Optical characterisation of an additively manufactured freeform optical system

Abstract

The growing demand for high-performance satellite optical payloads with low size, weight, and power requirements, has driven the need for advanced optical systems capable of delivering superior imaging performance within strict mass and volume constraints. Freeform optical designs are well suited to compact sensor payloads due to their ability to correct for high-order aberrations while maintaining high-sensitivity, resolution and wide field-of-view. Additive Manufacturing (AM) further enhances these systems by enabling lightweight structures and integrated mounting features; however, achieving the required surface finish and form accuracy for freeform optical mirrors remains a significant challenge, particularly when Single-Point Diamond Turning (SPDT). This work builds on previous research into AlSi10Mg Laser Powder Bed Fusion (L-PBF) post-processing by implementing an optimised heat-treatment procedure prior to SPDT. An exemplary 6-inch off-axis 2-mirror system was designed, manufactured, and characterised to demonstrate a 14 % lightweighting factor enabled by AM. The complex mirror surface shapes were designed to push the manufacturing limits, by incorporating off-axis shapes, positive and negative curvatures, and high-order terms. The mirrors were successfully manufactured with 5 nm – 7 nm average surface roughness and minimal surface defects suitable for short-wave infrared applications. The surface form error results present an opportunity for AM to manufacture optimised stress-dissipating lightweight mirrors to minimise sensitivities to mounting-induced surface deformation. These findings demonstrate a future for AM to enable compact, lightweight freeform optical systems.