Wavefront reconstruction with elongated sodium laser guide stars

Details Wavefront reconstruction with elongated sodium laser guide stars Wavefront reconstruction with elongated sodium laser guide stars

Nov 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sf2a.conf...77t&link_type=abstract

"SF2A-2008: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: C. Charbonnel, F. Combes

Astronomy and Astrophysics

Astrophysics

Scientific paper

When forming a sodium laser guide star (LGS), the lightened volume in this layer can be seen as a cylindrical source ˜1m in diameter and ˜10 km in length. Because of the parallax, the LGS looks significantly elongated when observed from the edge of the pupil of an Extremely Large Telescope. This effect prompts the lasers to be launched from behind the secondary instead of from around the telescope, but still degrades significantly the accuracy of wavefront sensing for adaptive optics. Further, the measurement uncertainties are no more uniform across the pupil and correlations are introduced. From numerical simulations, we analyze the benefit of taking into account these structured correlations and the effect of priors in wavefront reconstruction algorithms. We found that priors are effective for a single LGS launched behind the secondary. When combining the measurements from several LGSs in a Ground Layer adaptive optics system, we found that taking into account the noise covariances and launching from the edge yields the best reconstruction. Further, in this configuration, we can discard the worse measurements along the elongated direction and reduce the field of view (thus use a smaller detector) without any significant loss of accuracy.

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