focus

[cardconvict]

hi i don't know if anyone can help me, i have just noticed on my c9.25 that when i'am doing images that in the corners the star look like donuts like they are out of focus but i take it that they are and it is only in the corners is this normal for this type of scope?

[BlueAstra]

If you're using it without the focal reducer, then you may get reduced performance at the corners of the FOV of a large CCD chip. I was told once by a chap from SX cameras that a SXVF-H9 chip (9x6.7mm) was about the largest you could use on a C8/11 without a FR.

You could try focussing in the corners and then examining the centre focus quality. If this shows good corners and bad centre then you have field curvature. Or try again with a FR if you are not using it.