Re: [ATM] Field derotator or equatorial?

[Andy Saulietis <iss@pvtnetworks.net>]

Bob May writes,

>Field derotation of a seperate guidescope won't work.  The
>problem is that you will be guiding a single star and if that
>star isn't in the center of the photo, the stars in the photo
>will rotate about the guide star.


Not nessessarily so..if one mounts the guide scope in a rotating tube whose
axis is accurately alined to the axis of the main scope. The
guide scope can then be offset relative to the inside of the tube to
find guide stars. Chuck Shaw and I have both built these
guiders and they do work. Getting the axes alined is not
trivial and the usual problems of flexure of the guide scope
can still be a problem.

I'm designing a variation of this guide system in which
the lens and rotator are fixed relative to the main scope,
and a motorized x-y carriage inside the rotator is used to move the guider
camera in the focal plane to acquire guide stars. One needs a
long f-ratio objective for this approach to increase the usable size
of the guiing field.

One issue with guiders of this type is that the guider axes will rotate
relative to the mount axes, eventually xausing an x error to look like
a  y guiding correction. This effect is corrected for in the Bartels alt az 
controller
software, which keeps track of these 2 axes system and resolves
the error measurements into the appropriate alt-az guiding correction axes.

Another way to fix this issue is to use an x-y guiding carriage inside the
main field rotator to make the error corrections instead of  moving the
entire scope & mount. The x-y corrections can be an x-y motorized carriage
or a tip-tilt mirror guider. Either of these would have a better correction
bandwidth than using the mount axes.

Finally, a better solution to using an external guide scope in an alt-ax-fr 
(or equatorial)
system is to simply have a strap-down (fixed) guide telescope with a guiding
field large enough to guarantee a useable guide star without having to
move the scope or the guide head. S/W would have to be developed to
translate the measured x-y guiding errors into resolved guiding corrections
appropriate to the guide correction system being used. This is not too
feasible currently because of the small size of affordable CCD imaging chips.

All of the above applies only to external guide scopes..if one uses pickoff
guiders on the main scope, you have a different set of problems, such
as finding the guide star and the correction axis rotation in alt az scopes 
with field
rotators.






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