Optical Element Thermal Modeling for JWST to Support System Level Ground Tests

Abstract

The James Webb Space Telescope (JWST) observatory currently under construction is scheduled for launch in 2018. This telescope has a large aperture, and an infrared-optimized telescope passively cooled to below 50 Kelvin. As a part of the JWST test program, optical elements of telescope assembly will be exposed to the simulated operational environment within the newly renovated Chamber A at NASA Johnson Space Center. These optical elements will be cooled to their cryogenic operational temperature from normal ambient temperature. During the cool-down and warm-up transitions of each test, maximum allowable temperature gradients are imposed to minimize thermally induced stress that may potentially harm the hardware. This paper describes the evolution of the JWST optical element thermal models needed to predict these gradients during system level tests with the intent of minimizing test durations without exceeding allowable stresses during the temperature transition phases of the tests. These environments include helium cooled shrouds and the use of rarified gas to accelerate transitions. The thermal models have been matured using data from high vacuum as well as low-pressure helium gas environments during subsystem level testing. This paper also describes the system level thermal model of the test article developed from sub-system optical element thermal models. Applying chamber cool-down and warm-up profiles to this model will provide temperatures, temperature rates and chamber pressures to be used in system level testing based on allowable temperature gradient limits of the hardware under test.