First Tests of Wavefront Sensing with a Constellation of Laser Guide Beacons

Abstract

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 ′ 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 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 ′ 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.

Tomographic reconstruction of stellar wavefronts from multiple laser guide stars

“…Wavefront recoveries using this approach, under seeing conditions of ~ 1.1 arcsec (r 0 = 10.1 cm at 500 nm wavelength) in September 2004, are described in detail in our earlier paper 17 . In June 2005, we obtained additional data in seeing of ~ 0.5 arcsec (r 0 = 22.5), at the 15th percentile for the site measured over the past two years.…”

“…al. 17 which examines the anisoplanatic behavior of the stellar wavefront with respect to the individual LGS wavefronts. The RMS difference between the two Beacon 1…”

“…For a full description of the laser beacon and tomographic wavefront sensing systems, we refer the reader to previously published accounts 17,18 . Earlier work on laser-guided AO has been confined to a single beacon, usually created by resonance scattering in the sodium layer at 90 km altitude 2,19,20 .…”

Exploiting Adaptive Optics with Deformable Secondary Mirrors

“…Wavefront recoveries using this approach, under seeing conditions of -1.1 arc second, are described in detail in Lloyd-Hart et al (2005). Since then, we obtained additional data in seeing of -0.5 arc second (ro = 22 cm at 500 nm wavelength), at the 15th percentile for the site measured over the past two years.…”

Laser guide star projection for large telescopes

“…Moreover, each such pixel provides information on a separate turbulent volume and they are not mixed. 4. Height imaging, relaying each section along the beam (and turbulence volume) onto a different pixel of the detector and using it as a separate beacon 6 .…”