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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usenet *at* davidillig dawt com

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

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

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