Re: ATM Aluminum, Split Ring, Astrograph

[Andy Saulietis <iss@pvtnetworks.net>]

Hi Dave,

>I am not against the FrAltAz mount, I just haven't been able to convince 
>myself that it works for me.  I take very long (up to 2 hour) exposures on 
>medium format film using an autoguider.  I live in LA and must drive to a 
>dark site to use the scope.  The best part of the sky for film photography 
>is near the zenith, but I would be willing to give up a 20 degree region 
>around the zenith if I could prove to myself that an open-loop field 
>de-rotation algorithm was sufficiently good to get the job done.  By 
>open-loop I mean 
>one that uses only one autoguider, not two.

We are currently experimenting with a 32" scope
that has field derotated off axis guiding (separate
guide scopes) that only needs one single star
guider. See Chuck Shaw's prototype setup
at http://www.ghgcorp.com/cshaw/
The 32" also has its own field derotator.

I recently built a smaller guide scope derotator
for Al Kelly's 17.5", this one uses thin
section ball bearings and a 3" offsettable guide
scope inside the 4" dia rotator, which
is boresighted to the main scope's derotator.

We'll be testing Al's scope & gude scope derotator
at TSP.

Both these scopes use the tangent Alt-az-fp drive system,
but this technique would work as well on the Bartels
drive system, which Chuck Shaw is putting on his 14"
and I'll eventually install on another 32", currently
located.

Al's scope is 'portable' (if you're his size..:) but
the 32's definetly are *not*. Doing imaging with
large alt-az-fr scopes is definetly doable but not
a trivial exercise.

Andy 

Andy Saulietis
ISS Enterprises
39 Silver Fox Trail
Mayhill NM 88339
505-687-3067 voice
505-687-3021 fax, call voice or e-mail to set up
e-mail: iss@pvtnetworks.net