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It is very hard to differentiate between field curvature and field rotation in your shots. To check for minute field rotation the best way is to shoot wide field in Ha so you get tiny stars and pick a guide star right in the corner of your frame and look at the opposing corner. On a 20min subs you're bound to see any rotation for sure. Mike BJ (sculptor) has done the maths on field rotation and he has developed a technique of misaligning the scope by a couple of arc seconds from the pole in a line that is perpendicular to the DEC axis of your scope. He routinely does 3h subs with pinpoint stars in narrowband. I'll have to dig the old email he sent me at the time and post it when I find it.
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