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I noticed that I'd gone on auto pilot and had rotated my OAG about 90 degrees which is what I think was causing that shadow on the right. Hadn't really thought too much about that, because that's where a gap in the amp glow on the ASI 1600 is, so it's usually darker there, but it shouldn't have been that dark on subs this short. I'm usually careful not to put the OAG there because I know it obscures the sensor a bit in that position.
So, I fixed that before I did anything. Then I verified that the flip-flat can cause similar issues by shining my torch on the flip-flat panel and seeing reflections come up in SGP.
So, not sure whether it might have been light reflecting off the prism on the OAG, or the flip-flat, or maybe both, but I'm hoping it was one of the above. I've removed the flip flat because I was planning on using it, essentially, as an automated lens cap when I start going "semi-remote" but I don't need it right now. Fingers crossed I can actually get some usable data tonight.
Thanks again everyone!
In other news, looks like I've mostly fixed my "tilt" - focuser collimation. Thanks to Colin for his posts on how to sort that out. Previously it was bad enough that I'd get as bad as 4" stars in one corner, while another was 2.5". Attached is FWHM map (in pixels, not ") from this same data (about 30mins of L data integrated). Looks like the tilt is almost gone but I still have what I assume is some curvature from not-quite-perfect flattener spacing. Still, a big step in the right direction!
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