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zAmbonii
Carpal Tunnel


Reged: 01/19/08

Loc: Ypsilanti, MI
Re: What's Wrong with the Celestron CGEM Mount? new [Re: gmartin02]
      #5413994 - 09/10/12 07:47 PM

Was wondering, is this just a problem with the CGEM or is it indicitive of the CG-5 ASGT mounts also. I get something similar with my CG-5, but always thought it was a sticky gear problem (even after cleaning and buffing the gears etc.

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Peter D.
sage
*****

Reged: 02/09/12

Loc: Central New York
Re: What's Wrong with the Celestron CGEM Mount? new [Re: zAmbonii]
      #5414850 - 09/11/12 09:52 AM

Quote:

Was wondering, is this just a problem with the CGEM or is it indicitive of the CG-5 ASGT mounts also. I get something similar with my CG-5, but always thought it was a sticky gear problem (even after cleaning and buffing the gears etc.




I've been wondering too. I've been lurking here for a while, and while I don't own a CGEM I've seen this problem with my CG5. I find that I can usually resolve it by either "perfect" polar alignment (drift alignment refining the All-Star routine, which theoretically eliminates the need for DEC corrections entirely) or by deliberately inducing DEC drift (by mis-alignment) while unbalancing with force in the direction of the drift.

I believe that this issue is caused by the mount "springing back" after a correction, when the DEC is imperfectly balanced in the direction opposite the correction. By unbalancing in favor of the correction, the problem goes away.

I don't think that "motor cogging" has anything to do with it. Maybe the geartrain is "cogging" due to binding or other interaction in the spur gears, but motor cogging only relates to a periodic error that is related to the number of poles in the motor's rotor. Since the motor's instantaneous rotation rate exceeds 30 RPM even at the slowest correction rate and its cogging rate would therefore be at least 1 Hz (since there are at least two poles), active guiding cannot possibly act fast enough or precise enough. It's usually not a problem, except possibly in the most precise drives that strive to obtain accuracy to within an arcsecond. As an aside, some audiophiles can hear the effects of motor cogging even in the best direct drive turntables (which perform a function similar to that of a mount's RA drive), but only because the human brain can be quite sensitive to audio clarity (flutter/jitter).

A well-engineered analog DC servo should be capable of exceeding the performance of any equivalent-sized stepper motor in terms of both torque and flutter, but I don't think that the Celestron servos are optimized. Their control loop is digital rather than analog; that introduces digital jitter even if the software CONSTANTLY monitors the encoder and controls the shaft position: the software might not be capable of that in this case because it's doing many things at once. But the software could certainly stand further development; it should at least be able to hold the servo output stationary after a correction.


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airydisc
member
*****

Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? [Re: Peter D.]
      #5435481 - 09/23/12 04:02 AM

Pete, I didn't observe cogging on my CGEM, but another user posted a video that showed it clearly - with no load on the motor. I can also verify that the firmware (at the time I wrote the article) did NOT constantly monitor the encoder position. That is, after reaching the target position in DEC the motor drive would go to zero. At that point, if no further guide commands were issued you could manually turn the motor shaft as much as you like and the controller would do nothing to correct it. Essentially, it an open loop control system in the long term.

Greg


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lsfinn
member


Reged: 01/28/13

Loc: State College, PA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: airydisc]
      #5658285 - 02/02/13 02:42 PM

Any updates on this problem?

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Ozy



Reged: 06/15/07

Loc: Goodyear, AZ
Re: What's Wrong with the Celestron CGEM Mount? new [Re: lsfinn]
      #5720879 - 03/08/13 05:27 PM

Did this issue get resolved with a firmware update yet? I just ordered a CGEM today.

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Jeff2011
Carpal Tunnel
*****

Reged: 01/01/13

Loc: Sugar Land, TX
Re: What's Wrong with the Celestron CGEM Mount? new [Re: Ozy]
      #5992133 - 07/26/13 10:40 PM

I have observed the same problem with my AVX. I agree with Peter's second method. The perfect polar align takes too much time. Just do an ASPA. It will usually be close but not perfect. Then monitor which way it is drifting in Dec. Then set PHD to only correct in the opposite direction. You may need to initially have set the pulse kind of high to get the direction change to take effect, then when it takes effect you can set the Max Dec to a lower value. The alternative is to use the hand controller direction button to get the dec change to take effect. Ever since I have figured this out, it has worked for me every time and my Dec line has smoothed out. Now if I could only get the RA line to be that smooth. I recently recorded and averaged 5 PEC runs. Perhaps that will help with the RA.

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chijesep
newbie


Reged: 04/14/13

Re: What's Wrong with the Celestron CGEM Mount? new [Re: gmartin02]
      #6346360 - 01/31/14 12:07 AM

I have the same problem but I use a CG5 Advanced GT mount.

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airydisc
member
*****

Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? new [Re: chijesep]
      #6390561 - 02/22/14 01:34 PM

More than 2 years after the article was published here (and much longer since it was first reported to Celestron), I have confirmation that the firmware update does indeed seem to fix the problem. I haven't seen it myself, as I got rid of my CGEM long ago, but a friend has one and reported his experience to me.

So I'm happy to recommend the CGEM again, although I'm still wary of Celestron's customer service and quality control.


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stevecoe
"Astronomical Tourist"
*****

Reged: 04/24/04

Loc: Arizona, USA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: airydisc]
      #6390700 - 02/22/14 03:11 PM

Airy;

My CGEM is four months old and has no problems. I just posted an image taken with a DSLR of M 46 in Puppis. The tracking is excellent with no guiding at all. I have also used it at 250X on Jupiter for several hours at a public viewing session, the mount kept that planet in the middle of field of view with no trouble.

I am very happy with the performance of this mount;
Steve Coe


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