Unfocused stars

[blueskies123_89]

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)

[multiweb]

That's exactly what it is: big field curvature. You need a flat(ter) field to be in focus all across the camera sensor if it's large enough. I'd check your MPCC spacing first.

[blueskies123_89]

Thanks, a few questions:

1) I thought MPCC was a coma corrector, not a field flattener.
2) How do i change spacing on it, I only bought the MPCC, not the set. Do I need to get the set of spacers and stuff?

Thx

[multiweb]

The MPCC is primarily a coma corrector but it will significantly flatten your field too. In my measurments I got a 20% improvement on my newtonian in field curvature with the MPCC. Ideally the back of the MPCC (glass surface) should be 55mm +/- 1~2mm from your sensor plane. You can get different spacers to achieve the correct distance that works for you.