You may find this interesting re the central obstruction sizes:
https://www.cloudynights.com/topic/5.../?fromsearch=1
Basically astrograph can be 40...50% by aperture,
SCTs at 30...35%
Maks around 23...32%
Mak Newtonians well below that.
There's another thread from someone testing celstrons interferometrically who stated they're all marginal to very ordinary and he has yet to see one with straight bands.