Notice oval stars in upper right right/left corners, while bottom half doesn't show this behavior or not as pronounced. Is this flexure in the imaging train? Or could it be loose focuser? Regardless, my take is why wouldn't one get flexure after adding this much backfocus, even if it wasn’t a loose focuser. I doubt this is field curvature since that should have been all around and equal. While we are on the subject I would like to know how much of field curvature would/should there be in an APO? I presume it will be much less compared to SCTs?
In short, I am looking for your analysis of this image and seeing if my conclusion is correct (i.e., it being a flexure in the long awkward imaging train, primarily, and/or possible lose focuser, secondarily). Note: I’ll do more tests after tightening things (primarily the focuser, since I had the extensions/camera well tightened for the image attached).
Image specs are as follows:
•17x3 min (guided)
•50 min integration
•Canon 60Da/TOA-130/Paramount ME
•Full frame (no cropping)