Cryogenic design and predicted performance of the James Webb space telescope beryllium aft optics subsystem optical bench

Abstract

With a planned launch of 2013, NASA's James Webb Space Telescope (JWST) will be the premier space observatory for astronomers worldwide. This infrared space telescope will be passively cooled to cryogenic temperatures in its solar L2 orbit. The JWST Optical Telescope Element (OTE) features a 6.5 meter, segmented Primary Mirror, which focuses light onto a Secondary Mirror and finally redirected into and through the Aft Optics Subsystem (AOS). The AOS consists of an optical bench which aligns and supports the telescope's Tertiary Mirror and Fine Steering Mirror Assemblies. This paper describes the unique cryogenic requirements and design of the JWST Beryllium AOS optical bench. Key performance requirements are reviewed including: launch environment, the cryogenic operating environment (nominally 39K), and optical alignment stability at cryogenic temperatures. The mechanical design approach utilizing Beryllium as the structural material for the AOS Bench is described relative to meeting the driving requirements. Material property verification, low and predictable material variability, and low thermal gradients across the structure are also discussed.