CEM40G - Tracking / Guiding issues - help needed

[Aayush Nair]

Hi all,

Looking for some help with my 1 year old CEM40G please.

I have 2 scopes – a WO FLT91 and a WO Z61 – with a 533MC pro. I use an OAG setup with the FLT91 and have also tried with a 120mm WO 32mm Uni-guide scope on both main scopes with the ASI 120mm mini camera.

The mount has developed some guiding issues, with what looks like stiction and bad belts.

1.       I can image up to 10 minute exposures, stars are pin-point after a lot of effort; many times there's an elongation in RA in subs. I cannot image without guiding, as tracking performance is poor despite good PA. I can do a max. of around 10-15s exposures at 360mm / 432mm focal length. 

2.       When observing the guiding graph, I see multiple pulses being sent to RA with no resulting effects until suddenly there is a change suddenly and the same thing happens on the other side of the graph; resulting in a sine wave format. Also see a lot of spikes in RA and DEC

3.       The Dec axis remains better than RA

4.       Normally, I've read that a larger guiding exposure would help - I've tried those and it shows the above effect amplified. I cannot use 2s or 3s or more. During such longer exposures i've observed the RA drift sometimes completely off the graph to then finally register pulses being sent (again the same sine wave form continues here too, but more pronounced). I've had better results with 1s and 0.5s exposures and this along with 0.90x guiding rates and aggression values of 100-130% help it to remain in the center of the graph, but with many corrections, as you can see in the guide log (attached and linked)

5. I've tried with 0.5x , 0.75x and 0.90x guiding rates (sidereal) and found 0.90x better with high aggression values in RA, with 0.5x and 0.5s exposures it usually does ok with about 120% RA aggression 

 

What I’ve done so far: 

1.       Meshing: adjusted and re-adjusted gear meshing from tight and loose, via inputs on various threads on SGL/CN and even the ioptron documents - didn't help much. Right now sitting at almost 3/4 turn loose from full lock - which worked a little better

2.       Belts: these were replaced by Ioptron as they were shedding. The overall behavior didn't change much though (what I described above). I've also tensioned the belts tight and a little loose (1mm to 3mm per spec) and this too didn’t effect the overall behavior

3.       Main Gear: I've cleaned and re-greased the gear and worm (without removing the worm)

4.       Calibration: I've calibrated many times, changed various settings on the ASI Air plus, aggression, guiding rates, etc. Works better with higher aggression, and 0.90x guiding rate with 1s exposures. Also saw that the last calibration wasn’t that good, and this still didn’t make a difference to what was happening above (the RA axis movement)

5.       Polar Alignment: PA has been done via Drift alignment every time, which reports it to be good

6.       Guiding: I use an OAG right now. Faced similar issues even with a 30mm guide scope with the WO FLT91 and also a WO Z61 - changing the scope and adding a guide scope didn't make much of a difference - and could see the same type of graph for the mount. 

7.       Balance: Balance is good on RA and DEC and even in 3D; I’ve tried East heavy and even camera heavy and this didn’t make a difference

8. Firmware version of the mount and HC is the latest one (it came with the October 2021 version). I don’t have the HC connected when guiding. 

 

Here are 2 guide logs from AAP for review please - click -  https://drive.google...SWO9Dj3cS8ngKyi

Also attached below. 

What else should I be doing to help improve this. I've also written to Ioptron and haven't received a reply yet. 

 

Much appreciated,

Aayush

PHD2_GuideLog_2023-01-21_191607.txt PHD2_GuideLog_2023-01-20_221935.txt

[Clarkey]

I am no expert on mounts, other than the occasional strip down and re-build, but there is a clear periodic error cycle of around 2 minutes in the RA axis. To me this suggests a mechanical issue of some type in the mount. The worm period is around 400 seconds - so it is not that. I wonder if something internal is slightly bent, such as a motor shaft?

[Aayush Nair]

17 hours ago, Clarkey said:

I am no expert on mounts, other than the occasional strip down and re-build, but there is a clear periodic error cycle of around 2 minutes in the RA axis. To me this suggests a mechanical issue of some type in the mount. The worm period is around 400 seconds - so it is not that. I wonder if something internal is slightly bent, such as a motor shaft?

Thanks, this looks like that to me too. iOptron has reached out and asked me to do a star backlash test in RA and DEC, and also check if the star remains on the cross-hair lines and if any deviations are noticed when slewing at 1x speeds. 
Will check this tonight I hope (not very clear right now). 

Much appreciated,

Aayush

[alacant]

Hi

Yeah... For guiding to be effective, the mount has to be better than perfect. Unfortunately, lower end models out of the box usually aren't.

Just one more thing comes to mind which is PPEC for RA at a fixed period of 200s. <--- EDIT. corrected PE.

But as you've tried just about everything else without success, maybe the best course of action would be to pull apart, clean, regrease and adjust.

Cheers and good luck.

 

Edited January 29, 2023 by alacant
200 no 120

[michael8554]

Hi Aayush

I can see you've tried hard to improve RA guiding.

"tracking performance is poor despite good PA."

Poor PA causes Dec drift, not RA, your Dec behaviour is fine.

Looking at the GuideLog, I see that there are two RA problems.

1. The main one is the failure of guiding to correct the cyclic Periodic Error, even at 100% Aggression:

That could be due to tight RA bearings, or a loose RA worm.

The mount has PE correction built in, but may be turned off, or have a poor correction model.

Have you tried training the PE ?

If you run for 10 minutes with guiding off you will see if the PE is as good as the iOptron spec of better than 7arcsecs.

The graph does show 3arcsecs peak to peak, but mat be far worse with guiding off.

2.  A lesser problem is the continuous spikes superimposed on the RA graph, which from other's posts is due to belt-drive problems.

Michael

 

[alacant]

** correction to this post.

I gave the wrong PE.

Apologies.