The need for JWST's metering structure to be stable over time while at cryogenic temperatures is derived from its scientific objectives. The operational scenario planned for JWST provides for the optical system to be adjusted on regular intervals based upon image quality measurements. There can only be a limited amount of optical degradation between the optical system adjustments in order to meet the scientific objectives. As the JWST primary mirror is segmented, the structure supporting the mirror segments must be very stable to preclude degradation of the optical quality. The design, development and, ultimately, the verification of that supporting structure's stability rely on the availability of analysis tools that are credibly capable of accurately estimating the response of a large structure in cryogenic environments to the nanometer level. Validating the accuracy of the analysis tools was a significant technology demonstration accomplishment. As the culmination of a series of development efforts, a thermal stability test was performed on the Backplane Stability Test Article (BSTA), demonstrating TRL-6 status for the design, analysis, and testing of Large Precision Cryogenic Structures. This paper describes the incremental development efforts and the test results that were generated as part of the BSTA testing and the associated TRL-6 demonstration.
Marshall Space Flight Center (MSFC) has been performing optical wavefront testing at cryogenic temperatures since 1999 in the Space Optics Manufacturing Technology Center's (SOMTC's) X-ray / Cryogenic Facility (XRCF). Recently the cryogenic optical testing capability has been extended to a smaller chamber. This smaller horizontal cylindrical vacuum chamber has been outfitted with a heliumcooled liner that can be connected to the existing helium refrigeration system bringing the kilowatt of refrigeration capacity to bear on a 1 x 2 meter test envelope. Cryogenic cycles to 20 Kelvin, including setup and chamber evacuation/backfill, are now possible in only a few days. Since activation and chamber characterization tests in September 2001, the new chamber has been used to perform a number of proprietary cryogenic tests on mirrors, adhesives, and actuators. A vibration survey has also been performed on the test chamber. Chamber specifications and performance data, vibration environment data, and optical test capability will be discussed.
Marshall Space Flight Center has modified the X-ray Calibration Facility to test Next Generation Space Telescope developmental mirrors at cryogenic temperatures (35 Kelvin) while maintaining capability for performance testing of x-ray optics and detectors.
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