I use a colour camera and am convinced its gama that causes onion rings.
I did a test on 3rd April on Jupiter in steady 6/10 conditions, on one avi I used 12 gama (10 gama is minimum with my DFK) which isn't very much and the image was ruined with onion rings.
All the other avis with minimum gama had no sign of the onions. Exposure in all cases was with the histogram almost at maximum.
http://southcelestialpole.org.au/for...ic,7504.0.html