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Equipment Discussions >> Mounts

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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: CGEM motor cogging new [Re: Ron (Lubbock)]
      #5882877 - 05/24/13 06:41 PM

Don't forget to provide feedback on your experiences with the beta upgrade - good or bad - to the Celestron beta site. Apparently the vast majority of people who have downloaded the software have not done so.

Chris


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Raginar
Postmaster
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: CGEM motor cogging new [Re: Ron (Lubbock)]
      #5883728 - 05/25/13 10:34 AM

Ron,

That's beautiful. Nice medium focal length image of M101.

Greg,
I misunderstood my CGEM then. So you're telling me that 8/3 harmonic issue is not due to the worm gear cycle? Or is that still there and this just fixes the 'sawtooth' many of us experienced in dec?


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Ron (Lubbock)
sage


Reged: 08/17/12

Loc: West TX
Re: CGEM motor cogging new [Re: Raginar]
      #5883754 - 05/25/13 10:46 AM

I got a chance to try MC v 31.15 last night. It was windy as heck and I only had 30 minutes to try it out before clouds rolled in, so I can't wait for a better night to do a more thorough assessment. However, here's my first impression.

I deliberately misaligned the scope enough that I would expect a pretty bad sawtooth pattern with a period of maybe 2-3 minutes. Not only was there no sawtooth in three different parts of the sky, but the RMS value of the Dec. guiding was ridiculously low! Dec. guiding is indeed noticeably smoother and quieter than RA guiding, as Derik from Celestron mentioned over on their site. The slewing of the scope is also smoother. There was no change to the RA guiding or its RMS value, except that it settled down after a few minutes and wasn't disturbed by skips in the Dec. guiding. I think I'm going to enjoy imaging with this mount a lot more now.

I'll wait for a second imaging session under better conditions, then post my assessment over on the Celestron beta tester site.


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gmartin02
scholastic sledgehammer
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Reged: 04/11/05

Loc: Santa Clarita, CA
Re: CGEM motor cogging new [Re: Raginar]
      #5884282 - 05/25/13 04:19 PM

Quote:

Greg,
I misunderstood my CGEM then. So you're telling me that 8/3 harmonic issue is not due to the worm gear cycle? Or is that still there and this just fixes the 'sawtooth' many of us experienced in dec?



Hi Chris,
This explanation may not be the best, but here goes: The "8" is the period of rotation of the worm gear. The 8/3 harmonic issue is due to gear ratio of the first spur gear to the worm gear. This is inherent to the design of the gearbox, and has nothing to do with firmware. This design makes it very hard to train PEC for the mount because there are 2 different "significant" periodic errors (the 8 minute one and the 3 minute one), it can cause quite a large "total" periodic error, and one that is not consistent worm cycle to worm cycle. Also, some mounts show more error in one of the other periods than others, so the problem may be worse on your mount than in others mounts.

I think the only way this could be corrected would be with a gearbox replacement with a 1 to 1 gear ratio between the worm gear and the first spur gear (and MC board/motor replacement to match the new gear ratio).

Note: This 1 to 1 worm/spur gear ratio has already been done in the newly released AVX mount. I would guess that Celestron will eventually release a new version of a CGEM class mount with this gear ratio, thereby eliminating the problem.

Celestron has not announced if they are going to spend the time & money to make an upgrade package to fix the already released CGEM mounts. Since they know about the problem and have customers complaining about it, in the perfect world they would make an upgrade package to correct this, but that would be a lot of engineering time and money spent on an existing mount that could be used instead to apply to a new mount (with other improvements as well). Also, it is not just a case of replacing the hardware in the existing mount - once the new hardware was in place, the MC firmare would also have to be changed to match the new hardware, tested extensively, and tuned before the mount would work with the new hardware - again, even more time and money. We will have to wait & see on this one for the existing mounts.

All that being said, the slopes of the periodic errors are fairly smooth, so they should be able to be guided out as long as the guiding corrections are issued ofter enough (every couple of seconds).

For long exposure times at longer focal lengths, there are a lot of other things that can prevent images from producing round stars. Things like seeing conditions, non-periodic mount errors, quality and sensitivity of the guide camera, guide scope focal length, appropriate selection of guide star, guiding software setup, flexure between guide scope & imaging scope, etc., etc., etc.

Probably the biggest culprit of the above for long focal lengths is flexure between guide scope & imaging scope. The best way to deal with this is to use an off axis guider with a high quality guide camera. I am currently getting some flexure between my AT8RC & my guide scope when imaging directly overhead - this is only significant enough to ruin images when the scope is pointed within about 15 degrees of the zenith, so with my current setup I avoid targets that will enter this area. The very first thing on my shopping list is to get the SX Lodestar guide camera and the TS-OAG9 off axis guider to eliminate flexure problems.

In summary, then 8/3 problem is still there - the new beta firmware fixes the "sawtooth" in Dec issue (that was preventing me from getting any usable images with long focal length scopes).

Anyone that knows has a better explanation of the 8/3 issue - feel free to correct any inaccuracies in my above post.

Final notes:

In general, I think it would be hard for any manufacturer to make a really, really accurate mount for AP at the price point that these mounts are being offered for. At these price points, many engineering and manufacturing compromises need to me made, and the mounts are too inexpensive to spend the time testing and tuning each mount individually. You get what you pay for - that why Astro-Physics mounts cost so much more: High quality (expensive) motors, more time spent machining, and individual mount testing/tuning before shipment. I would be shocked if any company (including Astro-Physics) could produce a mount for under $2000 that could handle a 30-40 pound imaging load and be super accurate for AP.

Greg


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svtdoug
sage


Reged: 02/07/11

Loc: Gig Harbor, WA, USA
Re: CGEM motor cogging new [Re: Raginar]
      #5884377 - 05/25/13 05:20 PM Attachment (15 downloads)

Quote:

Don't we need to see pictures over 480secs to prove that it's actually gone? Without it, you've just confirmed that it works within a worm gear cycle. I always had success at 5 minutes or so, I never got much beyond that.

Can you guys remove your reducers too? It'd be nice to see long focal length shots. 1000mm and longer exposures would be great .

Chris




Hi Chris,

My image of M51 on the previous page was taken without a focal reducer - at the full 2000 mm fl for the C8.

I've also imaged at 600 seconds with the firmware fix. This image was taken 4/11/13 at Camp Delany, with v30.91 of the beta firmware, using an Orion 8" Astrograph at 800mm fl. It consists of 12 - 10 min frames for 2 hrs total. Granted, its shorter focal length. The photo is also cropped about 60%.

Greg - nice images! And nice explanation of how the firmware fits in.

One other note - just to set the record straight: The majority of CGEM users have never had this dec guiding problem, your motors were manufactured correctly. For those people, great, this firmware is not a big deal, and it is possible to get great images without upgrading. But for those who have this dec cogging issue, myself included, it has made imaging extremely difficult and frustrating. For us folks, this firmware fix is very good news, is very welcome, and gives new life to our CGEM's. We are very grateful that Celestron has addressed this issue.

Doug


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Stew57
Carpal Tunnel
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Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM motor cogging new [Re: gmartin02]
      #5884523 - 05/25/13 06:51 PM

You close with your explanation. For every 3 exact rotations of the worm gear the spur turns 8 exactly. Form many mounts the 8/3 rerror is very large. For some mounts this error is greater than the worm pe. The problem guiding for some is the 8/3 error is far from smooth. With an integer gear box or a 3 worm rotation PEC it would help immensely. I have been able to use the work arounds for the cogging but the 88/3 error gets me. Besides unguided video would be nice if the pec would work.

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Raginar
Postmaster
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: CGEM motor cogging new [Re: Stew57]
      #5885356 - 05/26/13 10:43 AM

Grey/Mark,

Thanks . For some reason I thought the two were intertwined and I guess I hopped the firmware would fix it for ya'll.

Doug,

That's beautiful. Mechanical issues or not, that's something to be proud of .


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Ron (Lubbock)
sage


Reged: 08/17/12

Loc: West TX
Re: CGEM motor cogging new [Re: Raginar]
      #5892262 - 05/30/13 12:03 PM

I was able to spend a few more hours with the new MC firmware last night, and the results were less promising this time. The Dec. cogging issue is not gone on my mount, it turns out. If there is any misalignment in Dec., the sawtooth pattern is still there, as bad as ever. I was only able to get rid of it by doing accurate drift alignment. That being said, the mount worked really well when I did get it drift aligned, and the RMS value in PhD was as low as I've ever gotten it.

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Stew57
Carpal Tunnel
*****

Reged: 05/03/09

Loc: Silsbee Texas
Re: CGEM motor cogging new [Re: Ron (Lubbock)]
      #5892547 - 05/30/13 02:09 PM

Was this the beta firmware from teamcelestron? If so post on the site as feedback is needed. If it a release version there is no cogging fix present.

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Ron (Lubbock)
sage


Reged: 08/17/12

Loc: West TX
Re: CGEM motor cogging new [Re: Stew57]
      #5892669 - 05/30/13 03:09 PM

It is beta version 31.15 and my findings have been posted on teamcelestron. Derik says I am the first lucky person to observe cogging with 31.15.

Edited to add: I was able to do a 4 hr. session a couple nights ago, and I did a very accurate polar alignment. I did not have any Dec. cogging during the whole session. Thus, I am thinking that the sawtooth pattern requires poor polar alignment. I am pretty happy with the way my mount is functioning right now, despite the trouble on the one night.




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jrbuete
member


Reged: 02/10/13

Loc: SoCal
Re: CGEM motor cogging new [Re: Ron (Lubbock)]
      #5926652 - 06/17/13 11:58 PM

Just got a CGEM 1100HD a few months ago and noticed some issues with a movement in the eyepiece about every second while viewing objects... depending on the sky location is was either unnoticeable or annoying... just did the check posted above and yep... looks like the cogging issue but one question:
Is it normal for it only to be in one direction? I could get it to have several times per ' going up but going down, never a hitch...

Guess I'm waiting for the firmware before more pics... oh well..


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orlyandico
Postmaster
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Reged: 08/10/09

Loc: Singapore
Re: CGEM motor cogging new [Re: jrbuete]
      #5926671 - 06/18/13 12:11 AM

if your drift alignment is very good, and you're not getting any DEC corrections, then you wouldn't see any cogging because there wouldn't be any DEC corrections...

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rmollise
Postmaster
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Reged: 07/06/07

Re: CGEM motor cogging new [Re: jrbuete]
      #5927134 - 06/18/13 09:56 AM

Quote:

Just got a CGEM 1100HD a few months ago and noticed some issues with a movement in the eyepiece about every second while viewing objects... depending on the sky location is was either unnoticeable or annoying... just did the check posted above and yep... looks like the cogging issue but one question:
Is it normal for it only to be in one direction? I could get it to have several times per ' going up but going down, never a hitch...

Guess I'm waiting for the firmware before more pics... oh well..




Are you saying you are observing visually? If so, this is another problem and you need to contact Celestron. Dec cogging only affects the dec axis when corrections are being applied to it. During normal tracking, NOTHING happens with the dec axis. The motor does not move.


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EFT
Vendor - Deep Space Products
*****

Reged: 05/07/07

Loc: Phoenix, AZ
Re: CGEM motor cogging new [Re: jrbuete]
      #5927297 - 06/18/13 11:21 AM

Quote:

Just got a CGEM 1100HD a few months ago and noticed some issues with a movement in the eyepiece about every second while viewing objects... depending on the sky location is was either unnoticeable or annoying... just did the check posted above and yep... looks like the cogging issue but one question:
Is it normal for it only to be in one direction? I could get it to have several times per ' going up but going down, never a hitch...

Guess I'm waiting for the firmware before more pics... oh well..




I agree that this is not likely the DEC cogging issue. Since it is happening while "going up" it is likely that it is a balance and stiction issue. If the axis is not well balanced, then when the mount is struggling to move up against the off-balance, the axis can begin to show signs of sticking due to the force being applied to the worm and ring gear. Essentially, the faces of the worm and ring gear are being pushed together hard enough to cause them to stick slightly and then quickly release giving you little jumps. When going the other direction, the mount is no longer struggling and as a result may move more smoothly. If this were happening in both directions, then the issue would most likely be one of worm and ring gear spacing. But when it only happens in one direction, it is almost always a balance issue.


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Raginar
Postmaster
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: CGEM motor cogging new [Re: EFT]
      #5927374 - 06/18/13 11:53 AM

I was going to guess balance too.

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jrbuete
member


Reged: 02/10/13

Loc: SoCal
Re: CGEM motor cogging new [Re: Raginar]
      #5928275 - 06/18/13 09:18 PM

Thanks! I'll pay particular attention to my balance this weekend and see what happens, I know I was off a little bit since I balanced with my DSLR and then was doing visual at the end so...

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Stargazer78
professor emeritus
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Reged: 03/05/13

Loc: Susanville California
Re: CGEM motor cogging new [Re: jrbuete]
      #6057081 - 09/01/13 04:13 AM

I think I am experiencing this issue

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Raginar
Postmaster
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Reged: 10/19/10

Loc: Rapid CIty, SD
Re: CGEM motor cogging new [Re: Stargazer78]
      #6057504 - 09/01/13 11:41 AM

I didn't think the DX suffered this problem.

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