GMT ASM prototype dynamic and optical tests results

Abstract

The GMT Adaptive Secondary Mirror (ASM) has recently completed the final design phase and the construction of the first segment has begun [1]. For the purpose of risk reduction, we developed a prototype with 72 actuators and a diameter of 354 mm, featuring the four innermost rings of actuators of the on-axis ASM segment. The prototype, named P72, has been conceived with the main goals of validating several new technical solutions adopted for the GMT ASM and not yet deployed in the current generation of large, contactless, VCM driven adaptive mirrors, in particular on LBT 672 and on the VLT DSM. These include larger actuators spacing, new external membranes to provide the in-plane constraint for the thin shell, improved connection to the capacitive sensors that measure the gap between reference body, RB, and thin shell and a completely new generation of control electronics. P72 has undergone a comprehensive test campaign including the electromechanical tests intended to identify and optimize the dynamic response of the adaptive mirror, and the optical tests characterizing the actual optical correction performance and the behavior of the new mirror flexures. The tests have been performed using a dedicated optical test setup with variable elevation in a climatic test facility at Microgate, which also allowed verification of the sensitivity to temperature changes. We report the prototype design and results of the test campaign, that included matching the experimental results and numerical results obtained by means of internally developed multiphysics numerical simulators.