Chris
Download the CCD calc from here
It works very well. You just need to know the pixel size for your camera.
It is 3888 x 2592 with pixels of 5.7 microns for a 400D
I was trying to figure out if an f/3.3 reducer would be ok to use on my 8" SCT with the 400D attached, and comparing that FOV to the 300mm APO Sigma lens im getting this week!