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maybe check that the spider vanes form straight lines from one side of the ota to the other (check by eye with a ruler for reference). If the secondary is slightly offset from the centre, you may get spiders on each side of the secondary that are not parallel with each other and will give two different diffraction spikes (eg the vertical vanes in the rough illustration attached).
cheers Ray
Last edited by Shiraz; 27-03-2023 at 08:55 PM.
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