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Misshapen star images from decentered element ?
I have suffered misshapen star images ever since I purchased a 12 inch F5 scope as a Dob. from Andrews in Greystanes,( Not his fault).
I also replaced my mount and auto guider at around the same time but kept shifting the blame for the elongated star images between the mount and guider. Peering down into the scope I noticed the mirror cell looking offside. Removing same I found the center mark on the mirror was off-center by a couple of mm, and came to the conclusion that the manufacturer 'cleverly' stuck the sticker, not by measuring the center of the mirror, but in order to place the mark in the center of the scope, by measuring the center of the entire mirror assembly, hence, in this case placing it off-center on the mirror.
So,if anyone else suffers misshapen star images like I, you might consider this same possibility in your case.
On Monday the mirror assemble goes back to Andrews for, hopefully, a replacement of the mirror cell containing the mirror, that was welded off center.
On page 166 in 'Astrophotography for the Amateur' by Michael A. Covington one can see just such an image with the wording: "Misshapen star images from a lens with a decentered element. This looks like a guiding problem but isn't."
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