Re: [ATM] Field derotator

[David Whysong <dwhysong@gmail.com>]

> Don Clement <clement.focuser@verizon.net> wrote:
> >No reason an AO system couldn't include a de-rotator, in fact I believe the
> >Keck does just that and was not easy to get working.  If you need to
> >control the image plane that closely, you need 2 guide stars for rotation
> >and those can double for Alt/Az tracking and fast tip/tilt won't help much.
> >At some point it is easier to use an equitoral mount.

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

(Just as a point of interest, the Keck AO system DOES account for the
rotatior position, as it rotates the pupil relative to the wavefront
sensor. Since the Keck mirrors are not circular, the pupil is not
invariant under rotation.)

Mel Bartels <mbartels@bbastrodesigns.com> wrote:
> This is where a single off-axis guide star with calculated guiding rates
> will work as well as 2 guide stars without the calculation.  See the
> previous images that I posted.

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.

I have considered writing autoguider software that tracks multiple
stars simultaneously in order to improve the guiding S/N, but that's
different...

> In any event, the 3rd "derotating" axis can be built into the mounting
> design in a surprisingly large of ways.
> 
> 1. rotate in imaging software

That isn't a good general purpose solution, as it effectively prevents
you from imaging near zenith (at lowest airmass). Larger cameras have
more stringent limits to exposure time (and where you can point) in
order to avoid noticable field rotation.

> PS Remember that field rotation technically occurs as you track near the
> horizon due to refraction, even in an equatorial mount.

Yeah, but it's slow, and who in their right mind images at such a
large airmass anyway?

-- 
David
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