Correlation of Effective Emissivity of Light-weighted Beryllium Mirrors during JWST Thermal Vacuum Testing

Abstract

The James Webb Space Telescope (JWST), scheduled for launch in 2018, must be cooled to temperatures below 50 K as part of a rigorous set of ground tests to verify operational performance. Some of the components that drive the time required to cool down the assembly to this temperature are the beryllium optics. Two properties of these optics that determine their cool down time are mass and surface emissivity. The mass of these optics, including the primary comprised of 18 hexagonal segments, a secondary and a tertiary, have all been reduced by machining pockets into the back side of the substrates. One difficulty in predicting the time required to cool these optics is estimating how their pocketed back sides effects their emissivity. Component level tests of these optics at operational temperatures has allowed for verification of their cryogenic optical performance. These tests have also allowed for estimates of mirror back effective emissivities based on correlating thermal models of the tests. This paper will describe the subassembly tests and show correlated results for mirror back effective emissivities.