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Olivier Biot
Amused
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Reged: 04/25/05

Loc: 51°N (Belgium)
What's Wrong with the Celestron CGEM Mount?
      #4949784 - 12/04/11 03:03 PM

What's Wrong with the Celestron CGEM Mount?

By Greg Marshall.


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Raginar
Postmaster
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: What's Wrong with the Celestron CGEM Mount? new [Re: Olivier Biot]
      #4950585 - 12/04/11 11:13 PM

Greg,

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


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stevecoe
"Astronomical Tourist"
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Reged: 04/24/04

Loc: Arizona, USA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: Raginar]
      #4950862 - 12/05/11 06:11 AM

Greg;

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


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Greg K.Administrator

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Reged: 12/11/03

Loc: Clifton Park, NY
Re: What's Wrong with the Celestron CGEM Mount? new [Re: stevecoe]
      #4951065 - 12/05/11 09:29 AM

Nice article.

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


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Goran Strand
sage


Reged: 03/05/08

Loc: Jamtland, Sweden
Re: What's Wrong with the Celestron CGEM Mount? new [Re: Greg K.]
      #4952632 - 12/06/11 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.

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airydisc
member
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Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? new [Re: Goran Strand]
      #4964031 - 12/13/11 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.

Greg


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highfnum
Carpal Tunnel
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Reged: 09/06/06

Loc: NE USA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: airydisc]
      #4969115 - 12/16/11 07:11 AM

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


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boeingfan
super member


Reged: 08/28/08

Loc: El Dorado Hills. CA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: airydisc]
      #4976997 - 12/20/11 11:29 PM

Have you looked in to having it "hypertuned" ?

http://www.deepspaceproducts.com/HyperTune-Service_8_4226.html


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airydisc
member
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Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? new [Re: highfnum]
      #4981894 - 12/23/11 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.

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airydisc
member
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Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? new [Re: boeingfan]
      #4981902 - 12/23/11 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.

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Ed J
member


Reged: 05/31/07

Loc: Tujunga, California
Re: What's Wrong with the Celestron CGEM Mount? new [Re: airydisc]
      #4994135 - 12/31/11 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.com/photos/edhiker/6204459052/ )

Ed Johnson


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FLYcrash
Pooh-Bah
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Reged: 08/29/09

Loc: Chicago, IL, USA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: Ed J]
      #4996561 - 01/01/12 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.


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svtdoug
sage


Reged: 02/07/11

Loc: Gig Harbor, WA, USA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: FLYcrash]
      #5011725 - 01/11/12 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.

Thanks!
Doug


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airydisc
member
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Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? new [Re: svtdoug]
      #5022698 - 01/17/12 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.

Greg


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svtdoug
sage


Reged: 02/07/11

Loc: Gig Harbor, WA, USA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: airydisc]
      #5029774 - 01/21/12 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.

Doug


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DeMar
newbie


Reged: 03/02/11

Re: What's Wrong with the Celestron CGEM Mount? new [Re: svtdoug]
      #5044817 - 01/29/12 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.

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Jason B
Proud father of 5!!
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Reged: 06/21/04

Loc: Mid-Michigan
Re: What's Wrong with the Celestron CGEM Mount? new [Re: DeMar]
      #5071645 - 02/14/12 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.


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sabin_fota
member


Reged: 04/08/12

Loc: Romania
Re: What's Wrong with the Celestron CGEM Mount? new [Re: Jason B]
      #5163041 - 04/09/12 01:56 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

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MichaelAK
member


Reged: 06/11/12

Loc: Las Vegas, NV (suburb)
Re: What's Wrong with the Celestron CGEM Mount? new [Re: sabin_fota]
      #5347743 - 08/01/12 09:36 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?

Michael


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gmartin02
scholastic sledgehammer
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Reged: 04/11/05

Loc: Santa Clarita, CA
Re: What's Wrong with the Celestron CGEM Mount? new [Re: MichaelAK]
      #5412601 - 09/10/12 12:15 AM

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.

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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
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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
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Reged: 01/23/07

Re: What's Wrong with the Celestron CGEM Mount? new [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
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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
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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"
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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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