Heuristic design of lightweight homologous structure for Large Submillimeter Telescope

We present the optimization of the backup structure (BUS) of a primary mirror for the Large Submillimeter Telescope (LST), a 50-m class single-dish submillimeter telescope project proposed in Japan. This telescope enables extensive surveys of huge volumes in the Universe. The LST project is currently developing a highly precise (<45 µm RMS) surface without an astrodome, which would be difficult to design with conventional methods. Additionally, the telescope aims to be lightweight for cost-effective construction and efficient surveys. Structural optimization for the BUS design effectively minimizes both static surface deformations and the total BUS mass, which involve trade-offs. In this study, finding optimum structures from vast combinations of nodal positions and cross-sectional areas of truss elements comprising the BUS is required. We applied a multi-objective genetic algorithm, an efficient method of combinatorial problems, to BUS design. This optimization was performed to minimize the actuator stroke lengths of the primary mirror panels and the BUS mass simultaneously under the constraints of antenna optics and architectural feasibilities, mitigating gravitational deformation of the surface. The optimization produced various kinds of optimal solutions that met the constraints. The surface accuracies of these structures were also optimized to 32 – 440 µm RMS. The optimized BUSs maintain the accurate surface of the primary mirror at any elevation angle (EL). Specifically, the circumferential truss members function as hoops at high ELs to restrain the outward deformation of the primary mirror surface, while radial truss structures lift the lower part of the primary mirror at low ELs to prevent gravitational deformation. This foundational design serves as a basis for further research under actual environmental conditions and for potential design enhancements.