Quite a long time ago there was a discussion about "FLT110 DDG strange star shapes" with the issue being strange, some what asymeterical spikes showing in bright stars. I noticed this effect recently in someone else's otherwise excellent image taken thru an AP scope, so I asked Roland what would cause this. He gave me such a useful reply, I thought I'd rejoin this group long enough to post this, as it seems to be a curse that might bite any one of us sometime in the future. This info will hopefully help you track down the problem. The bottom line of what he explained to me is;
If every star in an image has some effect, it is coming from the front of the scope (the main aperature lenes), the obvious expample is how the spider on the front aparture of a Newt relector scope causes diffraction spikes on every star.
Whereas if you see some effect like darkening or flare or spike in one part of the frame, but not in another, its a problem with something in the light path further back in the scope towards the camera. The closer to the focal plane, the more localized the effect will be. It could be a little screw head or something protruding into the light stream, that causes a diffraction effect. It could also be a piece of lint on a filter, on a cover glass or on the telecompressor that is throwing up a diffraction pattern locally
Hope this saves some people some time and frustration.