I recently purchased a used ASI1600MC-Cool and have been running into some issues. No matter what spacing configuration I tried, I ended up with some crappy stars at the edges. The middle of the frame was sharp, but as you got to outer half of the image the stars gradually get worse. I do have a set of pretty cheap calipers, but they aren't easy to read, so I resort to measuring with the calipers and then holding them next to a ruler with millimeter markings. A little crude to say the least.
Yesterday I got desperate and made a trip to Starizona to see if they could help me out. The scope in question is my Vixen ED81S with the dedicated .67 reducer, which requires 63.5mm of back focus. After a few minutes they got me squared away with the back focus and I tried again last night. I imaged the Double Cluster which was pretty close to zenith when I stared imaging, and much to my displeasure, the stars still were crap at the edges. Pictured below is my current imaging train as well as the star field. Any suggestions would be greatly appreciated, thanks.
-Mike