Re: Suddenly stars are dashes with good guiding

Geof Lewis

It sounds like differential flexure to me. Have you check that the guide scope hasn’t worked loose from the main OTA, it won’t need much and you probably won’t even see it. Guide performance may well be nigh perfect, but if the guide scope is slowly shifting about then you’ll see that in the images taken through the main OTA.

From: <> on behalf of Chris Carlton <pselaphid@...>
Sent: Monday, July 13, 2020 9:07 pm
Subject: [ap-gto] Suddenly stars are dashes with good guiding
So, I have experienced an odd problem, to me anyway, the past two nights of imaging. I doubt it is related to the mount, but I won't rule anything out and wanted to run it by the group here before presenting it elsewhere. I have used basically the same setup for a year and a half since I got my 1100AE and before with other mounts. I use PhD2 for guiding through a 400 mm guidescope mounted on the focuser of my TEC 160fl. I guide through the RJ12 guide cable and Starlight xpress camera. PA using Polemaster. Pretty basic set up that has worked well for me with round stars and decent guiding depending on seeing. 

I was completing an otherwise successful image run of planetary NGC 6781 when stars went from round then elongate, then dash shaped and unusable through a series of about three 5 min. exposures. I checked the usual suspects....cable tangle, loose connection, recal PhD2, redo PA, etc. Odd thing was, guiding was very consistent, based on the graph, at 0.27-0.35 rms both before, during, and after the observed problem. The images looked like unguided images with PA drift. I bumped the OTA and watched the guide graph spike and then settle right back to where it was, so it was working and responding to upsets. I've had much worse guiding during bad seeing, up to around 0.80 rms, which is about my limit, and still had round stars....fat, but round. And it happened rather suddenly. It was late, so I quit.

Next night I moved to my dark sky site and set everything up again, this time paying close attention to getting everything set up correctly. I had great guiding and good results for a while, then the same thing happened, just as I was finishing up. Still, quite good apparent guiding at 0.35-0.40 based on the guide graph, but elongated stars. I went ahead and finished the session. I don't know if it's relevant, but both times it happened at around the same elevation, around 50 degrees under warm, muggy conditions thanks to our current heat wave. I had planned to gather a bit more data last night, shut off guiding to check for obvious drift, save some example frames, and a log file. I found another guide cable and had planned to switch them out. But a storm blew through and foiled plans. 

I'll bounce this off the PhD forum and maybe the brain trust on Cloudy Nights if necessary. But I wanted to exclude anything obvious other than guide performance that might cause it. This group is probably just as good with guide problems anyway. Why is guide performance good, apparently, but stars are bad? If something as simple as a bad cable is responsible, why is guiding indicated as good or excellent on screen? I hope to be back up there in a couple nights to troubleshoot and perhaps resolve the issue.

Sorry for the long explanation, but here is a picture to look at while you think about it:

Chris Carlton

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