I've been scratching my head with this one and can't figure it out. I ran a sequence on M42 last night and there is some progressive strangeness happening towards the end. The sequence was shot on a Skywatcher ED80 with an ASI294MC Pro.
There's a video at this link that shows a looping blink run of the calibrated and debayered frames. At the end you can see dark arcs appear around the image and they grow progressively to the end of the sequence. Oddly for a couple of images right before the sequence ended the image clears up, then the artifacts return for the last couple of frames. The artifacts are present in the uncalibrated data as well.
The marks start appearing at 7 seconds into this video, just to the left of center of the frame. It's helpful to play it at half speed (gear icon at bottom right, playback speed).
It was starting to cloud over and transparency wasn't great for most of the night. It feels to me like these are cloud-related, but I don't see what could cause them. They don't show up in the flats I took later on.
For reference, here's the timelapse from my all sky camera last night. The first occurrence of the artifacts was at 04:20, and unfortunately M42 is just out of frame at that point. It's still a ways from setting, even if it is quite low - but there's nothing to obstruct it at that time.
Any thoughts? I'm mainly curious as most of the images are good, and cloud definitely negatively impacts the images in question - I just have no idea what would cause those black marks.