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A good book to loan or buy is Star Testing Astronomical Telescopes: a maual for optical evaluation and adjustment, Suiter, published by Willmann-Bell Inc 1994-2001.
It is a comprehesive manual and will solve this issue for you. I have found it to be most interesting.
Another consideration, and one that is covered by the book; is the possibility that you may have pinched optics. That said though there is no way of knowing without comparing your images with that in the book.
Best of Luck.
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