Laser guide stars for large telescopes: cone effect and astrophysical implications

Details Laser guide stars for large telescopes: cone effect and astrophysical implications Laser guide stars for large telescopes: cone effect and astrophysical implications

May 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........3l&link_type=abstract

PhD Theses

Physics

Optics

3

Adaptive Optics, Laser Guide Stars, Multi Conjugate Adaptive Optics, Cone Effect, Sky Coverage, High Resolution Imaging, Extremely Large Telescopes

Scientific paper

The performance of an Adaptive Optics (AO) system equipped with a Laser Guide Star (LGS) on 3.6~m and 8~m telescopes are evaluated. The use of an LGS allows to significantly increase the sky coverage (fraction of the sky which can be observed). Indeed, 99 % of the sky is accessible with a LGS (whereas 10 % only is accessible with a Natural Guide Star (NGS), at 2.2 μm, average galactic latitude and longitude with a Strehl ratio of 0.2). The number of quasars which can be observed with a Strehl ratio greater than 0.2 increases from 357 to 6803. The performances of an LGS-AO decrease dramatically towards shorter wavelengths (<1 μm), due to the cone effect (i.e. focus isoplanatism). A 3 dimensional study of atmospheric turbulence allows to solve this problem. Four LGSs provide a good correction quality in the visible (Strehl of 80 % for an 8 m telescope). The corrected Field of View can be significantly increased(100'', Strehl of 30 %). Some low order modes (forms of tilt, defocus, astigmatism) must be measured from a NGS. Due to the finite number of deformable mirrors being used, anisoplanatism appears in the corrected field. The performances of an AO system for correction in the visible on a 100 m diameter telescope are estimated, using 4 laser (and natural) guide stars. The are no physical limitations preventing a high sky coverage, with a milli-arcsecond resolution in the visible. The last chapter is devoted to the study of a few Mira-type stars, with a AO system and an integral field spectrograph, with the aim of detecting shock waves in their atmosphere.

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