Re: ATM Alt/Az Autoguiding

["Tom & Lou Krajci" <krajcit@3lefties.com>]

>From:    Chuck Shaw <cshaw@GHG.NET>
>
>Folks that use an Alt/Az system know the problems that field rotation can
>cause for astrophotography.  Installing a field rotator that de-rotates
>the
>camera/focuser takes care of that, but to autoguide you need to de-rotate
>the autoguider also on an alt/az mount or you get a translation >error that
>gets introduced.
>
>We took a quantum leap ahead in capability this weekend with our 32"f/4 we
>have at Danciger, Texas. We installed an early design for a >guidescope
>rotator, and it seems to work quite well!!!    We instantly went from
>shooting 15-30 sec unguided shots with the 32"  and only >keeping between
75
>to 80% of them to being able to shoot 4 minute shots and keeping 100% of
>them!!!!
>
>These initial images were not great, but the alignment of the guidescope
>rotator was not really tweaked in, the seeing was terrible (shooting >thru
>sucker holes in the clouds), the settings on the ST-4 were not optimized
for
>the conditions, the bugs were carrying us off, and it was 80 >deg and 100%
>humidity (in other words only a slight below average night on the Houston,
>TX Gulf Coast!<G>).  We are looking forward to >getting the system tweaked
>in and seeing what an autoguided 32"f/4 can really do!!!!!!

Have you evaluated your main scope/guide scope rig for differential flexure?
One way to do that is to put a crosshair eyepiece in each scope, and point
at a star near the zenith.  Adjust so both scopes have their crosshairs on
the same star.  Now...move to different parts of the sky and see how much,
and in what direction your differential flexure is.

This may be a source of trailing, even in four minute exposures (especially
considering the size of the main instrument), so it's worth analyzing this
aspect of your system's performance.  (I'm ashamed to admit, but in early
attempts with my puny, six inch, f/3.75 astrograph and guidescope...there
was an amazing amount of differential flexure...I can only wonder what a 32"
scope and guidescope flexure will be like.)

Hmmmmm, if your flexure is repeatable/smooth...you may be able to model it
with software and cancel it out in the autoguider.  Paul Mortfield wrote
some software that I've tinkered with that lets your PC talk to the
ST-4...and tell it to shift the desired pixel value that the ST-4 is trying
to correct towards.  Paul initially wrote the software so you could ST-4
track on a star, yet compensate for the motion of a comet that's too
faint/fuzzy to autoguide on.  I've used it to experiment with correcting for
differential flexure.  Initially it's tedious, but it can work.

Hope this helps,
Tom Krajci

PS.  One of these days I hope to have a rig like you guys are developing!