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Celestron CGX Mount Problem on RA axis

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#1 robmogford

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Posted 20 June 2019 - 05:12 AM

Hi, just wondered if anyone else had experienced the following problem which started last night:

 

My CGX is mounted on a permanent pier and the problems started as soon as I’d finished polar alignment and tried to do a new star alignment.  As soon as I tried to navigate to the first star there was a clear jumping back and forth when the mount settled at the location of the first star.  To get started I was only using my guide scope and camera at low resolution (ASI120 camera and Skywatcher ED50 Guide Scope) but the jumping was still very noticeable - see video 1 attached.  There was also a clicking sound coming from the mount (which you can hear in both videos and on closer inspection of the RA axis you could see the belt drive moving back and forth constantly which seems to correlate with what I was seeing on screen - see video 2 attached.

 

Anybody got any ideas what might be causing this and possible fixes worth trying?

 

Thanks Rob

 

Links to short 5s videos here on YouTube:  

 

Video 1:  https://youtu.be/7UwjFH4DIIw

 

Video 2:  https://youtu.be/aze52xlRdPM

 

 


Edited by robmogford, 20 June 2019 - 05:47 AM.


#2 skaiser

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Posted 20 June 2019 - 07:11 AM

Hi rob

 I have a cgx mount also.

i have experienced a number of quirky things over the last year or so.

some questions.

 Was this a one time occurrence  or has it continued to do this the rest of the evening   

 Most of my “events” did not repeat after a reboot.

not a satisfactory answer as to why it happened but it seems the controls/software can hiccup once in a while

 

if this is a repeating event you might want to reset your hand control and reload software.


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#3 Dynan

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Posted 20 June 2019 - 07:24 AM

I suspect that your 'worm to ring gear' mesh is too tight. Found much the same problem with mine when the mesh was adjusted too tightly.

 

This image was taken with a gear mesh that was found to be too tight (look closely at the brighter stars and NGC 6543 to see the jump):

 

CGX JUMP.jpg

 

I've said it before, and I'll say it again... Celestron is doing a great disservice to CGX owners by not providing a COMPLETE and repeatable method of mesh adjustment of the CGX. They used to have a section on mesh adjustment, which was only partially complete, in the old manual. Even that is now COMPLETELY removed. Something as important as this adjustment should be shared in good faith. Bad adjustment now means that a mount has to be sent to them in California, causing stiff shipping costs and sky-time.

 

This adjustment changes with temperature. I found this out the hard way last July when my previously accurate CGX started sounding like a wounded animal. I've since found an adjustment method and target that works for my mount...but I have no idea if it is correct. I use my image results to judge that, which is probably the best measure. But no set procedure has evolved for when it happens again.

 

I will say this about mesh adjustments:

There are TWO that need to be correct, one affecting the other, namely worm-ring mesh DEPTH (by positioning the motor 'sled' squarely and at the proper depth), and worm-ring spring tension (by adjusting the variable spring tension screws). The spring tension was the ONLY one illustrated in the OLD manual. A number of videos can be seen here:

 

https://www.youtube....=cgx adjustment

 

But these were graciously shared by fellow CGX owners who had to figure it out by themselves.

 

If/When I find out something definitive and reproducible, I'll reply here.

 

 

 


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#4 skaiser

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Posted 20 June 2019 - 09:31 AM

Other CGX adjustments that I have noted/collected here and on youtube can be found as noted:

Hope this helps.

 

Here is where I discuss the alignment of the motor drive belt system. 

https://www.cloudyni...es#entry8881541

 

Some Suggestions/information I have received from Celestron over time:

-----------

  Give up trying to guide Declination in both directions until you have mastered guiding Declination in just one direction. A tiny bit of Polar alignment error will create enough Dec drift that you should never need to guide in both directions. There are some very special circumstances where perfect polar alignment is crucial. But in general, if you are getting between 1 and 2 arc seconds of drift in declination per minute, the improvement to your guiding will be worth the penalty paid in field rotation. Unless you have actually observed field rotation in your sub frames, you probably don't need to worry about it.

 

  Remember that no matter what you do with the gear backlash on the worm wheel, a Celestron mount will always show about 60 arcseconds of backlash from the gearbox. This is going to require special handling from your guide software. A properly adjusted CGX-L mount will have between 60 and 150 arc seconds of gear backlash (total). Which means you will need to have 60 to 90 arc seconds of gear backlash from the worm block. Reducing the total amount of gear backlash from 150 arc seconds to just 60 will not eliminate the eliminate the special handling requirement. So don't risk screwing it up just to make this smaller. Always remember, that while your mount is trolling through the gear backlash, a process that will take between 15 and 150 seconds depending on how your guide software handles it, your mount can not guide in Declination in Either directions. If you stop half way through that processes, it will take a comparably long time to exit the gear backlash on either side before guiding can resume. You definitely shouldn't be trying to change directions any more frequently than once per minute. If you find yourself changing back and forth constantly you are probably not guiding at all, just tooling around in the gear backlash.

 

  Do not make RA too tight. Gear backlash does NOT affect RA guiding. Even huge amounts (10 arc minutes) wouldn't matter because the RA tracking motor never stops turning - regardless of the direction you are guiding the mount. Making the RA gear mesh too tight, can cause microbinding which will be hard to see in visually, but will show up as stretched stars in RA.

 -------------- Also from Celestron

(with reference to the CGX-L manual    91531_CGX-L_EQ-Mount-and-Tripod_Manual_5lang_Web.pdf)

 

The mount's behavior could be the cause of the worm gear binding. However, the spring loaded worm block should accommodate this. So it may simply be too tight to where the spring is not recoiling properly.

1. Using a Phillips head screwdriver, remove the gear cover for the axis you intend to adjust (Figure 13).
2. Test the mesh by grabbing hold of the motor and worm block with your hand and pulling it away from the worm wheel. The worm block should recoil slightly, causing the worm gear to back away from the worm wheel slightly.
3. Using a 2mm and 3mm Allen wrench, make very slight adjustments as needed to optimize the gear mesh (Figure 14).

• If the worm block does not recoil, the mesh is too tight, or the limit screw is pushing the block against the worm wheel.
- Turn the Range Limit Screw clockwise and check for recoil.
- Turn the Distance Adjustment Screw clockwise and check for recoil.
• If the worm block recoils but there is noticeable backlash when the axis is locked.
- Turn the Distance Adjustment Screw counterclockwise very slightly, only 1/16th of a turn or less and retest.
- Tighten the Range Limit Screw slightly if the recoil exceeds approximately 1mm.

It’s OK to err on the side of having a little amount of backlash. After the adjustments are made above, try this test before recovering R.A. motor:

Lock down the R.A. axis and gently push against the counterweight shaft in R.A. If they feel a slight “click” in the mount, that is OK, and generally ensures that you will not bind. If you do not feel any movement at all, that should still be OK since they adjusted the mount so that the spring will give, but it still runs a slight risk of binding. Again, side on having a tad of backlash and from that point the spring should accommodate a wide temperature range, eliminating the need to constantly fine-tune the mount.

 

------------------------

Daniel, above has a nice collections site of youtube videos .

Good luck


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#5 robmogford

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Posted 23 June 2019 - 04:43 AM

Thank you to Skaiser & Dynan for your help in pointing me in the right direction.  I've been reading the threads and watching the videos you recommended and seem to have identified my problem. 

 

As many others seem to have experienced, with the change in location and temperature, the worm-ring mesh had started to bind.  This was causing the backward and forward motion in the gear as it tried to track an object but struggled with stiction.  I hadn't mentioned but I was also getting  a squawking noise from the mount when moving in anticlockwise on the RA axis. 

 

The problem I had was that the basic adjustment described briefly and badly in the Celestron Manual wasn't solving the problem.  Tightening the Distance Adjustment Screw in the Manual simply wouldn't lift the worm gear high enough off the block to prevent binding.  The solution seems to have been using the second adjustment not referred to in the manual but identified by others on here.  By loosening off the block using the large allen bolts under the worm gear and then slightly loosening the distance adjustment screw for the block things finally freed up - the binding and squawking noise have both gone.  I now have a small amount of play in the RA axis which I guess may give backlash issues but I'll have to wait for a proper clear night to test.  At least now I think I have enough room to adjust in either direction and fine tune things.

 

Thanks again for your help and I'll let you know how I get on after a proper test.

 

Rob


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#6 RazvanUnderStars

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Posted 23 June 2019 - 06:17 AM

As a future CGX owner, I'm grateful to all who contributed to this thread, being aware of the issues and fixes ahead of time is invaluable. Thank you all.



#7 Dynan

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Posted 23 June 2019 - 08:58 AM

I now have a small amount of play in the RA axis which I guess may give backlash issues but I'll have to wait for a proper clear night to test.  At least now I think I have enough room to adjust in either direction and fine tune things.

Remember to adjust your balance 'Heavy East' to take the RA backlash out. [Viewing East of meridian, heavy counterweight side...West of meridian, heavy scope side] Not a lot of imbalance...just enough to keep the worm and ring always engaged.

 

I slide only the closest counterweight to the mount from true balance position, about 1/2 inch either way.

 

[Ex. Viewing East of meridian, {Scope west of mount} slide the weight AWAY from the mount 1/2". At Meridian Flip and viewing West of meridian {Scope east of mount}, I reposition the weight to true balance position (I have marks in the c/w shaft at true balance position) then slide the weight TOWARD the mount 1/2".]

 

To clear any confusion, with my C-11, I have three counterweights on the mount. I position the closest between the aforementioned marks and adjust the others for true balance. That way I have a repeatable movement of the closest weight.

 

And as directed by Celestron, Always finish your Goto's and image starts with UP and RIGHT on the HC.

 

Thanks for the follow-up and please keep us posted! Ya never know who need this info!


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#8 RazvanUnderStars

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Posted 23 June 2019 - 09:11 AM

Hi Dynan,

 

Given that the mount can observe up to 2 hours after the meridian without the flip, if you do that, do you need to adjust the balance once the object is past the meridian or only if there was a flip?

 

Thanks.

 

 

[Ex. Viewing East of meridian, {Scope west of mount} slide the weight AWAY from the mount 1/2". At Meridian Flip and viewing West of meridian {Scope east of mount}, I reposition the weight to true balance position (I have marks in the c/w shaft at true balance position) then slide the weight TOWARD the mount 1/2".]

 

 


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#9 Dynan

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Posted 23 June 2019 - 10:29 AM

Consider it this way: The RA gears should always be LIFTING AGAINST gravity. This will cause the gears (and hence the motor) to always be engaged on one side of the gears...not searching back and forth. It's a 'pre-load' situation.

And since RA is always East to West, you want it LIFTING on the East side of the mount.

 

When you go past meridian with the same balance (previously East Heavy), the weight will shift to the West side of the mount. That will mean the RA is LOWERING against gravity...not a good situation, since any 'stiction' in the mount will be allowed to affect the tracking. Gravity is your friend here...

 

It's NOT where your counterweights are, or your scope, relative to the meridian. It's the BALANCE POINT that you set up that passes the 'meridian' of your mount (the center of your RA axis).

 

So, to answer your question the best I know how, is to shift weight as balance point passes RA center, independent of any flip that happens, either manually or automatically.


Edited by Dynan, 23 June 2019 - 11:03 AM.

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#10 RazvanUnderStars

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Posted 23 June 2019 - 02:15 PM

Thanks, Dynan! Clear now.


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#11 Dynan

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Posted 23 June 2019 - 09:12 PM

You're Welcome. Anything for our wonderful northern neighbors! wavey.gif

(And anyone else, for that matter...)



#12 robmogford

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Posted 24 June 2019 - 05:14 AM

Hi Dynan

 

Looks like I might have clear sky's tonight so going to try and do a proper test of the mount.  Nothing too stretching so will use my Skywatcher 80ED and 50mm guide scope.  As you've been very helpful in explaining stuff in concise non techno babble, I just wondered what guiding accuracy you typically get from your CGX under 'normal' conditions.  Hopefully you use PHD2 in which case I'm referring to the RMS Error px/ arc-sec and RA Osc - I know there are lots of variables but just a rough ball park would be great.  Anyone else reading this thread also feel free to contribute if specific to CGX mount!!

 

Cheers

Rob



#13 Dynan

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Posted 24 June 2019 - 08:04 AM

This is with a 60mm x 240mm guide scope with an ASI224MC camera. It showed me what the CGX was capable of:

 

PHD2 LOG 0506.jpg

 

The image came out great.

 

M52.jpg


Edited by Dynan, 24 June 2019 - 08:19 AM.

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#14 robmogford

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Posted 24 June 2019 - 08:52 AM

This is with a 60mm x 240mm guide scope with an ASI224MC camera. It showed me what the CGX was capable of:

 

attachicon.gif PHD2 LOG 0506.jpg

 

The image came out great.

 

attachicon.gif M52.jpg

Great, thank you and nice image!


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#15 robmogford

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Posted 29 June 2019 - 06:39 AM

A Quick Update

 

Well I had a 4 hour run of clear skies to test the worm gear adjustments to the CGX mount on Thursday night.  Skies were clear but we've been experiencing a heatwave in France with night time temperatures around 30C so I suspect there was some atmospheric turbulence which may have impacted on guiding.  However, I achieved the most stable results I've ever managed with the mount so far.  Using a Skywatcher50 guide scope (240mm focal length) with a ZWO ASI120mm I saw guiding in PHD2 with RMS errors between 0.9 and 1.2 arcseconds.  I understood the mount should be able to consistently deliver sub 1 arc sec accuracy but this is still a big improvement from what I was achieving previously.

 

So far I've had to loosen the worm gear off enough to leave a little play in the RA gear as if I remove all the play the worm gear then starts to bind on certain parts of the RA gear.  I've tried to deal with the backlash implications of this by making the balance slightly heavier on the East side of the mount.

 

Sample 240s sub in Ha from last nights session attached - stars look pretty round to me at 2.3 arcsec/px resolution? The real test will come when I try mounting the Celestron 8"HD with its OAG and a max resolution of  0.386 arcsec/px...

 

Thanks for your help.

 

 

Attached Thumbnails

  • CGX Guide Test Exposure Ha 240s LowRes.jpg


#16 Dynan

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Posted 29 June 2019 - 07:46 AM

Excellent image and adjustment! Nice round stars speak volumes for this mount.

 

I know the CGX is well suited for the task. If the clouds aren't a problem tonight, I hope to emulate your effort. I have adjusted mine the same way you describe. The image will tell.

 

CS!




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