Hello Richard,
I am not an expert in astrophoto at all but there is something that I don't understand in your question: how do you base backfocus calculation on chip diagonal?. As I understand the way newtonians (or any telescope) produces an image, the image is at the focal point whose distance is fixed (for objects at an infinite distance). You can look at this image, even without an eyepiece, with a sheet of paper moving back and forth looking at the moon for example. So backfocus issue is for me a pure mechanical question that has no relation with size of the CCD or CMOS chip. It is related to the position of chip in the body of the camera and the mechanical adapter you use to attach your camera to the focuser, if this distance put the chip beyond the achievable move in of the focuser, you cannot focus ie put the chip at the focal point. The size of the chip is related to the field of view that you will get with different cameras when the chip is placed exactly at the same physical point that is the focal point.?
If my understanding is right you should compare where the chip in your present camera is placed from the edge attached to the focuser with the same measure of you future camera, the size of the chip will change the magnification.
But I might be wrong. Hope an expert will clarify it.
Frederic
Fr¨¦d¨¦ric Ruciak