Bad guiding after many years

[Maxastro59]

Hello everyone, I rarely post on this forum because I've almost never had any problems with my PHD2 guiding sessions. However, I have to report some unusual behavior that has occurred in the last two months or so. For many and many years, I've been guiding without any problems using a 60/240 guide refractor with an Orion Starshoot camera (yes, it's old, but it's always worked great) in conjunction with a Takahjashi FSQ 106. Over the years, I've almost never discarded images from a session (70, even over 100) due to stars that weren't guided well. When it has happened, it's been one or two images at most, but rarely. Recently, however, a good quarter of the images in any sessions have been thrown away. There's no warning sign that the guiding isn't going well, especially since the session is remote and I only notice everything the next morning. But at a certain point (with a clear sky, constant seeing, etc.) and nothing to suggest a bad guide, the stars gradually become more and more elongated, then return to normal after a while, but ruining 10, 15, or even 20 exposures. I've always been reluctant to upgrade the PHD2 (it worked fine...) version, but at this point I tried installing the latest available (dev7) and the result remained unchanged. I also recalibrated, used the calibration wizard, and followed its suggestions, but there was nothing to be done. Looking at the log from last night's session (which I'm sending you via the link below), I noticed that at a certain point the guide stopped working a couple of times and then started again. And last night, 14 out of 39 images had to be discarded. And those were just 5-minute exposures.
This is strange because, as I said, I've never touched anything for years and it's always worked great. There were guiding sessions where the corrections were almost nonexistent, and the total error averaged around 0.5". I doubt the problem is related to the mount (but in any case, I also sent the log to 10Micron), and I hope you can understand something from analyzing the logs provided.

My setup:
10 Micron GM2000 QCI mount
Takahashi FSQ 106N - ZWO ASI 2400MC Pro
Tecnosky 60/240 Guide Refractor - Orion Starshoot Camera

This optical setup is side-by-side with another setup using the same mount, which is as follows:

Officina Stellare RC Pro 10" F8 - Playerone Poseidon M Pro
ONAG (On-Axis Guider) from Innovation Foresight - ZWO ASI 1600M

The next test will be to do a session with the RC and see what happens, because the problem has been present for a few months and during this period I have only used Takahashi.

Thanks and best regards

Massimo Bernardi

www.mbernardi.it

https://openphdguiding.org/logs/dl/PHD2_logs_QgYN.zip

[Maxastro59]

Sorry, forgot to mention that the session is under NINA

[Bruce Waddington]

With only a few exceptions, the guiding sessions in your two logs were reasonable and show no evidence of guiding or mount tracking problems.  I think the likely explanation is that you're seeing differential flexure, probably because something in the guiding assembly has become loose or because you have a cable that's pulling or dragging.  You can read about differential flexure in the "Supplemental Info" section of the User Guide  - that section also describes a simple test you can do to confirm that differential flexure is the problem.  

https://openphdguiding.org/man/Supplemental_Info.htm#Differential_flexure

Coming back to the "exceptions" I mentioned, there are a few places that show this kind of behavior (RA in red):

This is a huge excursion, certainly large enough to create elongated stars in whatever image was being acquired.  It was not caused by guiding, nor is it likely that the mount did this.  It's consistent with my earlier conclusion that the guide scope/camera assembly is moving around.  To remind you of the small images scales we are talking about, this 50 arc-sec excursion is equivalent to an unwanted movement of the guide camera sensor by 50 microns - about the thickness of a human hair.

Hope you can track it down quickly,

Bruce

[Maxastro59]

Thanks Bruce, your suspicion actually seems the most reasonable, although this setup has been in place since 2020 and nothing has ever been touched since. I can also reasonably rule out a problem with cables pulling because all the wiring goes to USB hubs and power systems that are positioned on the optical tubes and therefore move with them while the distance between the device and the USB or power port remains fixed. What I noticed when comparing the images from the last two nights (14 rejected in the first and 22 in the second) is that the problem seems to start at a certain time, around 3:20 am, and lasts until about 4:30 am. Since the object being pointed at is always the same (IC1805) and therefore the optical tube is in the same position, it is plausible that at that moment something in the setup slips; this could explain why the images before are guided perfectly. But I can't explain why the later ones are the same, which continue until the end of the session around 5:20 in the morning. If something moves as soon as it reaches a certain position after, say, an hour and a half or so, does it stop and everything goes back to normal? Anyway, I'll visit the Observatory (which is about 150 km from where I live), but tonight I'll do a test that could be decisive. In fact, side-by-side, I have another optical system with a different guide camera and a different setup. I'll aim at the center of the same subject and see what happens. If the images are all reasonably well guided, then the hypothesis of something moving in the other setup would prevail; otherwise, I suppose I need to look at the mount. In the meantime, thanks for the support. PS - I forgot to point out that, since I have the better data since the upgrade to the latest version of PHD2 (dev. 7), I went back to the previous one last night to see if this could have any value, but as I said, the result hasn't changed.

RGDS

Max

www.mbernardi.it