Hey Dennis, so sharp you could shave with it!
If the onion ringing is due to under-exposure what does the K3CCD light meter indicate when you're capturing (I usually sit around 150-180).
I actually run with the brightness fairly high at the expense of gain. I'm still playing around with this, but am still a little wary of too much gain = grain - but then it depends a lot on seeing an number of frames I've got to stack.
beautiful shots!