On Sat, 25 Sep 2004 09:54:57 +0000 (UTC), (Rob Johnson)
wrote:

How does a wider field of view help an autoguider? Don't I first have
to find a guide star and center the guide scope on it? Once the guide
star is in the center of the autoguider's CCD, I don't see how a wide
field of view helps. If I used the autoguider to find a guide star, I
can see how a wide field of view would help to locate it; but once it is
found, a wider field of view would only seem to reduce the sensitivity
of the autoguider to movement. Am I missing something?

A wide FOV can reduce or completely eliminate the need to aim the guidescope.
For example, my 200mm focal length guiding system is fixed to the main OTA. With
a FOV greater than one degree on a side, there are always suitable guide stars
available. A fixed guidescope eliminates many of the sources of flexure that
exist between it and the main OTA- a primary source of differential movement
when guiding.

I image at a scale of 0.8"/pixel, and guide at 7"/pixel. The guiding accuracy is
ultimately determined by the S/N of the guider image. Any cooled CCD camera
designed for astroimaging or guiding should easily be capable of producing an
image where a stellar centroid can be measured to better than 1/10 pixel
precision- probably much better. So this kind of scale ratio between the imager
and guider is perfectly reasonable.



By the way, I looked at the Cloudbait Observatory site. I am suitably
impressed. You have some very nice equipment. Putting together a
computer controlled, autoguided scope using a photodiode, a computer,
and a CCD in the late 1970s is astounding.

Thanks. I've always enjoyed the instrumentation side of astronomy as much as the
observation side.

_________________________________________________

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