On Mon, 30 Jun 2008 04:38:09 GMT, 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.

What you are saying seems so obvious that I'm not positive just what it
is you are asking g. If I understand correctly, however, then I'd
agree with what you are saying. It's a bit confusing, though, to talk
about "focusing", because that can mean different things with different
optics. For example, many SCTs are focused by adjusting the spacing
between their primary and secondary mirrors; in that case, the distance
to the focal plane with respect to the rear port is not fixed. But with
a rigid optical system, where focusing just means positioning an imager
so it is aligned with the focal plane, your assessment that seems
(obviously) correct. You could, for example, construct a set of
parfocalizing rings so that different imaging devices could be swapped
in and out with no need to ever refocus the instrument (in theory,
anyway; in practice I doubt many scopes are mechanically stable enough
to be entirely focus-free).
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com