That’s astigmatism and collimation can’t fix it.
As per above it’s possible one of the elements is under strain, from a clamping ring, retaining ring or lens cell that is too tight. This can happen to the corrector, the primary, the secondary.
If you have been tinkering with the scope think carefully what you touched and whatever it was, loosen it.
Retaining screws should not be tight, and if you are worried they’ll fall out, secure them with a dab of women’s nailpolish (use black, red looks hideous on a scope).
Secondly, the thermal expansion of glass and metal is different. In particular the corrector should be sitting on paper or cork shims that act as little spacers, for two reasons - to stop the glass making contact with metal, and to allow gaps so that the tube can expand and contract without squeezing the corrector laterally. If it does, the image will be degraded - I have seen this in a few SCTs where to owners did not understand the purpose of the shins and renived them - and screwed down the retaining ring tight to the point I was surprised the glass had not cracked.
I’ve also seen one where someone decided it was a good idea to pack the space behind the secondary mirror with wadding (probably to stop it rattling). Mistake; the pressure (while seemingly tiny) was enough to ruin star images.
Edited by luxo II, 20 September 2021 - 07:32 AM.