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guiding issues with CEM26

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#1 ofergol

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Posted 22 September 2021 - 11:58 AM

A few months ago I purchased  the iOptron CEM26 mount.
Things seems to work fine. Recently I started to observe significant  oscillations of the guiding.
I’m using a 6’’ classic cassegrain as the main telescope and a 60mm/240mm guiding scope with ZWO ASI462 as the guiding camera (the problem exist with different rigs as well).
I control the system with ASIAIR (although the guiding issues exist also with phd2 on a laptop).
I use a very stable pier. The polar alignment is perfect (sub minute error in both axis). The focus is optimal. The weather is great (nice seeing and no wind)
The problem is that the guiding is poor with unexplained error peaks of 2,3,4 and even 8’’ once every minute or so. Even between such peaks the guiding is not as I expected.
I was wondering if anyone has a clue about the problem.
Thanks
Ofer

 

I attach picture of a guiding example (taken from phd2 log viewer).



#2 mehdymo

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Posted 22 September 2021 - 12:16 PM

Normally these peaks are related to balance problem at different points in the sky. We normally do 2d balancing, while the balance is 3D and when you change the scope direction it becomes significantly unbalanced. Try to balance on the object direction instead of balancing at zero position. On some mounts, slight unbalancing helps (just move the scope and counterweights a few milimeter from balance point).


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#3 ofergol

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Posted 22 September 2021 - 01:07 PM

Thanks.

that's a good advice and I will check it.

However, I'm quite balanced (although not at the target, but my system is quite symmetric so I'm not  significantly changing the balance).

Moreover, the problem exist even with 6 pound telescope and camera. my mount should carry such weight with ease.



#4 mehdymo

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Posted 22 September 2021 - 04:58 PM

you are welcome. On some mounts, the perfect balance causes ocsillation and you should make it slightly unbalanced.

Thanks.

that's a good advice and I will check it.

However, I'm quite balanced (although not at the target, but my system is quite symmetric so I'm not  significantly changing the balance).

Moreover, the problem exist even with 6 pound telescope and camera. my mount should carry such weight with ease.



#5 Kevin_A

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Posted 22 September 2021 - 07:16 PM

This is a common issue with this mount and the gem28. It is caused by worm mesh stiction. Your meshing needs to be loosened. I had a discussion with ioptron on this matter. I would guide at 0.3 to 0.5 most of the time and then out of nowhere 4 to 8 then back to great guiding… rinse and repeat. The key is to set the mesh to where the shaft wiggles very slightly between the worm teeth due to a slightly loose mesh and it is then totally elliminated just slightly before the switch clicks into the lock position. Mine wiggles slightly at the arrow near the lock position and all backlash is elliminated once it clicks into the locked position. Cheers! 


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#6 Aerions

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Posted 25 September 2021 - 11:31 PM

This is a common issue with this mount and the gem28. It is caused by worm mesh stiction. Your meshing needs to be loosened. I had a discussion with ioptron on this matter. I would guide at 0.3 to 0.5 most of the time and then out of nowhere 4 to 8 then back to great guiding… rinse and repeat. The key is to set the mesh to where the shaft wiggles very slightly between the worm teeth due to a slightly loose mesh and it is then totally elliminated just slightly before the switch clicks into the lock position. Mine wiggles slightly at the arrow near the lock position and all backlash is elliminated once it clicks into the locked position. Cheers! 

Loosening the mesh as in doing what's described here in the instruction manual?

https://gyazo.com/7d...648df048ea47d35

https://gyazo.com/2d...7dbca3d6d30ad71



#7 ofergol

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Posted 26 September 2021 - 05:23 AM

I loosened the mesh and it certainly improved the situation. however, it is still unsatisfactory (RMS on both DEC and RA above 1'') and I'm hesitating about additional loosening.



#8 Aerions

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Posted 26 September 2021 - 11:21 AM

I loosened the mesh and it certainly improved the situation. however, it is still unsatisfactory (RMS on both DEC and RA above 1'') and I'm hesitating about additional loosening.

Yeah probably don't wanna loosen it too much. I solved my issues through phd2 by turning off dec backlash compensation. Was over correcting big time and throwing off my Ra as well.. 



#9 Kevin_A

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Posted 26 September 2021 - 04:18 PM

I call this and gem28 finicky mounts. They need to be tweaked to your specific weight and cant be either too loose or too tight. It is further complicated be very light scopes and when you use asiair. They all seem to guide well then spaz out for no reason then guide well. Calibration of your guide scope doesnt seem to change much. I wish ioptron would step up to the plate and say exactly the correct meshing technique and belt tension as they are very vague.


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#10 Kevin_A

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Posted 04 October 2021 - 12:03 PM

Here is a link that is supposed to fix erratic guiding errors on these mounts.

https://www.ioptron....nAdjustment.pdf



#11 rgsalinger

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Posted 05 October 2021 - 10:01 AM

The first thing to do is to post a guiding log when you want guiding assistance. Without that, it's just guesswork on the part of anyone as to what the problem actually is. It may well be that a "loose mesh" is somehow necessary, but when you do that, you are effectively substituting the possibility of backlash in place of stiction.

 

Whenever I hear about large occasional excursions I wonder about the overall environment and the cabling of the system. It is quite remarkable how easy it is for a bad cabling choice or a bit of wind of barely visible high cloud to make one think that there's some mechanical issue with a mount. 

 

If the mesh were, in fact, too tight what you would see is quite different from what you would see if you have, say, bad seeing or a cabling issue. The guide log is the best way to trouble shoot any guiding issue. 

 

My own view is that one should start with PHD on it's default settings and then, if it's actually necessary, make variations in cadence and aggression if and only if the stars are not round. If the stars are round then avoiding the temptation to fiddle with the guiding is really the best advice you will ever get IMHO and YMMV. 




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