Re: Autoguider correction frequency


Woodwind
 

Bob,

Two questions - and thanks for your comments. I am the guilty one for setting this hare running.

Would not the the best of both worlds be a system that allows you to select both the exposure time and the frequency of exposure. That will give you the chance to minimise loss of definition of brighter stars, but also avoid over-correcting the mount. Alternatively, the mount makers could allow you to choose settings to filter corrections and eliminate very small inputs attributable to poor seeing and other "noisy" variables.

The other question is whether the Orion SSAG and similar cameras can be used as a "CCD" imager. The .05sec exposure setting indicates that it might be a very useful CCD subsitute - even if it is actually a CMOS device as someone pointed out on this thread.

Murray

Bob Piatek <bobtek@...> wrote:
This is an interesting thread. Though I've gotten in late on the
discussion, a few points come to mind:

1) Although a guide camera may be able to take exposures as short as
20mS, this will result in the system trying to 'chase the seeing' as
others have pointed out. Long exposures of 1 second or more will
average out the perceived position of the guide star due to seeing
conditions. The problem with longer exposures is that you will tend
to saturate the image sensor when using bright guide stars. A
saturated guide star image will cause problems with the guider
software routines that compute the star centroid when measuring star
position. One way around this is to use a shorter exposure but to
average several pictures together before computing the guide star
position. That way you don't saturate on the bright guide stars and
also average out the seeing induced variations. Not many guider
software programs provide for this type of operation, however.

2) Auto-guiding is a closed loop system in that a measurement of the
guide star's position is made and then a correction is sent to the
mount to correct for any error. The process repeats indefinitely or
at least until the sun rises. The key premise here is that you cannot
make a guide correction faster than you can see the result. Otherwise
you have an unstable system. So, if you send a guide correction to
your mount, you better wait for the telescope movement to complete
before taking another picture. If you try to take a picture of your
guide star before the previous guide correction has finished, you will
erroneously conclude that the last correction didn't correct for the
error and you will probably do the wrong thing. Most telescopes have
a lot of mass and can't respond very quickly. That is what AO systems
promise. They can respond faster since they are moving an optical
element which has far less mass than your telescope mount.

3) Auto-guiding attempts to correct pointing problems regardless of
their cause. These can be any or all of problems with polar
alignment, periodic error, flexure, etc. So, if for instance you
have poor polar alignment, this will be detected by the auto-guider
and a large correction will be sent you your mount. Better polar
alignment will result in less drift between guide star pictures and
smaller corrections being made. Your stars will get progressively
rounder with smaller and smaller guide corrections. Likewise, auto-
guiding will correct for periodic error in the mount's drive train but
it would be better to have the mount's drive electronics perform the
periodic error correction. It is, after all, a very predictable error
and is best handled by the drive electronics. The auto-guider would
just try to make a correction after the error has occurred whereas the
drive electronics would be pro-active and not let the error occur in
the first place. The moral of the story is that there is no
substitute for addressing the sources of error where they lay. Your
auto-guider will have an easier job and your pictures will show better
results. I always say... ' the best auto-guider correction is one
that doesn't have to be made'.

4) I'm the designer of the Starfish Guide camera and, yes, it uses the
same image sensor as the Orion Starshoot Autoguider. Apart from using
the same image sensor the two cameras are distinctly different as the
following references point out:

http://www.fishcamp.com/starfish_shootout.html

http://www.astrophotoinsight.com/node/162

Regards,

Bob Piatek

fishcamp engineering
105 W. Clark Ave.
Orcutt, CA 93455

http://www.fishcamp.com
TEL: 805-937-6365
FAX: 805-937-6252

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