Design optimization of a lightweight secondary mirror assembly for a 1-m class ground telescope for satellite laser ranging

Large-sized ground telescopes have been developed to meet the high demands for opto-mechanical imaging systems in space and military applications. In line with these advancements, we developed a 1-m class ground telescope for astronomical imaging and satellite laser ranging (SLR). In a ground telescope, mirror deflection is mainly induced by gravity and temperature change. In particular, the gravity vector varies depending on the pointing direction of a telescope, so the surface deformations of the mirrors due to self-gravity need to be managed in different observation directions. This study introduces a mechanical design for an optical tube assembly (OTA) and suggests an optimized design for the secondary mirror (M2) assembly. For a kinematic positioning of the M2, its lightweight was achieved based on the partially open-back structure with hexagonal pocket cells. Then, we optimized the flexure mount design with a bipod structure to minimize the surface errors (SFEs) of the M2 in both the horizontal and vertical pointing directions. Additionally, we simulated the deflections of the primary mirror (M1) and M2 assemblies when installed on the telescope. Based on our design, the M2 was fabricated and processed, and we demonstrated its assembly process and surface quality test.