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I was just about to post the same comment that Ray made. The guide star determines how the rotating field looks. The rotation is around the guide star. You might want to keep that in mind if you are comparing images from night to night and have not used the same star.
That thought leads naturally to a question I've not seen considered. Just how important is it to select the same star from night to night (or year to year!) in terms of image registration? I imagine the effect would be smaller when polar alignment is quite good, but it's always present to some degree. Two different guide stars will give 2 different fields of rotation. This might preclude perfect registration. Obviously the effect must be quite small to vanishing or we would all be concerned with this I suppose.
Peter
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