I have a new 8" f/4 imaging newtonian, and a new Baader MPCC coma corrector that I use with it. Last night I acquired some images that contained a relatively bright star (~mag 3) for the first time with this gear. In the images I see a prominent third diffraction spike, and I am trying to determine what causes it. I went back and looked at some other images with the same gear from a few weeks ago, and I see a trace of this extra spike on the image of a mag 7 star, but not on the image of a mag 3 star aquuired a few days earlier. So it appears that something "happened" a few weeks ago.
I have examined the equipment - no spider webs in the OTA, no scratches on the primary mirror, no residue I can see on the MPCC. There is a small (3mm) blob of something stuck to the primary mirror about a half inch from the edge, but nothing else I can see. The only glass between the imaging sensor (a Canon T2i DSLR) and the secondary is the coma corrector.
I am suspecting the MPCC. Should I be looking at something else?
The star is Propus, in Gemini. All subexposures contain the extra spike.
Edited by DonR, 08 March 2015 - 12:25 PM.