Thanks Rob and Rodney for comments :-)
As to distance from flattener... My setup is C11 and FF-FR.
I believe the image Rodney supplied is applicable to refractors only..
After hours spent playing with varying sensor to FF-FR distance by +/- 10mm, and using both Celestron and Meade FR, there was no significant differences in behavior that I could detect. So my conclusion was it was FR thing, and sudden increase in field curvature away from centre (both C and m Meade behave almost the same way), in the area already affected by vignetting (~6mm away from sensor centre).. So I decided to crop out the corners of the frame and keep only central square where those aberrations are not visible.
https://www.iceinspace.com.au/forum/...75#post1558975
Generally there is not much info on the subject out there.. just a lot of confusion..
Funny thing, the best results re filed curvature I obtained with my "own" flattener - 50mm doublet (200mm FL) that came from old russian binoculars, but the coma and CA is slightly increased. (BTW, I have thread on this subject on forum, link is
here.
Anyway.. at the moment I put this project (tilter) on hold for a while, but I will revive it when my circumstances allow.