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iOptron CEM40 - strange guiding fault

  • Posts: 212
  • Thank you received: 39
Hey Max,

I had the mount out last night but did not notice any "crazy" oscillations like you experienced. Very windy but still managed sub arc 0.7 arc sec guiding. The only problems I had were from the failure to park after polar aligning. Pressing "go to home" returned the mount to the park position but I think it seems that one needs to tell EKOS that it is parked by setting the parking in "site management tab" otherwise it cannot calculate and perform an automated flip it seems. So just another thing to remember to do. It would be nice if the mount parked after doing PA though :).
Due to the extraordinary cloudy weather over the last few months this is the first time I have been able to get the mount under the skies. Hope it lasts!

Cheers
iOptron CEM40G, SharpStar CF90IIApo with Moonlight focuser, ASI294MCpro, ASI120MM-S, DIY MyFocuserPro2, RPi4 8Gb 480SSD running KStars/Ekos/Indi under Ubuntu Mate 22.04, DIY flat box
Skywatcher Flextube 12" Dob, Meade LX75 8SCT, Skywatcher NEQ6Pro

2 months 5 days ago #83448

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So, I removed the flat ZWO USB3 cable from the through-the-mount channel and routed it outside the mount.
Aesthetically it doesn't look bad at all.

Testing the setup right now.
Guess what? Everything is going great.
Meridian flip went smoothly.
Guiding is better than ever. And it's getting better as it gets darker. (10:40pm right now).
Just a more or less flat line without any run-aways or oscillations. Around 0.5" RMS total.
Cloudless and quiet night. But the Moon is there. Bortle 8.

So, that flat but still thick/stiff USB3 cable was making too much disturbance in the through-the-mount channel.
Problem solved. Shame on iOptron. The through-the-mount cable management worked a generation ago.
Finally my CEM40 is working well. Hope it will stay this way.

Max S.

RPI 4 B (4Gb) with SMate
Askar FRA400, TPO RC6, Nikon D5500 full spectrum (with IDAS D1 clip-in filter).
ASI2600MC Pro
ZWO ASI120MM Mini on ZWO 30F4
ZWO ASI224MC-S on Orion 50 mm f/3.2
ZWO EAF x 2
SW AZ-GTi
SW HEQ5
RST-135
iOptron CEM40G
Last edit: 2 months 5 days ago by maxthebuilder.
2 months 5 days ago #83464
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Hey Max,

Guess what. I have just had the crazy oscillations that you reported. The mount was guiding fine up to the meridian flip. The mount flipped as expected then automatically re-aligned, re-focused and re-calibrated the guiding. All this appeared to go OK. The first image was taken then after the first dither the oscillations in RA stated. This eventually responded to reducing aggression in RA in the guiding options and reducing prediction ratio to 0.1 (from 0.5). This settled it down but then after the next dither it started up again so I tried reducing the max adjustment down to 0.1 from the default. This seemed to help but the next dither off it went again albeit less dramatic. Eventually I changed to one pulse dither and this seemed to stop it. Maybe it was unable to reach the set dither after the flip so the one pulse does not have a set target dither amount. I didn't have time to explore if setting all the other changes back to how they were would make a difference but I will try next time. I might leave one pulse dither on from now on though. If your oscillations occur again try just changing to one pulse dither as it seemed to have the greatest effect of all the changes that I made. Out of interest I have also noted that the SEP focus algorithm does not seem to detect any stars so I have reverted to gradient. Have you noticed anything similar?

Cheers
iOptron CEM40G, SharpStar CF90IIApo with Moonlight focuser, ASI294MCpro, ASI120MM-S, DIY MyFocuserPro2, RPi4 8Gb 480SSD running KStars/Ekos/Indi under Ubuntu Mate 22.04, DIY flat box
Skywatcher Flextube 12" Dob, Meade LX75 8SCT, Skywatcher NEQ6Pro

2 months 3 days ago #83495

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Do you have a screenshot (or log) to share? "Crazy oscillations" could have many different causes. Max's were quite specific - for some reason (maybe usb related?) the guide algorithm lost the ability to lock onto the star (SNR went to zero).
2 months 2 days ago #83537

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Hi Ron,Thanks for the interest. Unfortuanately I did not take any screen shots and the weather has closed in again. I have attached the guiding logs. Everything was fine until the meridian flip which on the logs is line 2145. The guider did a recalibration but at the next dither it takes much longer to complete the dither. The oscillation were in RA mainly. During this period I was making on the fly changes to RA aggression and predictive components. The next few dither had a similar prolonged adjustment period then the settling to normal post dither behaviour coincided with the change to one pulse dither. After which I ran out of time. I dont remember any issues with loss of SNR etc but the guiding log is the only log I have. I will need a bit more time to explore if this behaviour repeats.
Cheers
Mike

File Attachment:

File Name: guide_log-...2-08.txt
File Size:196 KB
iOptron CEM40G, SharpStar CF90IIApo with Moonlight focuser, ASI294MCpro, ASI120MM-S, DIY MyFocuserPro2, RPi4 8Gb 480SSD running KStars/Ekos/Indi under Ubuntu Mate 22.04, DIY flat box
Skywatcher Flextube 12" Dob, Meade LX75 8SCT, Skywatcher NEQ6Pro

2 months 2 days ago #83552
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I had several sessions since that weird guiding experience (and I had some other strange guiding issues).
Everything was working fine without any issues.
As I mentioned before my problem was related to a USB3 cable fed through the mount.
Once I removed that cable and routed it outside the mount all guiding problems that I had disappeared.
With no clouds and no wind I get around 0.5" RMS total which is pretty good in my books.
Max S.

RPI 4 B (4Gb) with SMate
Askar FRA400, TPO RC6, Nikon D5500 full spectrum (with IDAS D1 clip-in filter).
ASI2600MC Pro
ZWO ASI120MM Mini on ZWO 30F4
ZWO ASI224MC-S on Orion 50 mm f/3.2
ZWO EAF x 2
SW AZ-GTi
SW HEQ5
RST-135
iOptron CEM40G
2 months 1 day ago #83558

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Hi Mike,

It looks to me like that 2nd calibration failed in some way. The mount appears to have not moved at all during the first West step and it winds up thinking that the RA only moves at 1.7 arcsec/second. So, when it sends a correction to the RA that correction is way larger than really necessary. So it overshoots, then overcorrects in the other direction and you are off to the races.

Why the new calibration after the flip? Are you using ST4 for guiding?
2 months 1 day ago #83571

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Hi Ron,
Thanks for looking at the log. Your analysis is very valuable. I am guiding with ASI120 using a OAG via the mount not ST4.
I have always recalibrated after a flip but have never had this oscillation issue afterwards. I don't want to over-react about this as it has only happened on the one night. I will just monitor the situation once the weather improves and I can get out under clear skies. It may be just a one off issue related to the failed calibration. I was not monitoring the re-calibration in real time but I will when I retest. If it appears to be a persistent issue I will start another thread as it now appears different to Max's problem.
Cheers,

Mike
iOptron CEM40G, SharpStar CF90IIApo with Moonlight focuser, ASI294MCpro, ASI120MM-S, DIY MyFocuserPro2, RPi4 8Gb 480SSD running KStars/Ekos/Indi under Ubuntu Mate 22.04, DIY flat box
Skywatcher Flextube 12" Dob, Meade LX75 8SCT, Skywatcher NEQ6Pro

Last edit: 2 months 1 day ago by Spartacus. Reason: Extra info
2 months 1 day ago #83577

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On recalibration after the meridian flip.
I also always recalibrate after meridian flip.
My other three mounts (AZ-GTi, HEQ5 and RST-135) would guide terribly without recalibration.
I think I did try reusing calibration with CEM40 and was not happy with the outcome.
Will give it another try.
Max S.

RPI 4 B (4Gb) with SMate
Askar FRA400, TPO RC6, Nikon D5500 full spectrum (with IDAS D1 clip-in filter).
ASI2600MC Pro
ZWO ASI120MM Mini on ZWO 30F4
ZWO ASI224MC-S on Orion 50 mm f/3.2
ZWO EAF x 2
SW AZ-GTi
SW HEQ5
RST-135
iOptron CEM40G
2 months 1 day ago #83578

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Interesting. As I understand, the primary thing learned by calibration is "how far does the mount move per second of guide pulse?". Why would crossing the meridian change that?. It does change as you move to different declinations, but the software calculates that effect.
2 months 12 hours ago #83589

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Hi Ron,
Like Max I have not been happy with reusing calibration due to worse guiding results. I suppose I always thought that the moving of the counterweight to the opposite side changes the backlash characteristics and this might change the guiding. Hence the need to recalibrate.
As I do quite a lot of my imaging without closely supervising the mount, I figure that it is better to be safe than sorry. Doing a recal seems like a small price to pay for peace of mind.
Anyway I will see if this was a one off or something that is repeated once the clouds clear.
Thanks for looking at the logs :)

Mike
iOptron CEM40G, SharpStar CF90IIApo with Moonlight focuser, ASI294MCpro, ASI120MM-S, DIY MyFocuserPro2, RPi4 8Gb 480SSD running KStars/Ekos/Indi under Ubuntu Mate 22.04, DIY flat box
Skywatcher Flextube 12" Dob, Meade LX75 8SCT, Skywatcher NEQ6Pro

2 months 10 hours ago #83592

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I did try to reuse the calibration after the meridian flip last night.
It didn't work well. The DEC was running away.
I think, Mike's idea about CW being on the opposite side is right.
In any case, that's another topic and we can close this thread since my problem is solved (was related to a USB3 cable fed through the mount).
Thanks!
Max S.

RPI 4 B (4Gb) with SMate
Askar FRA400, TPO RC6, Nikon D5500 full spectrum (with IDAS D1 clip-in filter).
ASI2600MC Pro
ZWO ASI120MM Mini on ZWO 30F4
ZWO ASI224MC-S on Orion 50 mm f/3.2
ZWO EAF x 2
SW AZ-GTi
SW HEQ5
RST-135
iOptron CEM40G
1 month 4 weeks ago #83595

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