Davoud wrote:
To me this is obviously true, but I need it confirmed or refuted: for
any given optical train behind a telescope the focal distance -- the
distance to a CCD or other flat electronic detector -- must always be
the same if the telescope is in focus. If I remove a CCD camera from a
'scope and replace it with an SLR I have to refocus only for the
purpose of moving the SLR's detector surface to the same distance from
the back of the telescope as that of the CCD.


Differences between the detectors is likely to cause grief with this as
well as manufacturing tolerances. As an example... the coverslip of the
CCD detector on an SBIG camera is going to move the focus. And you
should also be aware that the faster the optical system the more
critical this small amount is going to be.

Are you trying to avoid having to refocus ? Personally I think you'd
have much better results with a knife edge focuser set up to match the
DSLR. Or if your focuser is precise enough you could measure the
difference between the two cameras and then just use that difference as
an offset.

Bill