OPTICAL FABRICATION: Beryllium makes ultrathin precision mirror strong

Engineers inspect beryllium primary mirror of the Infrared Telescope Technology Testbed (ITTT), fabricated by Hughes Danbury Optical Systems (HDOS; Danbury, CT) as part of a Jet Propulsion Laboratory (JPL; Pasadena, CA) technology development and demonstration program for NASA’s Space Infrared Telescope Facility (SIRTF). To minimize weight while maximizing performance, the 85-cm-diameter primary mirror tapers rapidly from a thickness of 50 mm in its central region to 6 mm at the outer edge. The bare mirror weighs 15 kg, while the integrated primary mirror assembly weighs 21 kg; the stiffness-to-weight ratio of beryllium allows engineers to fabricate a mirror weighing only half as much as a glass version with comparable self-weight deflection.

The mirror assembly is shown mounted on an aluminum plate for its initial cryogenic testing at 5 K in the SIRTF. The ITTT telescope is a Richey-Chrètien configuration designed for diffraction-limited performance at 6.5 µm. Hughes engineers have completed the secondary mirror assembly, which will be integrated with the primary mirror after JPL finishes the final cryogenic testing; a single-component beryllium metering tower that will support the secondary mirror assembly is still under fabrication at HDOS.