Re: ATM Aluminum, Split Ring, Astrograph

[Aplanatic@aol.com]

Clive wrote:

> >>>
>     I am quietly surprised that nobody has suggested an
>  Alt/Az/Fr mounting as a more efficient alternative to
>  the traditional split ring in this instance.
>     Surely it is sufficiently proved as a viable, real
>  world option as to not be found wanting further
>  validation...?
>  <<<

Mel responded:
  
>  It's surely been validated by the pros for decades, and by amateurs in
>  the past few years.  I certainly recommend it as more flexible and
>  easier to make than an equatorial, particularly in larger sizes.  The
>  field rotator in particular turns out to be very straightforward to
>  make.
>  
>  But as to efficiency, that also encompasses difficulty of concept and
>  of construction, and here, I think people tend to fall into one camp
>  or the other, and not cross over between the two so easily.  So, if
>  someone starts talking equatorial with me and asks questions, I often
>  will not mention the "fraltaz" mount.

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.

The autoguider pick-off will be 3" from the farthest corner of the film and I 
want images in this corner that are trailed by no more than 0.001".  Thus, 
over a two hour exposure I want less than 1/3000 radian of 
uncompensated field rotation, i.e., less than 1.5 arcminutes error over 
a 2 hour exposure.

I have, over the years, played the game of simulating the field rotation 
error under various assumptions of mount and optical axis 
non-orthogonality, imperfect star calibrations, etc..  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.

I know that with a well made split ring equatorial I can expose for two 
hours, 20 degrees from the pole, and have untrailed star images.  I 
have done it time and again.

If you can show me how to do this with a portable FrAltAz, I'm all ears.

Thanks,
Dave Rowe