Toward high-dynamic active mirrors for LGS refocusing systems

Abstract

In the frame of the E-ELT-EAGLE instrument phase A studies, we designed a convex VCM able to compensate for the focus variation on the Laser Guide Star (LGS) wavefront sensor, due to the elevation of the telescope and the fixed sodium layer altitude. We present an original optical design including this active convex mirror, providing a large sag variation on a spherical surface with a 120mm clear aperture, with an optical quality better than lambda/5 RMS up to 820μm of sag and better than lambda/4 RMS up to 1000μm of sag. Finite element analysis (FEA) allowed an optimisation of the mirror's variable thickness distribution to compensate for geometrical and material non linearity. Preliminary study of the pre-stressing has also been performed by FEA, showing that a permanent deformation remains after removal of the loads. Results and comparison with the FEA are presented in the article of F.Madec et al (AS10-7736-119, this conference), with an emphasis on the system approach.