Optimizing Wavefront-Deformation Sensor Placement for Active Radio-Telescope Surfaces

Abstract

Next-generation high-frequency radio telescopes require primary-surface accuracies that passive structures alone cannot reliably achieve. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a 50 m single-dish concept operating up to $\approx$ 1 THz, imposes a $\approx$ 20 $μ$m rms surface-accuracy requirement across its full aperture. This is practically unattainable for a purely passive reflector subject to gravitational, thermal, and wind-induced deformation. Closed-loop active collimation and surface control are therefore imperative, which in turn requires the deformation field to be known across the full aperture in real time. Measuring it directly at the necessary resolution across the complete surface is, however, hardly feasible; instead, the current AtLAST concept development foresees reconstructing the surface from a limited set of discrete sensor positions. An algorithmic framework is presented that optimizes the number and placement of these sensors to maximize the reconstructability of the deformation field. Finite-element analysis (FEA) load cases (gravity, thermal, wind) define the deformation space, from which a data-driven Proper Orthogonal Decomposition (POD) basis is derived; sensor positions are then chosen by a greedy optimization algorithm and then assessed via leave-one-out cross-validation. Applied to FEA deformations of AtLAST's Back-Up Structure (BUS), and assuming a sensor noise of 5 $μ$m rms, the method reconstructs all load cases with 50 sensors to below 2.7 $μ$m rms (worst case) residual (BUS-) surface error.