ATM Alt/Az Autoguiding

["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!!!!  Now, the camera cooling and skyfog and blooming on dim tiny 
little stars are our upper limits!!!! (yeah, I know all you guys that shoot with fork 
mounted or GEM mounted scopes don't have these problems, but a 32"f/4 scope does not lend 
itself easily to those type mounts!!!)

Al Kelly used his CB245 to image M4 and NGC 5490 (a tiny little fuzzy near Arcturus),  
and Ed Grafton setup the ST-4.  It took all of us to get things going this first time, 
but we are QUITE excited!!!!

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!!!!!!

I took a few snapshots of the guidescope rotator system I built,  and I wrote up a short 
article with a couple of snapshots for those that are interested.  Its at:

http://www.ghg.net/cshaw/guiderot.htm

I have the links screwed up that go from the thumbnails to the larger photos, and its 
getting late here, so I will wait to fix that till tomorrow, but you can get a good idea 
of what things look like.

If anyone else is autoguiding an alt/az mount I would like to swap thoughts with you.  I 
know Berthold Hamburger has made a derotator for a piggyback 35mm camera on his alt/az 
system and has his film at the developer......

Next stop is a slightly different and more compact design to use on my portable 14.5"f/5 
newtonian that is driven by Mel Bartel's wonderful scope.exe software!!!

See ya,

Chuck

Chuck Shaw
cshaw@ghg.net                   (home e-mail)
cshaw@ems.jsc.nasa.gov    (work e-mail)
http://www.ghg.net/cshaw  (S4:  Stars, Sailing, Spaceflight, & Stuff)