Re: guiding relays vs scope controls

The reason this can be bad is because Windows is not a
realtime operating system, and the timing between the two commands can
end up quite different from what the guide correction duration should
actually be. Another problem is that the commands can get fragmented
during transmission, creating a timing error. This is particularly <<

This is very interesting and explains some of my observations.
With the ST-4 I use a very old Compact laptop (about 15 yrs old) in
the DOS mode with CCDTrack. I never had any problems. Ive tried newer
laptops in the Windows mode and coulden't get any performance
reliability.







Chris L Peterson wrote:
On 07 Sep 2006 23:49:28 GMT, Pierre Vandevennne
<pierre@xxxxxxxxxxxxxxxxx> wrote:

I seem to have problems with the autoguiding relays of my sbig camera.
Haven't really objectivated the issue but it shows up with two different
mounts using two different cables, so I guess there not much room for
doubt.

I have therefore switched my guiding from guider relays to telescope
control. Is the performance going to be markedly different? So far,
everything is fine, but I am using a very short focal length (540mm) and
have had to cut the aggressivness in half, all other things being equal.

There are two ways the guider logic can work. The "bad" way, which is
very common, is that the guider software sends a command to start a
correction, and then after some amount of time it sends another command
to stop it. The reason this can be bad is because Windows is not a
realtime operating system, and the timing between the two commands can
end up quite different from what the guide correction duration should
actually be. Another problem is that the commands can get fragmented
during transmission, creating a timing error. This is particularly
possible if the transmission interface is networked: Ethernet or USB
(including serial over either of these).

The "good" way of sending guide commands is to send a single command
containing the direction and the time. This can't get fragmented, and
isn't dependent on Windows for timing. Newer mounts increasingly have
this type of guiding built into their interface.

AFAIK, SBIG cameras don't manage guider timing internally. That means
that they use the first sort of control, with all its possibilities for
error. Whether you will do better using scope control depends on whether
your mount supports internally timed guiding. If it does, your
performance may be better. Otherwise, it should be pretty much the same
either way.

_________________________________________________

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

.

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