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I never thought of sag.
When I have my DSLR on the back with a wider FOV I notice the stars are much worse in the corners and elongation tends to be more fanned out in different directions. A flattener may help with this but I always had a sneaking suspicion something else is at play.
Is there anyway to diagnose flop, maybe rotating the camera to see if the orientation of the elongation changes?
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