I notice that when I focus in or out slightly, the star shapes change from the comet shape to the crescent shape at the edges of the frame. The shapes change consistently so it's not tilt, and my OTA is collimated. When I saw the comet shapes I thought okay too close add some space. I changed the 5mm spacer at the front of my train (that threads onto my reducer corrector) to an 8mm spacer. Then I got crescent shapes when the stars were slightly out of focus. When I ran the autofocus routine in NINA, it completed, then the stars became comets. Has anyone ever experienced this?
Yet another backfocus question
The in focus star shape is the relevant one to judge your backfocus. If you shift the plane of focus while you are close to the proper backfocus what you experienced can happen. Instead of just trying out you could ask the vendor or the community for the expected backfocus of your setup. For sure you will need to increase backfocus further based on what you told us.
Good luck and clear skies
Wolfgang
Good luck and clear skies
Wolfgang
Concise
Don't trust star shapes regarding back focus. For the "simple" case where tilt and all other parameters are fine, you can check your backfocus by this simple method:
Center a fairly bright star and focus it. Move the star into a corner. Check if you need to move the focuser in or out in order to get the star focused. If you need to move it in, your working distance is too short and you need to add a bit. If you need to move the focuser out to get the star in focus, you're too far away from the reduer/flattener and need to get closer.
Björn
Center a fairly bright star and focus it. Move the star into a corner. Check if you need to move the focuser in or out in order to get the star focused. If you need to move it in, your working distance is too short and you need to add a bit. If you need to move the focuser out to get the star in focus, you're too far away from the reduer/flattener and need to get closer.
Björn
Helpful Concise