ATM (no subject)

[GregJ888@aol.com]

Date: Sun, 08 Nov 1998 23:47:05 -0800
 Mel Bartels <mbartels@efn.org> writes--

>I have been told that SBIG's 'adaptive optics' unit on the 3.5 meter WYN
scope
>made significant improvement - at least 50% in resolution.... clip...

Andy Saulietis  writes--
>Maybe we should take a moment to define terms here, to me 'adaptive
>optics' ...clip...

Both are right, I would call the SBIG unit an AO system within the frame work
of these discussions, correcting only 2 Zernike terms, tip/tilt.  You can even
argue that it has a single aperture Shack Hartman sensor if you really want
to, and since it is the only thing close to an AO unit available to us...
Take a look at the paper--

    Roddier, Northcott, Graves-- "A Simple Low-order Adaptive Optics System
for Near-Infrared Applications"; Astronomical Society of the Pacific, Jan 91,
pp 131-149

Think you can download it from-

        http://adswww.harvard.edu/article_service.html 

Or go to ASP's website and follow the links.

The graph showing performance relates D/Ro  to normalized Strehl ratios. There
D = clear aperture, and Ro= Fried's parameter.  For D/Ro<3 tip/tilt is almost
as good as  9-Zernike correction.  Peak for Tip/tilt only is at about D/Ro= 4
and for Diffraction limited photography is always required.  Tip/tilt is
always the largest component up to D/Ro=30 (largest value graphed).  5-Z terms
peaks at D/Ro= 6 and 9-Z terms of correction peaks at D/Ro= 8.

If we assume for a minute that Ro = 4" for us visual- flat landers (this is
the figure of merit number I've been using, just remember it's pulled out of
the hat) then less than 12"-16" tip/tilt is all that is needed and at 20" it's
a lot of bang for the buck.  So the first question is, what is the real value
for Ro?  If it's 2" that means one thing and 10" quite another.  

Another tidbit from the graph, at CFHT with a D/Ro=4 the difference between
normal uncorrected and Diffraction limited imaging has a detection level
difference of 3 stellar magnitudes.

A very interesting paper... let me know what you think.


Greg Jones