC axis
....* turning it counter-clockwise will loosen the DEC axis

WARNING: It will not take much to affect the tension on the DEC axis! You do not want it to be too loose!

8. When you are satisfied, replace the black cap, tighten the DEC axis lock knob, add the counterweight(s) you believe will help balance the scope in the DEC.

9. Add your OTA.

10. CAREFULLY loosen the DEC locking knob slowly and adjust the position of the counterweight until the telescope is balanced (see your instruction manual).

IF you need to adjust the tension on the RA axis see this blog post.

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