CCD imaging and secondary size

I am needing a longer focal length telescope and have been wanting to use my home made 8" f/7 newt for CCD imaging with an ST10. It presently has a 1.52" secondary and with the turned down edge mask installed it delivers super planetary views.I am confident that with a rigid spider it would prove to be a great imaging platform.


I will be machining the focuser and spider myself so I can exercise quite a bit of leeway with the dimensions. My dilemna is sizing the secondary large enough to fully illuminate the ST10 while minimizing the loss of contrast from a large CO. After running my numbers in Newt 3.0 I come up with a secondary size of 1.83" The resulting fully illuminated area just fits within the fov of the ST10 at this focal length. I realize this is adequate but am concerned about possible edge defects with secondary mirrors. Would it be better to go up to the next size or stay with the 1.83"?

Design Parameters:

8" f/7
tube od 10.5" (sonotube)

focuser height 2.5"
travel 0.75"
extra height for CCD/cfw 1.0"

ST 10 fov=24.5' x 36.4' @ 1400mm fl, 1.0"/pixel resolution

Thanks,

Mike

You didn't mention how wide your edge mask is. This will change the numbers slightly (making the diameter a bit smaller and the f-ratio a bit larger - for example, if you have a 1/4" edge mask, it becomes a 7.5" f7.46). This will make the 100% illumination zone a bit bigger than you calculated. I would stick the with 1.83". As for edge defects, rather than getting a larger secondary and masking it, just get a higher quality 1.83" secondary.

Jarad