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Active Optics: stress polishing of toric mirrors for the VLT SPHERE adaptive optics system
Emmanuel Hugot1, Marc Ferrari, Kacem El Hadi
1Université de Provence Aix-Marseille I, Laboratoire d'Astrophysique de Marseille, Centre National de la Recherche Scientifique / Institut National des Sciences de l'Univers, 38, rue F. Joliot Curie, 13388 Marseille Cedex 13, France. emmanuel.hugot@oamp.fr
Abstract:
The manufacturing of toric mirrors for the Very Large Telescope-Spectro-Polarimetric High-Contrast Exoplanet Research instrument (SPHERE) is based on Active Optics and stress polishing. This figuring technique allows minimizing mid and high spatial frequency errors on an aspherical surface by using spherical polishing with full size tools. In order to reach the tight precision required, the manufacturing error budget is described to optimize each parameter. Analytical calculations based on elasticity theory and finite element analysis lead to the mechanical design of the Zerodur blank to be warped during the stress polishing phase. Results on the larger (366 mm diameter) toric mirror are evaluated by interferometry. We obtain, as expected, a toric surface within specification at low, middle, and high spatial frequencies ranges.
