Freeform mirror based optical systems for nano-satellites

Abstract

INTRODUCTIONHigh resolution satellite imagery is essential in a wide variety of applications such as urban monitoring, environmental protection, disaster response, precision farming, defence and security. State of the art microsatellites are able to reach sub-meter ground resolution. These instruments are based on classical two, three or four mirror optical systems and the surfaces are rotationally symmetric or sections of rotationally symmetric surfaces. For nano-satellites, with very limited volume and mass budget, the primary mirror segments are needed to be deployed in order to create the final operational primary configuration that spans over a sufficiently large area. As a result of packaging, mass and space allocation requirements, the primary mirror segments don't deploy into a complete optical aperture, instead sub-apertures are formed with arbitrary (usually rectangular) shapes. Synthetic aperture telescope systems, such as the Michelson or the Fizeau type can be used to achieve sub-meter resolution, the latter being generally more practical due to its relative simplicity. Due to the highly truncated optical apertures (creating non-circular shapes), it is worth to investigate how freeform surfaces (without any rotation symmetry) can be used in these optical architectures. In the current paper we study various freeform mirror based optical system concepts for nano-satellites and investigate what is the performance increase with respect to image quality, field of view and compactness. We present various freeform based systems with significant performance gain. We also investigate convergence during the optimization process and the robustness of the created solutions are further studied. We use freeform optimization techniques to create different freeform based optical architectures that we compare with conventional optical designs. We discuss the necessary manufacturing techniques to create the freeform optical surfaces and detail the metrology to verify the manufacturing and alignment of the complete system. We build our work on the experience that we gained in the OPTICON FP7 Freeform Active Mirror Experiment (FAME) project [1].