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What's Wrong with the Celestron CGEM Mount?

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#1 Olivier Biot

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Posted 04 December 2011 - 03:03 PM

What's Wrong with the Celestron CGEM Mount?

By Greg Marshall.

#2 Raginar


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Posted 04 December 2011 - 11:13 PM


Great article. My mount does it too. I wish there was a good solution to it.

#3 stevecoe


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Posted 05 December 2011 - 06:11 AM


My CGEM worked just fine, but I a fortunate enough to live in Arizona so I have Starizona near by. When I bought it from Starizona, Dean Konig looked it all over then lubed and set the gear adjustment very carefully. I never had a cogging problem and once I got good with the polar alignment routine I got lots of great images and enjoyed my CGEM.

After taking all the images I wanted to take, I was done with astrophotography and returned to just viewing. I have a simple tracking mount and I enjoy some wide field imaging, but I am happy that I returned to just looking at the sky.

It is unfortunate that there is not one simple "fix it" for this problem.

Clear skies to us all;
Steve Coe

#4 Greg K.

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Posted 05 December 2011 - 09:29 AM

Nice article.

Tnakfully my CGEM does not exhibit this behavior when guiding - works great!

#5 Goran Strand

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Posted 06 December 2011 - 02:16 AM

Thanks for a great article. My mount acts exactly likes this. Now I've contacted my supplier and asked him to get in contact with Celestron about this.

#6 airydisc


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Posted 13 December 2011 - 03:20 AM

Thanks for the feedback. I recently heard that Celestron has released a firmware fix to address the problem. Have you heard about that? I stopped paying attention after upgrading to an AP Mach1GTO.

I was asked whether a mount that does not exhibit the problem should be upgraded and I think the answer is "no" - at least until it has been proven to work reliably. I say that because I believe that the changes needed were extensive.


#7 highfnum



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Posted 16 December 2011 - 07:11 AM

when you said swapped moters
did u get a new motor from celestron or just
swap RA motor with dec motor?

#8 boeingfan


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Posted 20 December 2011 - 11:29 PM

Have you looked in to having it "hypertuned" ?


#9 airydisc


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Posted 23 December 2011 - 07:19 PM

I swapped the RA and DEC motors. You can get a replacement motor (it's the same as the CG-5 motor), but there's no guarantee that the new one will be any better.

#10 airydisc


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Posted 23 December 2011 - 07:24 PM

Yup. I talked to Ed at Deep Space Products. Once he understood the exact nature of the problem he said that a hypertune would not help. It would be different if he could find a suitable replacement motor, but so far that has not happened.

#11 Ed J

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Posted 31 December 2011 - 01:40 PM

I am a member of "TeamCelestron.com", just looked, but if new firmwere exists, I can't find it there. Anybody know of its existence? It has been on my back-burner since my third motor DEC solution ( http://www.flickr.co...ker/6204459052/ )

Ed Johnson

#12 FLYcrash



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Posted 01 January 2012 - 09:41 PM

Hi Greg,

I'm no astrophotographer but still enjoyed reading your clear exploration of such a subtle issue.

I wonder if one could source high-end servos satisfying the specs of the CGEM servos. Seems like upgrading to motors that consistently avoid cogging would save a lot of APers a lot of hassle.

#13 svtdoug


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Posted 11 January 2012 - 04:22 AM

Hi Greg,

I enjoyed your fine review of the cogging problem. I have a "mild" case of this - in that my jumps are normally within +/- 1 pixel, but sometimes dec will wonder off 2 or 3 pixels. I upped the Guide rate on dec in the mount to 90% and upped the PHD's Min Motion to .40 - and both of these measured seemed to help - making the jumps less severe. And as you noted, upping PHD's Max Dec to 1800 or 2000.

I believe it was Ed who has communicated with a firmware engineer at Celestron who acknowledged the problem and is supposed to be working on a fix.

Would be nice - then it would be reasonably good mount.


#14 airydisc


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Posted 17 January 2012 - 12:19 PM

Sorry for my slow response here!

There have been serious efforts to find an alternative motor for the CGEM, but AFAIK, none has been found.

Doug, yes, the degree of cogging will vary from one motor to another. I'm skeptical about improving the situation with adjusted guide parameters. Have you measured the long-term performance with and without the changes?

Agreed, the CGEM would be a fine mount if they could solve this problem - or if you are lucky enough to get one with good motors. I've seen and tested some with good motors, but both of the units I owned had bad motors on the DEC axis.


#15 svtdoug


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Posted 21 January 2012 - 01:18 AM

Hi Greg,

Have not used the mount since - we've had lousy weather, so can't comment on long term performance. But it clearly dampened the dec excursions during the session when I made the adjustments.

Several days ago, I did receive an email from Derik of Team Celestron regarding the cogging issue. He responded - "As for the DEC guiding problem, I am working on it as fast as I can. I hope to have some alpha code out next month."

I've signed on for Beta testing, so hopefully we will see a fix in a month or two.


#16 DeMar


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Posted 29 January 2012 - 07:20 PM

I have the same issue mentioned here and have found that by adjusting some of the guiding settings it'll perform better for a little longer but eventually it'll start to drift in DEC again. A difference between 5 and 10 minutes before drift occurs. I've resorted to sitting there and watching the readouts with the hand controller at the ready so that when I start to see a drift, I overcome it with manual input. Obviously only works with wide field photography. I'm hoping Celestron comes out with a fix soon or the mount will be replaced.

#17 Jason B

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Posted 14 February 2012 - 07:34 AM

Nicely written article.

My CGEM has been flawless since day one. The only time I have had any issues with guiding, it's been a balance issue on my part.

#18 sabin_fota



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Posted 09 April 2012 - 12:56 AM

Great article Greg, I have the same problem with my CGEM. Hope I'm gonna fix it because I ordered the hypertune kit from deepspaceproducts.com

#19 MichaelAK



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Posted 01 August 2012 - 08:36 PM

Yes Greg, great article. I was thinking of buying the CGEM mount for astrophotgraphy for a load of no more than about 37 lbs.

Greg or anyone,

Any good alternatives?

Also, short of buying one, how would we have any assurance that Celestron has fixed the problem or minimized it?


#20 gmartin02


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Posted 09 September 2012 - 11:15 PM

Celestron is working on the problem. They are on their 3rd beta release of the motor control firmware with a fix for the problem, but the beta releases are still unstable. They are also working on other updates for a new firmware release, such as improved backlash compensation.

#21 zAmbonii



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Posted 10 September 2012 - 06:47 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.

#22 Peter D.

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Posted 11 September 2012 - 08:52 AM

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.

#23 airydisc


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Posted 23 September 2012 - 03:02 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.


#24 lsfinn


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Posted 02 February 2013 - 02:42 PM

Any updates on this problem?

#25 Ozy



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Posted 08 March 2013 - 05:27 PM

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

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