Russell Wallace wrote:
On Wed, 10 Dec 2003 12:10:53 -0500, Andrew Yee
wrote:

"HST is the only telescope on Earth or in space that can do this with the
required precision right now," Benedict said.

I'm curious - what prevents a ground-based telescope with adaptive
optics from being used for this? I remember reading that, for example,
the Keck telescopes can get diffraction-limited images with adaptive
optics, which should make their resolution four times as good as
Hubble's; or is it more complicated than that?

Small-field astrometry has been exploited magnificently in the
Galactic Center field. Other applications run into lack of suitable
reference stars within the field of good image correction. What's
worse, the point-spread function from AO doesn't just get bigger
as you look farther away from the reference object, the first-order
effect is that it becomes asymmetric (mostly elongated radial to
this distance). This comes about because a single adaptive element
can correct for phase scrambling at one atmospheric level, getting
progressively worse for distortions produced at some other level.
The buzzword for going beyond this is "multiconjugate adaptive optics",
which may require something like a hexagon of laser guide stars
around the direction of interest. There is a Canadian group, in
particular, trying to get this working (on a decade timescale?)
for instruments on Gemini.

Bill Keel