Atmospheric refraction sounds quite likely too.
I don't do a lot of imaging due to my limited setup, I've no drives and 70mm gives limited light capture but I try what I can. I should also add that I attach the camera using the afocal method through a good eyepiece and not the cheap one that came with the scope. I would also think that light going through all that glass (scope - eyepiece - multiple camera elements) would have an effect due to ad-hoc alignment. Can't really say without trying a camera in the prime position (no eyepiece or lens).
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