Mike, maybe it's a case that underexposure contributes to the intensity of onion ringing, but I've got some avi's that I've captured with the light meter in K3CCD reading between 230 and 255 (with some burnout in the white bits) and they Still have the rings. As Asi mentioned I think the problem with Registax quality sorting is it's not based on shape so much as sharpness so you can have sharp but badly distorted frames contributing to the stack and I can well imagine this would setup rings around the edge. When my LXD75 is not well balanced I also get a jittery image which might also be a factor. Only way to really test this is of course going through these avi's meticulously culling the out of shape ones and then retry.... need a spare couple of hours
cheers,