Re: [ATM] Field derotator

[gregj888@comcast.net]

David;
Think those quotes are mine, just to protect Mel and Don.  Derotation is another axis and has all the same issues as any other.  We know how the sky moves, but tracking requires a closed loop, below some error level.  The easiest way I know would be to track 2 stars in the same field.  As always, it depends on how tight your requirements are...  If the error limits are high enough for a given derotator, running it open-loop and keeping a single guide star centered will be fine.  
You're right, AO is not really relevant here...  
As near as I can tell, everything on the Keck is tied in...:-).  The clear aperture of their derotator is larger than many of our telescopes and is an impressive optic.  It's also too large to move at any speed (as in AO), but am sure it will track to required levels.  I was lucky enough to get to see/talk to a couple of their engineers a few years ago about my AO project.  The only real question they asked was how I was planning to mount the scope.  When I said equitorially, they replied "GOOD."  I took that to mean derotation had provided some challenges.  I did not ask them to elaborate.
Greg Jones

From: David Whysong <>

> Don Clement <clement.focuser@verizon.net> wrote:
> >No reason an AO system couldn't include a de-rotator, in fact I believe the

David
Keck has a rotator of course, since it has an alt-az mount. But AO
really has no bearing on this discussion.


Mel Bartels <mbartels@bbastrodesigns.com> wrote:
> This is where a single off-axis guide star with calculated guiding rates

David
I can't imagine why anyone would want to use a second guide star for
this purpose, and I've never heard of it being done.
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