[Martin Pugh]

Hi all. Thanks for the replies.

So the lens is an ASKAR 200mm f4.....allegedly designed for astrophotography with a backfocus of 55mm, M48 output thread.

I have of course adjusted the optical line up to compensate for the differences in the 2 cameras. I was initially using a Cyclops Optics UV/IR filter and that put the focus position significantly past the infinity mark. When I then put in a thicker Antlia 3nm Ha filter, the focus position moved closer to the infinity mark as you would expect (given that filters extend, or push out the backfocus), but still a good distance away.

Today I put on a much smaller sensor - ASI1600.....no filters, just a direct connection with M48 spacers to exactly 55mm. The lens came to focus very, very close to the infinity mark. So there is something very wrong with the specs on the ZWO camera...buyer beware.

Thinking that tilt and whatnot was the cause of the rather poor stars I am seeing, I used the smaller sensor to eliminate this. Now interestingly, when I mounted the QHY600 and ZWO6200, I aligned the camera to north. The stars, even on axis, have a teardrop shape. I didnt bother to align the ASI1600 today, and the first image off the camera following focus shows the teardrop shaped stars have rotated....indeed, when I rotated the lens/camera assembly to match the others, the teardrop shaped stars matched.

So if the 'optical defect' rotates with the lens, it has to be an optical defect and...it cant be a tracking problem (all of the test images are 10 sec unguided).

cheers
Martin