Lightweighted beryllium cryogenic mirrors for both monolithic and segmented space telescopes

Abstract

A series of developmental as well as flight mirrors have been in process over the last few years for IR cryogenic telescope applications such as the Space Infrared Telescope Facility (SIRTF) and the Next Generation Space Telescope (NGST) and for visible ambient systems such as Space Based Laser (SBL). We will discuss the performance of the 0.85-m SIRTF primary mirror (26.6 kg/m² areal density) and the 0.5-m Subscale Beryllium Mirror Demonstrator (SBMD) beryllium mirror (9.8 kg/m² areal density) as well as the current status of the 1.4-m Ball semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD). The AMSD mirror itself has an areal density of 10.4 kg/m² and is currently in polishing. The entire AMSD assembly including composite reaction structure, flexures, and actuators, has an areal density less than 15 kb/m². Cyrogenic test results of the SIRTF and SBMD mirrors will be presented along with test data on the AMSD actuators. The SBMD mirror wsa cryofigured based on ambient and cryo testing to achieve a wavefront quality of 19 nm rms at 35 K. In addition, the effects of optically coating SBMD with a protected gold multi-layer system will be shown - demonstrating that a lightweight mirror can be coated without adverse print-through due to coating stress at ambient or cryo operating temperatures.