Re: Adaptive Optics

From: Don Bruns (dbruns_at_stellarproducts.com)
Date: 09/30/04 

Date: Thu, 30 Sep 2004 00:03:19 GMT

Mitch Alsup wrote:
>
>
> But even if you have plenty of photons and therby a great S/N ratio for WF
> detection, the servo* tracking speed is necessarily slower than the speed
> at which the atmosphere moves/jiggles in the visual part of the spectrum.
> I don't understand it (why) all even after reading it a dozen times. But
> when someone with the credentials of R. Wilson states that it won't happen
> I tend to believe them even when I can't figure out exactly why.
> >...
> My point was and is that even the guys with big scopes believe that you
> will never correct visual wavelengths to diffraction limted seeing. However
> down in the NIR region, the atmosphere moves slower, and this enables the
> corrections to be practicable.

The turbulence that generates the wavefront errors is directly related
to the wind speed, so it is independent of wavelength. The reason that
the time constants are shorter in the visible region is that the "seeing
cell" size is slightly smaller, so that the cell moves across the
aperture in a shorter time. The servo bandwidth is indeed much smaller
than the sampling speed, but that is usually taken into account in a
good design. (Note that "visible" often means "wavelengths recordable
by a CCD", or out to about 950 nm, compared to the 1.5 micron or 3
micron band where expensive infrared focal planes are required.)

Large observatories might not be interested in visible adaptive optics
for many of the reasons already stated - it is much more difficult and
costly. In addition, since time is so precious on a large telescope,
the potential adaptive optics system needs to be designed for the worse
case observing so that no time is wasted. Designing any kind of system
for worse case operation makes it even more difficult. For
amateur-level adaptive optics on smaller telescopes, the trade-off is
that the system can be designed to work only on the best 25% of the
nights. On those nights where seing is good, an adaptive optics system
can do diffraction-limited imaging at relatively low cost.

Don Bruns