One of the key skills a new deep sky astrophotographer will need to develop is to be able to identify and correct problems with their telescope imaging train. The main way we do this is to look at the stars in the image and note if they are nice and round and pinpointed or if they’re deformed in some way. Deformed or misshapen stars is a key symptom of an underlying problem with the image train. Here are a few key symptoms of the main problems that can be encountered.
Problem: The stars in the image are all stretched equally and deformed in the same direction across the entire image.
Possible issues: Your mount is not properly balanced in both axis (RA and DEC) or there were serious wind gusts during the imaging session or there were problems with auto guiding and / or the optical tube or guide scope has flexure and deformed slightly during imaging. You’ll need to check mount balance at a minimum and review your auto guiding during the session.
Problem: The stars are stretched unequally and only in the corners of the image.
Possible issues: This could be a problem where the focuser is sagging or the camera sensor is tilted / askew from the axis of the image train. Check that your camera is properly aligned to the image train.
Problem: The stars in the center of the image are round but are equally deformed / stretched as you move away from the center.
Possible Issues: Check the back focus; if the stars are deformed radially outward, back focus distance is too long. If the stars are deformed radially inward, the back focus distance is too short.
Problem: The stars appear to trace small arcs, forming a circle.
Possible Issues: Here, you have a field rotation problem. The cause for this issue is an error with the mount’s polar alignment. You’ll need to redo the mount polar alignment.
Problem: The stars look like little comets or commas.
Possible Issue: This is a problem mainly seen with reflector telescopes and astrographs containing mirrors and it is due to problems with collimation. You’ll want to recheck and redo the collimation of the optical tube.