Paper: astro-ph/0508402
Date: Fri, 19 Aug 2005 18:59:38 GMT (101kb)

Title: First tests of wavefront sensing with a constellation of laser guide
beacons

Authors: M. Lloyd-Hart, C. Baranec, N. M. Milton, T. Stalcup, M.
Snyder, N.
Putnam, and J. R. P. Angel

Comments: 25 pages, 7 figures, uses aastex.cls, to be published in the
Astrophys. J
\\
Adaptive optics to correct current telescopes over wide fields, or future
very large telescopes over even narrow fields, will require real-time wavefront
measurements made with a constellation of laser beacons. Here we report the
first such measurements, made at the 6.5 m MMT with five Rayleigh beacons in a
2 arcmin pentagon. Each beacon is made with a pulsed beam at 532 nm, of 4 W at
the exit pupil of the projector. The return is range-gated from 20-29 km and
recorded at 53 Hz by a 36-element Shack-Hartmann sensor. Wavefronts derived
from the beacons are compared with simultaneous wavefronts obtained for
individual natural stars within or near the constellation. Observations were
made in seeing averaging 1.0 arcsec with 2/3 of the aberration measured to be
from a ground layer of mean height 380 m. Under these conditions, subtraction
of the simple instantaneous average of the five beacon wavefronts from the
stellar wavefronts yielded a 40% rms reduction in the measured modes of the
distortion over a 2 arcmin field. We discuss the use of multiple Rayleigh
beacons as an alternative to single sodium beacons on 8 m telescopes, and the
impact of the new work on the design of a multi-sodium beacon system for the 25
m Giant Magellan Telescope.

\\ ( http://arXiv.org/abs/astro-ph/0508402 , 122kb)