which barlow for an autoguider?

I am considering a 4" f/5 refractor (Celestron Wide View 102) as a
guidescope using a Pictor 216XT CCD camera as an autoguider. What is
the best focal ratio to use with the 216XT, which has a pixel size of
10 microns? I know that the Airy disk at f/5 is about 6.7 microns in
diameter, but I have a 2x barlow which would make it 13.4 microns. Do
autoguiders, the 216XT in particular, do better with star images smaller
than a pixel, about the same size, or bigger? Should I go without a
barlow, use my 2x barlow, or get a 3x (20.1 micron star) or 5x (33.5
micron star) barlow?

Thanks for your advice.

Rob Johnson
take out the trash before replying

On Fri, 24 Sep 2004 15:04:10 GMT, (Rob Johnson) wrote:

I am considering a 4" f/5 refractor (Celestron Wide View 102) as a
guidescope using a Pictor 216XT CCD camera as an autoguider. What is
the best focal ratio to use with the 216XT, which has a pixel size of
10 microns? I know that the Airy disk at f/5 is about 6.7 microns in
diameter, but I have a 2x barlow which would make it 13.4 microns. Do
autoguiders, the 216XT in particular, do better with star images smaller
than a pixel, about the same size, or bigger? Should I go without a
barlow, use my 2x barlow, or get a 3x (20.1 micron star) or 5x (33.5
micron star) barlow?

You cannot autoguide unless the star image covers several pixels. If your optics
and seeing are really such that the Airy disk is determining your resolution
(which I doubt) you will want to defocus the image very slightly. I would guide
at the shortest focal length possible. The 500mm you are considering is good-
you can even go shorter and get the benefit of a wider FOV. There is certainly
no need to increase the focal length with a barlow. (I guide with a 200mm focal
length scope.)

_________________________________________________

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

In article ,
Chris L Peterson wrote:
On Fri, 24 Sep 2004 15:04:10 GMT, (Rob Johnson) wrote:

I am considering a 4" f/5 refractor (Celestron Wide View 102) as a
guidescope using a Pictor 216XT CCD camera as an autoguider. What is
the best focal ratio to use with the 216XT, which has a pixel size of
10 microns? I know that the Airy disk at f/5 is about 6.7 microns in
diameter, but I have a 2x barlow which would make it 13.4 microns. Do
autoguiders, the 216XT in particular, do better with star images smaller
than a pixel, about the same size, or bigger? Should I go without a
barlow, use my 2x barlow, or get a 3x (20.1 micron star) or 5x (33.5
micron star) barlow?

You cannot autoguide unless the star image covers several pixels. If your optics
and seeing are really such that the Airy disk is determining your resolution
(which I doubt) you will want to defocus the image very slightly. I would guide
at the shortest focal length possible. The 500mm you are considering is good-
you can even go shorter and get the benefit of a wider FOV. There is certainly
no need to increase the focal length with a barlow. (I guide with a 200mm focal
length scope.)

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?

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.

Rob Johnson
take out the trash before replying

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

In article ,
Chris L Peterson wrote:
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.

Thanks for the advice.

At the f/5 focus of my 4" spotting scope, the 3.36mmx2.42mm chip in the
Pictor 216XT will only cover 23'x16'. So, it sounds as if I won't need
a barlow for guiding with that setup. However, I will keep the TeleVue
5x barlow for planetary photography. It gives a nice sized image of the
planets with my Rebel hooked directly to my 8" scope, and it shouldn't
be overpowered by the 14" scope that is coming in a couple of weeks.

Rob Johnson
take out the trash before replying