Optical thermal vacuum test for the FORMOSAT-8 optical system assembly

The interferometry is often adopted to check the optical quality and support the alignment in assembly for high precision demand optical system. The interferometer could acquire the system wavefront error (WFE) and obtain the delicate variation of aberration terms. For those optical systems applied to the space mission, the transition from atmosphere to vacuum environment would lead to the dimensional changes of mechanical structure within the scale of micrometer and eventually diminish optical performance. Consequently, as stated above, it is essential to validate the whole optical system in vacuum circumstance. Before the system actually being launched into the space, they are usually placed in the thermal vacuum chamber during ground testing in order to validate if the design could withstand the harsh environments such as high vacuum level and large temperature difference. Nevertheless, it is a big challenge to build up an in-situ optical measurement architecture for large aperture optical system in the thermal vacuum chamber due to the finite internal space of chamber, limited aperture size of transmission view port of chamber door and thermal dissipation problem of measuring instruments. In this paper, we demonstrate an innovative way of interferometry for monitoring the optical performance variation of FORMOSAT-8 (FS-8) optical system assembly (OSA) in our current vacuum chamber that the test telescope and the diverger lens were located in the vacuum environment, while the interferometer stayed in the ambient circumstance. The interferogram was successfully obtained thanks to the rigorous optical alignment process and the speical designed reference tools.