Stress polishing of E-ELT segment at LAM: full-scale demonstrator status

Abstract

The Laboratoire d'Astrophysique de Marseille (LAM) is involved in the prototyping of a full scale demonstrator for stress polishing of segments for the European Extremely Large Telescope (E-ELT). Stress polishing method is developed at LAM since more than 40 years, and this mature technology has recently been used with success for VLT instruments. Stress polishing is now considered as a promising manufacturing method for mass production of large off axis mirrors, specifically for ELT segments. This powerful method, based on elasticity theory, allows the generation of super-smooth off-axis aspherics with a minimal amount of high spatial frequency ripples by spherically polishing a warped blank with a full-sized tool. Thanks to the simple spherical polishing, the operation time can be strongly reduced compared to the time-consuming sub-aperture tool methods of grinding and polishing. The goal is to rapidly converge to less than 1 micron RMS of optical quality on a circular blank which will be finally cut hexagonally and finished using Ion Beam Finishing. In this paper we will present the status of the demonstrator and the design of the warping harness prototype that must be able to precisely warp the circular blank.