On Jun 29, 9:38*pm, 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.

Davoud

Hi
That is true but there is a problem. The normal focal point
of an optical telescope is about the end of the focuser when
at about mid point in adjustment.
The CCD is about 1 inch inside of the camera.
This means the focuser needs to adjust enough
to compensate for the camera mount plus the depth
of the mounting of the CCD in the camera.
This is one of the reason that SCT are often
used for camera work. The focal point can be moved
over a long range.
One could also use a Barlow but that also significantly
changes the effective magnification of the telescope.
Not that the focuser on a SCT doesn't change the
magnification, just that it is a smaller percentage
change for the same amount of change in focal
position.
Dwight