As you know from a previous post concerning collimation, I'm using an Orion 8 Astrograph like you. After one my latest sessions, I noticed in the images that the diffraction spikes of the spider vanes were not perpendicular.
I checked the scope during daytime over and over again and the spider vanes are perpendicular in the tube, just one was a little tilted (which was easily fixed with the knobs to tighten them), but they are perpendicular.
I've been breaking my head about this until I saw a picture of your setup with the Orion 8. You use a dovetail that is much longer than the one that comes with the scope, and so the distance between the tube rings is much wider than in my setup with the default Orion dovetail; in my case with the DSLR, coma corrector, guide scope and guide camera all at the top, the tube has been shifted completely forward in order to obtain balance. So I start to suspect that in some positions (I was imaging M52, which at the time - mid december - was almost in the zenith) there is some torsion on the tube; and seeing you're Orion 8 setup, a longer dovetail might overcome this. Now my 2 questions:
- what dovetail are you using
- with a longer dovetail the camera can no longer "hang down" in the home position - which is said the best position to obtain balance, so how do you obtain balance.