Re: ATM Aluminum, Split Ring, Astrograph

[Aplanatic@aol.com]

I wrote:

> >>>
>  I have never been
>  able to convince myself that one can do much better that 10 arcminutes
>  field rotation error over a two hour exposure that passes through the
>  meridian 20 degrees from the zenith.  I may not have found THE
>  ALGORITHM that does the job, so please correct my misconception.
>  <<<

Mel responded:
  
>  Ah, but it does exist!
>  
>  Check out Chuck Shaw's work along with Marty who is doing the
>  software.  With a $19 quickcam autoguider attached to a piggyback
>  scope that sits on an altazimuth mount, the Houston group has been
>  getting very good long exposure results.

How good is good?  A piggy back guide scope is a HUGE problem for 
long exposure photography with a 2 meter focal length instrument.  
Each arcsecond of flexure between the guide scope and the main 
scope causes 10-micron trailing in the photograph.  But that is another 
story.  I assume that software can be written that allows for off-axis 
guiding, i.e., using a pick-off mirror and an autoguider through the main 
scope, so that the troubling problem of differential flexure is essentially 
eliminated.

My problem is with the field de-rotation algorithm itself in light of errors 
in calibration induced by a) unmodeled defects in the mount, b) inaccurate 
measurements of star locations during the calibration process, c) flexure 
of the optical axis during calibration and photography, d) errors in the
drive system.

As I have said, this is one tough problem, but I will take a look at the 
references that you provided.  Before investing in the huge amount of time
needed to make a high quality FrAltAz mount, I must be quite sure that
it will provide the accuracy that I'm looking for.  Yes, a split-ring 
equatorial
is bigger, heavier, and less contemporary, perhaps, but I know from the
outset that it will satisfy my accuracy requirements.

Thanks,
Dave Rowe.