Opto-mechanical design, alignment, and test of the SPHEREx telescope: a cryogenic all-aluminum freeform system for the astrophysics medium explorer mission

Abstract

SPHEREx is a Medium Explorer astrophysics mission that requires a wide-field cryogenic short-wave infrared (SWIR) to mid-wave infrared (MWIR) telescope. The SPHEREx telescope has been designed and built at Ball Aerospace based on a JPL optical prescription and system architecture. The telescope has a 20cm entrance pupil, three freeform aluminum mirrors, a dichroic beam splitter furnished by Caltech, and forms two images in the SWIR and MWIR spectral bands. The "all-aluminum" architecture combined with a cryogenic space environment defines the opto-mechanical design approach for this system. Ball Aerospace performed developmental testing on a flight-like aluminum engineering development mirror for cryogenic surface figure and measured the differential thermal expansion of structural and optical aluminum materials. These development tests validated aspects of the athermal design prior to system-level integration and test. The telescope alignment process uses a laser tracker and system wavefront error (WFE) data to determine the adjustment of two mirrors and a focal plane assembly optical simulator (FPAOS). The FPAOS provides a retroreflection of interferometer light from each image location and an athermal focus position. The FPAOS was used in place of the SWIR band focal plane during system alignment and was moved to the MWIR focal plane interface to validate WFE performance in both bands. A cryogenic system WFE test was conducted to validate SWIR band performance at operational temperatures, and a post-vibrational WFE test demonstrates flight readiness. The SPHEREx telescope has been tested, delivered, and is ready for the next level of system integration.