[blueskies123_89 (George)]

Hi

I've been experiencing this problem for a while - when I focus on the centre of the field, the edge becomes unfocused. I assume this is because the field isn't flat, but I don't know if it should be as severe as what I am experiencing now.

I have attached an unedited 10min sub of M42 in OIII to illustrate the point. The scope is a meade SN8, via baader MPCC and thousand oaks OIII filter. Also attached is an original size crop of the edge of the frame, at the running man region.

The same problem exists without the OIII filter. Without the MPCC I have both the unfocused stars and coma.

Thanks for the help.

EDIT: Camera is Canon 400D. The shot taken at 400ISO (doesn't really matter)