Measurement of large cryogenic structures using a spatially phase-shifted digital speckle pattern interferometer

Abstract

The James Webb Space Telescope (JWST) Backplane Stability Test Article (BSTA) was developed to demonstrate large precision cryogenic structures' technology readiness for use in the JWST. The thermal stability of the BSTA was measured at cryogenic temperatures at the Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF) and included nearly continuous measurements over a six-week period in the summer of 2006 covering the temperature range from ambient down to 30 Kusing a spatially phase-shifted digital speckle pattern interferometer (SPS-DSPI). The BSTA is a full size, one-sixth section of the JWST primary mirror backplane assembly (PMBA). The BSTA, measuring almost 3 m across, contains most of the prominent structural elements of the backplane and is to our knowledge the largest structure ever measured with SPS-DSPI at cryogenic conditions. The SPS-DSPI measured rigid body motion and deformations of BSTA to nanometer-level accuracy. The SPS-DSPI was developed specifically for the purposes of this test and other tests of large cryogenic structures for JWST.

The James Webb Space Telescope Mission: Optical Telescope Element Design, Development, and Performance

“…A prototype of the composite structure, called the Backplane Stability Test Article (BSTA), was built by ATK and tested at MSFC's XRCF (Figure 8). Verification of the structure's stability made use of a new Electronic Speckle Pattern Interferometer (ESPI) metrology approach (Saif et al 2008), a technology development in itself, to confirm the structure was TRL-6.…”

Technology gap assessment for a future large-aperture ultraviolet-optical-infrared space telescope

“…As an example, the backplane stability test article (BSTA) on JWST demonstrated that the lightweight cryogenic composite could meet its required stability. The work demonstrated mirror pad mount interface motion of 28.5 nm/K over a 25 K static gradient from 30-55 K. 53 By comparison, ATLAST will need about 3 pm/K but over a gradient of a few mK at room temperature, and a subscale test will be critical to that demonstration. The inherent stiffness of silicon carbide (SiC) makes it an attractive option for the support structure, although additional effort will be required to maintain thermal stability; SiC has a higher coefficient of thermal expansion (CTE) than glass, requiring finer thermal control, on the order of 0.1 mK instead of ~1 mK for glass.…”

James Webb Space Telescope optical stability lessons learned for future great observatories

“…Designing a test to validate a model to these levels is as big a challenge as designing the flight element itself. The BSTA metrology system required development of a new type of speckle interferometer to properly perform the test 9 . GSE influence on the performance of the test article needed to be minimal, ideally zero.…”