Focal Distance?

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.

Davoud

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