TheSkySearchers

I've been using my OSC cameras on a Stellarvue 80mm which is piggybacked on my C11" in my observatory for 2 years. I usually do anywhere from 90s-5 minute guided exposures depending on the goal. I am using the Stellarvue flattener and the Optec autofocuser as always and backfocus of 55mm from the rear of the flattener to the camera sensor.. Here is an image using the ASI533 and the stars are tight and round all around the image as best my eyes can tell.




Now, switching to the ASI1600MM, I took 5 minute guided frames with a Ha filter and I noticed issues in 3 of the 4 corners with the stars. While seeing was below average, the guiding was working well. I re-measured the backfocus and it is correct- 55mm. Rotating the camera didn't change much and when I tried the 0.8x flattener/reducer (image attached), I got the same result. At this point, I am assuming camera tilt problem. Does that sound correct? If yes, has anyone tried the ZWO tilt adjuster? Otherwise, I need to crop which I hate to do.
Bruce

What's the chip physical size difference between the osc and the asi1600?, I have a few tilt rings, but I find adjusting my focuser tilt or secondary mirror good enough, I can save on space for back focus using this method, this is sounds undoubtedly like tilt error that you have

Somebody else ran my data on CCD Inspector, I believe, and found a 15% downward tilt.

The ASI533 OSC is:
Diagonal 15.968mm
Pixel size 3.76µm
Image area 11.31*11.31mm

The ASI1600MM is:
Diagonal 21.9mm
Pixel Size 3.8μm
Image area 17.6mm*13.3mm

Ahh I see the asi1600 is bigger and will show more collimation, alighnment error towards the edge of the chip. Does your focuser have collimation screws?

unfortunately not- guess I will just crop a little to get the worse edges removed

Tilt issue resolved once I replaced the compression rings with all threaded connections.

Great news Bruce. Good job of detective work and problem solving.

Congratulations!!

Steve