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iOptron CEM40 - strange guiding fault

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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).
1 year 9 months 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
1 year 9 months 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
ZWO AM5. RST-135. AZ-GTI. HEQ5. iOptron SkyTracker.
TPO RC6. FRA400. Rokinon 135 and other lenses.
ZWO ASI2600MC. D5500 modified with UVIR clip-in filter.
ZWO ASI120MM Mini x 2. ZWO 30F4 guider. Orion 50mm guider.
ZWO EAF x 2.
1 year 9 months 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?
1 year 9 months 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
Last edit: 1 year 9 months ago by Spartacus. Reason: Extra info
1 year 9 months 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
ZWO AM5. RST-135. AZ-GTI. HEQ5. iOptron SkyTracker.
TPO RC6. FRA400. Rokinon 135 and other lenses.
ZWO ASI2600MC. D5500 modified with UVIR clip-in filter.
ZWO ASI120MM Mini x 2. ZWO 30F4 guider. Orion 50mm guider.
ZWO EAF x 2.
1 year 9 months 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.
1 year 9 months 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
1 year 9 months 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
ZWO AM5. RST-135. AZ-GTI. HEQ5. iOptron SkyTracker.
TPO RC6. FRA400. Rokinon 135 and other lenses.
ZWO ASI2600MC. D5500 modified with UVIR clip-in filter.
ZWO ASI120MM Mini x 2. ZWO 30F4 guider. Orion 50mm guider.
ZWO EAF x 2.
1 year 9 months ago #83595

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