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20" Dob Coma
I've been using my 20" skywatcher dob sparingly over the last few months due to the rather crappy Brisbane weather, but I've noticed some odd and fairly pronounced coma becoming evident when the scope is pointing close to zenith. I normally collimate it at around 45 degrees, and it holds collimation well all night, apart from the coma, which disappears again when the scope moves out of the vertical. Before I start ginning about with it, does anybody know what might be causing this? I'm assuming it'll have something to do with the primary seating, but any other theories would be appreciated.
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