James Webb Space Telescope Thermal Pathfinder Test Development

The James Webb Space Telescope (JWST), scheduled for launch in 2018, includes a 6m primary mirror passively cooled below 55 Kelvin. The observatory cannot be optically or thermally tested as a system at flight temperatures due to its large sunshield, so the telescope portion along with its instrument complement will be tested as a single unit. This test is scheduled for 2017, and on the program schedule critical path, so a set of cryogenic tests with some flight-like hardware is being performed earlier to help mitigate this schedule risk. The JWST Pathfinder was used in two optical tests performed in 2015, and includes a test-only telescope structure segment, two flight-like primary mirrors, a flight-spare secondary mirror, and associated electronics. This paper updates plans and associated thermal analysis for the 2016 JWST Thermal Pathfinder (TPF) cryogenic test.

The primary TPF test objective is to run the flight telescope test cooldown timeline with a thermally similar Pathfinder configuration. Thermal hardware was added to the Pathfinder structure to more closely match telescope thermal characteristics, including flight-like insulation and blanks to simulate most mirrors. Because the flight telescope has very few thermal sensors, the Thermal Pathfinder has sensors at all critical locations to ensure that gradient requirements on structures and mirror assemblies can be met with the planned cooldown timeline.

Another TPF thermal objective is to provide insight into flight backplane heat flow characteristics, which are critical for accurate flight predictions. After the baseline stabilization, heat will be added to specific interface areas to raise local temperatures and generate additional steady state conditions.