Laser guide star elongation and distributed launch optics

Abstract

Laser beam shaping is particularly challenging with Laser Guide Stars (LGS) and large telescopes. In Adaptive Optics (AO), LGS, elongation becomes significant with TMT (30m) and E-ELT (42m). It significantly reduces performance of Schack-Hartmann and curvature wavefront sensors. To determine the dimension of the laser source, we need to know Na layer column abundance, centroid height, Na concentration, and Na layer thickness. The LGS spot elongation is a function of the vertical sodium layer thickness and the orthogonal offset of the observer of the laser beam. R Ragazzoni (2003) (3) suggested that LGS elongation might be reduced by distribution of the laung optics around the telescope primary or secondary, all beams focusing and combining at the required elevation of 93km. Although TMT and E-ELT deal with the same parameters with respect to AL LGS, they apply them differently. For TMT (B. Ellerbroek, 2010 (4)), there will be 6 LGS wavefront sensors. Both center and side launch configurations have been considered, but the former is preferred due to cost and current progress. For E-ELT, LGS< elongation was analyszed for over a year with nocturnal as well as seasonal Na variation. The orthogonal offset is 21 m. Assuming a laser of sufficient power, a telescope could observe a low latitudes for more than 250 days per year. Six laser guide stars and three natural guide stars are forseen with launch positions at the edge of the aperture preferred. Elongation then depends on the Na density profile. Performance of TMT and E-ELT will be compared for system complexity as well as anticipated performance.