Design of the multiple Laser Guide Stars wavefront sensor prototype for the EELT

Abstract

Almost all the scientific instruments foreseen to be installed on the European Extremely Large Telescope claim for the Sodium Laser Guide Stars for an effective adaptive correction, over a large fraction of the sky, of the wavefront aberrated by the atmospheric turbulence. These artificial sources present an elongated, irregular and time variable intensity profile due to the density variations of the Sodium layer, where they form. On the focal plane of a Shack- Hartmann wavefront sensor, the baseline sensor of the adaptive optics modules, the laser guide stars appear as elongated spots on the lenslet array focal plane. Thus the local wavefront measurements inside the sub-apertures require some investigations to evaluate the performance of advanced centroiding algorithms for the extended spots position measurement to reduce the residual wavefront error. The laboratory prototype presented in this paper reproduces the expected conditions, for an Extremely Large Telescope case, when measuring the wavefront of a Laser Guide Star by means of a Shack-Hartmann wavefront sensor. A simplified version of the prototype has been successfully integrated and tested in 2010. We describe the design of new components, that permit to achieve the following features: a programmable setting of the elongated source intensity profile, to be able to reproduce realistically the time variable Sodium layer density vertical profile; the simulation, one at a time, of a multiple laser guide stars launching system by means of an optical derotator that rotates the elongated spot pattern on the detector plane; the implementation of a two layers turbulence model that introduces a differential wavefront aberration according to the azimuth angle of the laser guide stars and simulates the time evolution of the turbulence. The scheduled test program is finally presented.