Stabilization of the sodium beacon: a proposal to achieve full-sky coverage for laser guidestar adaptive optics

Darryl J. Sanchez

doi:10.1117/12.742175

18 October 2007 Stabilization of the sodium beacon: a proposal to achieve full-sky coverage for laser guidestar adaptive optics

Darryl J. Sanchez

Author Affiliations +

Proceedings Volume 6711, Advanced Wavefront Control: Methods, Devices, and Applications V; 671109 (2007) https://doi.org/10.1117/12.742175
Event: Optical Engineering + Applications, 2007, San Diego, California, United States

Abstract

This document proposes a method to achieve full-sky adaptive optic coverage using a laser guidestar (LGS) adaptive optics (AO) system. This is done by stabilizing the sodium beacon by using information from the uplink beam so that the downlink tilt, i.e. the atmospheric tilt, can be measured. It is commonly known that tilt cannot be measured from a laser guidestar beacon because the uplink and downlink beams experience the same atmospheric tilt. This is true in itself, but disregards information that can be obtained from the uplink beam. Because the atmospheric structure function is a maximum near the exit aperture and decreases exponentially with altitude, the uplink beam accumulates most of its tilt near the exit aperture. For instance, when using the Hufnagel-Valley atmospheric model with A = 1.7 x 10^-14 and W = 21, ≳ 92% of uplink atmospheric tilt occurs at altitudes below 5Km and 97% by ∼ 20Km. So, the instantaneous position of the uplink beam along the uplink path can be a predictor of the instantaneous position of the sodium beacon; this is this paper's main result. Further, a means of measuring tilt from the uplink beam from its Rayleigh backscatter is presented and for some of the parameters and geometries considered, it is shown that the uplink tilt can be measured at sufficiently high altitudes and camera frames rates that a control system can stabilize the uplink beam, which in-turn stabilizes the sodium beacon, thus allowing atmospheric downlink tilt to be measured. Assuming that such a control loop can be implemented, such stabilization would allow for full sky laser guide star operation with AO performance similar to current LGS AO systems using off-axis natural guide stars.

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Darryl J. Sanchez "Stabilization of the sodium beacon: a proposal to achieve full-sky coverage for laser guidestar adaptive optics", Proc. SPIE 6711, Advanced Wavefront Control: Methods, Devices, and Applications V, 671109 (18 October 2007); https://doi.org/10.1117/12.742175

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KEYWORDS

Cameras

Adaptive optics

Sodium

Telescopes

Sodium guide stars

Atmospheric modeling

Laser guide stars

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