Optical design of MAORY Laser Guide Star Objective

The turbulence affects the angular resolution of images. Single Conjugate Adaptive optics, SCAO, is the simplest way to correct the atmospheric turbulence using a Natural Guide Star, but it has some limitations such as the anisoplanatism. Multi Conjugate Adaptive Optics is a technique which overcomes this issue by using as many NGSs, as Wavefront Sensors. In order to increase the sky coverage, Laser Guide Stars, LGSs, are also used. One of the first light instrument of the 40 meter European Extremely Large Telescope is MICADO, or the Multi-Adaptive Optics Imaging Camera for Deep Observations. It works with the post-focal MCAO relay, called MAORY, which uses six LGSs. A description of MAORY optical design is provided in this thesis. These LGSs suffer from aberrations and thus, after separating the light by a dichroic, an objective is designed to reduce these aberrations. In fact, since during sky tracking the zenith angle changes, the six LGSs launched at the Sodium layer vary their distances from the telescope primary mirror. As a consequence, the LGSs at different distances will focus on different positions along the optical axis. Through the procedure of optimization one can improve or modify the design in order to reduce or remove aberrations. Since no component can be perfectly manufactured and aligned, a tolerance analysis is important to ensure that the final, assembled instrument meets the requested performance. All in all, the goal of this thesis is to optimize and perform the tolerance analysis for the LGS Objective of MAORY and show the results. The mechanical constraints and the variations of the aberrations with the zenith and azimuthal angle are taken into account. The number of optical surfaces has been minimized keeping the LGS Objective requirements satisfied. Moreover, the residual aberrations of the LGSs have been kept as small as possible in order to reach the performance requirements of MAORY. The software Zemax has been used.