Measuring line-of-sight sodium density structure using laser guide stars

Author:

Hellemeier Joschua A12,Bonaccini Calia Domenico2,Hickson Paul1,Otarola Angel3,Pfrommer Thomas2

Affiliation:

1. Department of Physics and Astronomy, University of British Columbia,6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada

2. European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-8046 Garching bei Muenchen, Germany

3. TMT International Observatory, 100 West Walnut Street, Suite 300, Pasadena, CA 91124, USA

Abstract

ABSTRACT The performance of adaptive optics systems employing sodium laser guide stars can be improved by continuously monitoring the vertical density structure of mesospheric sodium along the line of sight. We demonstrate that sodium density profiles can be retrieved by amplitude modulation of continuous wave (CW) lasers. In an experiment conducted at the Large Zenith Telescope (LZT), ESO’s Wendelstein Raman-fibre laser was amplitude-modulated with a pseudo-random binary sequence and profiles were obtained by cross-correlation of the modulation pattern with the observed return signal from the laser guide star. For comparison, high-resolution profiles were obtained simultaneously using the lidar system of the LZT. The profiles obtained by the two techniques show noise contamination, but were found to agree to within the measurement error. As a further check, a comparison was also made between several lidar profiles and those obtained by simultaneous observations using a remote telescope to image the laser plume from the side. The modulated CW lidar technique could be implemented by diverting a small fraction of the returned laser light to a photon counting detector. Theoretical analysis and numerical simulations indicate that, for 50 per cent modulation strength, the sodium centroid altitude could be retrieved every 5 s from a single laser guide star, with an accuracy which would induce a corresponding wavefront error of 50 nm for the Extremely Large Telescope and less than 30 nm for the Thirty Meter Telescope and Giant Magellan Telescope. If multiple laser guide stars are employed, the required modulation amplitude will be smaller.

Funder

European School of Oncology

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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