Well, a DSLR is not really the tool for this kind of work, but your good self (& Malcolm) are really wringing the DSLR's neck in this respect, so great job guys.
The ringing is *basically* caused by under exposure. On a webcam, you'll have way better tools at your disposal to control this. Waveletts only
accentuate what is there, but not the cause. There is no 'one thing' that causes onion ringing. It's a combination of things hence my *basic* answer
I recommend the next step to be; pulling the trigger on a webcam if you truly want to get serious about planet imaging.