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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1523688" data-attributes="member: 68323">There are other posts on the forum about this ... it's diffraction from the knife edge of the roof not being as sharp as it could be.You only really see it on very bright point (or line) sources: minimaglite with the top off at 4m in a dark room is a good test. Real stars. The sun bouncing off a ball bearing or other CONVEX surface (don't try it unless you know what you are doing!).I just tried the minimaglite test and I see the diffraction spikes in the Swifts. Not huge but it's there (I've seen worse and better (none in the best case))</blockquote>