I'm getting something similar from my SCT, and I'm not using a field flattener. With/without the light pollution filter I get it. Was wondering if it's due to my Canon sensor being big, 24x36mm maybe (5Dm2)?
I've been looking through recent posts on the forum where someone might have has the same, but all I can find is coma problems in Newtonians? And here a come corrector lens is suggested Arif, not sure if it will help your problem in a refracting telescope?
(Click here for some info)
Advice/suggestions it would be appreciated.
Attached is a low res of what I get. Mine also seem to be worse upper right corner, although upper left also sucks.
(Unprocessed)