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CEM25P - not overloaded but won't rotate clockwise

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#1 sonshadowcat


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Posted 19 June 2019 - 07:53 PM

I recently purchased a 6" newtonian that weighs 10lb. The weight of the 2 cameras and guidescope is roughly another 4lb. This should be well within the weight limit of the CEM25P.


However, it will not rotate clockwise in RA (moves just fine counter and in DEC). It gives a horrid grinding noise and will only move if I softly help push it along. I've been balancing my mount for a while now with other scopes so I'm fairly sure it's balanced as it should be in RA. But I cannot for the life of my figure this out which is bothering me since I've got clear nights now.


Does anyone have sage advice that I'm too blind to see?

#2 Gipht


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Posted 19 June 2019 - 08:18 PM

My CEM-60 will not move if the load is imbalanced by more then a few lbs.  If  your telescope is balanced properly then that should not be the cause.  With such a light load, a balance problem seems unlikely.  With my heavier telescopes, I do try and balance with the telescope pointed in the vicinity of where the nights target will be.


I did have a problem with one of the allen screws which held the counterweight bar to  the RA axis loosened up, and that caused binding and stopped RA motion until I found and tightened the screw.  You may want to go through and make sure everything is tight.

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#3 scadvice



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Posted 19 June 2019 - 10:23 PM

Is it a new mount? I had about 16 pounds on my cem25P without problems. As Gipht said it's either your balance or something loose or binding.


I had a counterweight bar loosen up on me also and it made not only balancing a problem but caused imaging issues.Took awhile to find it as it was so very slight in movement.

#4 santafe retiree

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Posted 19 June 2019 - 11:15 PM

From the CEM25 Quick Start Guide:


10. Setting Tension Adjuster Position: Set both Gear
Switches to LOCK positions after balancing the
mount. Fully screw in the Tension Adjuster. Turn the
mount power on. Press #9 button on hand controller
to change the slew speed to MAX. Press the arrow
button to check the gear meshing. If the mount
motor has “grinding” sound (which is not harmful)
while slewing, the Tension Adjuster is too tight.
Release 1/8 to 1/4 turn and check it again. If there is
excess play in either RA or DEC axis, or even the gear
is skipping, the gear and worm is not meshed
properly. Turn the Tension Adjuster more clockwise.
You may need to readjust the Tension Adjuster for
different payload.


Good luck!



#5 OldManSky



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Posted 20 June 2019 - 08:10 AM

I'm carrying about 17lbs on mine currently, and it slings it around with no problems.

If it won't move one direction but does move the other way, that really does indicate a balance problem (and as mentioned above, possibly a tension issue as well).


Especially with a Newtonian, the RA balance can change as you move the scope in DEC -- because the non-symmetrical orientation of the scope's focuser, finder, etc. change as you move in DEC.  

When you balance your RA, try changing the DEC position of the scope and see if you wind up with a fairly big RA mis-balance situation.

The solution is to orient the focuser position on the scope differently (rotate the tube in the rings, if possible), to add some counterweights on the tube opposite the focuser, or other "creative" ways to try and keep the weight symmetrical as you move the scope around.

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