The optimum distance is 4", but it's not critical, especially for deep-sky
imaging, where the limiting factor is tracking/guiding accuracy, and the
"magnification" is low. It's fairly common to have a long train of
equipment on the back of the scope- focuser, filter wheel, focal reducer,
etc (and a focal reducer makes things worse by compressing the light cone
beyond it, so the primary/secondary spacing is decreased even more). In
fact, SBIG's AO-7 can increase sharpness considerably by providing much more
accurate guiding, even through it adds a few inches to the optical path
length.
-John

>
>Thanks John,
>The camera is a canon 300d (aka digital rebel in the US), I'll go ahead
>and get a T ring, I wasn't sure if the threads on the 2 adapters with
>the flip mirror were T-threads, but having them so makes life much
>easier.
>
>Of course the next question is, what is the optimum distance between the
>chip (film) surface and the back of the OTA (easier then measuring the
>focal length). I remember reading a while back that if the chip was too
>far away, then you can induce spherical aberration (presuming you can
>still focus it). I wonder does it really make a difference or is it a
>purely mathematical argument??
>
>Clear Skies
>Dave Lillis.

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