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CEM40 - Excessive RA Play - Worm Adjustment Attempt Made It Worse

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#1 rlillard

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Posted 24 June 2022 - 01:44 AM

I have very little time on this mount.  It is approx 6 mo.  with about than 1 hour of operational time.  I purchased it new. 

 

Last night I was setting up to capture some photons for reals and noticed  play in RA.  DEC is fine.  I have no means of measuring the play but I can say it is visible and I can feel it when I lay my finger over the surface.

 

I decided to use the worm adjustment procedure in Appendix D of the manual. Things have gone down hill from there.

I am no expert with mounts but have decades of experience with similar precision mechanical equipment and believe I am competent to make such adjustments, especially after RTFM.

 

I am using the term "set" screw to mean the 2mm  screw.  The "lock" screw is the 3mm screw.

 

1:  I found both set screw and lock screw tight.

 

2:  I loosened the set screw and it felt normal.

 

3:  I began tightening the lock screw by turning it CW in small increments.  The lock screw felt moderately tight and yet movable with intention.  During the adjustment increments I could feel no change in play.

 

4:  After about 1/2/ turn with no discernible  improvement I reversed direction.

 

5:  Continuing similarly  in the CCW direction I encountered stiff resistance to the point where it seemed inadvisable to continue.

 

6:  I reversed the lock screw back to its approximate original position and tightened the set screw.

 

 

Seeing no improvement  I tried to GO TO ZERO and the mount didn't move.  I the tried a SEARCH ZERO POSN.  The mount now make a high speed spinning noise.  Having done enough damage for one night, I shut every thing down and covered up.

 

I have two questions:

 

* Why did my adjustments have no effect on the worm adjustment? 

 

*  If the worm mesh adjustment  is not the source of the RA play, then what is?

 

 

 

 



#2 Seaquel47

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Posted 24 June 2022 - 11:44 AM

I have also have a CEM40 that I bought used.  It's a good little mount but I have had to make some adjustments occasionally to fix problems.  There have been three different problems that have caused me RA problems.  1) The worm adjustment, 2) a loose counterweight shaft, and 3) screws coming loose allowing the DEC assembly flop around.

 

I actually just got mine back from iOptron.  I had sent it there to eliminate a tiny wiggle in the RA.  They were very responsive and did a quick turnaround but unfortunately while they made some adjustments and replacements they didn't help the wiggle.  

 

My analysis is that the spring plunger that pushes the worm assembly down onto the worm or ring gear does not have enough spring tension to fully engage the worm to ring gear.  This is what the RA lock lever (iOptron calls it a gear switch) does.  I've included a picture of my gear switch.  You can see the spring plunger in the knob.  The second picture shows the spring plunger by itself.  The loose spring and ball you see in the cap fit into a hole in the lever and they serve to provide a positive stop when you lock and unlock the RA.  Check out this link from iOptron for more details - https://www.ioptron....ear_Switch.pdf 

 

The black circle you see on the assembly is the 2mm locking screw that holds the spring plunger so it can't move after adjustment.  The 3mm wrench goes into the hex socket in the end of the spring plunger.  What I found is when I tightened the plunger enough to stop the wiggle, I actually had totally compressed the ball into housing and the housing itself was pushing on the worm assembly.  When left in that position, you cannot unlock the RA.  iOptron is sending me a new spring plunger to see if that solves the problem.

 

I would suggest that you turn the spring plunger down until you can't unlock the RA (note you have to pull the hex wrench out) and then incrementally back off of the tension until you can unlock the RA.  Then tighten the locking screw.  That's about the best you can do for now.

 

 

 

 

 

 

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#3 rlillard

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Posted Today, 12:50 PM

Thanks for the the info,I've been busy and was slow to reply.  I have an RMA and am sending it back for repair.



#4 Seaquel47

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Posted Today, 02:32 PM

As an update, I have received the new spring plunger and I have installed it.  Unfortunately I still have the same tiny bit of wiggle.  Since my last post I did have one good night to shoot and the total guiding for 4 hours was between 0.35" to 0.65" so it does do a good job even with the wiggle.  I even had a meridian flip during that run and the guiding still did good.

 

Good luck with yours.



#5 BillHarris

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Posted Today, 09:22 PM

Seaquel:
Tiny bit of wiggle of 0.35" to 0.65".

Does that mean a periodic error of 0.35 to 0.65 arc-second. By most any metric, that is a low error.

--Bill


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