Bente Eegholm

Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

The JWST backplane stability test article: a critical technology demonstration

The unprecedented stability requirements of JWST structures can only be conclusively verified by a combination of analysis and ground test. Given the order of magnitude of the expected motions of the backplane due to thermal distortion and the high level of confidence required on such a large and important project, the demonstration of the ability to verify the thermal distortion analysis to the levels required is a critical need for the program. The demonstration of these analysis tools, in process metrology and manufacturing processes increases the technology readiness level of the backplane to required levels. To develop this critical technology, the Backplane Stability Test Article (BSTA) was added to the JWST program. The BSTA is a representative substructure for the full flight backplane, manufactured using the same resources, materials and processes. The BSTA will be subject to environmental testing and its deformation and damping properties measured. The thermally induced deformation will be compared with predicted deformations to demonstrate the ability to predict thermal deformation to the levels required. This paper will review the key features and requirements of the BSTA and its analysis, the test, measurement and data collection plans.

Calibration of spatially phase-shifted DSPI for measurement of large structures

We present a method for the calibration of a spatially phase-shifted digital speckle pattern interferometer (SPS-DSPI), which was designed and built for the purpose of testing the James Webb space telescope (JWST) optical structures and related technology development structures. The need to measure dynamic deformations of large, diffuse structures to nanometer accuracy at cryogenic temperature is paramount in the characterization of a large diameter space and terrestrial based telescopes. The techniques described herein apply to any situation, in which high accuracy measurement of diffuse structures are required. The calibration of the instrument is done using a single-crystal silicon gauge. The gauge has four islands of different heights that change in a predictable manner as a function of temperature. The SPS-DSPI is used to measure the relative piston between the islands as the temperature of the gauge is changed. The measurement results are then compared with the theoretical changes in the height of the gauge islands. The maximum deviation of the measured rate of change of the relative piston in nm/K from the expected value is 3.3%.

Technology demonstration of large stable cryogenic composite structures for JWST

The need for JWSTs 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 structures 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.

Cryogenic metrology for the James Webb Space Telescope Integrated Science Instrument Module alignment target fixtures using laser radar through a chamber window

The James Webb Space Telescope Integrated Science Instrument Module utilizes two fixtures to align the Optical Telescope Element Simulator (OSIM) to the coordinate systems established on the ISIM and the ISIM Test Platform (ITP). These fixtures contain targets which are visible to the OSIM Alignment Diagnostics Module (ADM). Requirements on these fixtures must be met under ambient and cryogenic conditions. This paper discusses the cryogenic metrology involving Laser Radar measurements through a chamber window that will be used to link photogrammetry target measurements used during ISIM structure cryogenic verification and the ADM targets, including evaluation of distortion introduced from the window.

Development of interferometry for testing the JWST Optical Telescope Element (OTE)

Instantaneous phase shifting interferometry is key to successful development and testing of the large, deployable, cryogenic telescope for the James Webb Space Telescope (JWST) mission. Two new interferometers have been developed to meet the needs of the JWST program. Spatially Phase-Shifted Digital Speckle Pattern Interferometer (SPSDSPI) was developed to verify structural deformations to nanometer level accuracy in large, deployable, lightweight, precision structures such as the JWST telescope primary mirror backplane. Multi- wavelength interferometer was developed to verify the performance of the segmented primary mirror at cryogenic temperatures. This paper discusses application of SPS-DSPI for measuring structural deformations in large composite structures at cryogenic temperatures. Additionally development of a multi-wavelength interferometer for verifying JWST OTE primary mirror performance at cryogenic temperatures will be discussed.

Spatially phase-shifted digital speckle pattern interferometry (SPS-DSPI) and cryogenic structures: recent improvements

The Spatially Phase Shifted Digital Speckle Pattern Interferometer (SPS-DSPI) is a speckle pattern interferometer in which the four phase-shifted interferograms are captured simultaneously in a single image. Designed to measure thermal distortions of large matte-surfaced structures for the James Webb Space Telescope (JWST) program, this metrology instrument has been used in two major cryo-distortion tests. This report will describe how differences in the vibrational motions of the test objects necessitated changes in basic algorithms. The authors also report operational upgrades, quantification of uncertainty, and improvement of the software operability with a graphic interface. Results from the tests of the JWST test structures are discussed as illustration.

Development of electronic speckle pattern interferometry for testing JWST composite structures

The stability requirements for the James Webb Space Telescope (JWST) optical metering structure are driven by the science objectives of the mission. This structure, JWST Optical Telescope Element (OTE) primary mirror backplane, has to be stable over time at cryogenic temperatures. Successful development of the large, lightweight, deployable, cryogenic metering structure requires verification of structural deformations to nanometer level accuracy in representative test articles at cryogenic temperature. An instantaneous acquisition phase shifting speckle interferometer was designed and built to support the development of JWST Optical Telescope Element (OTE) primary mirror backplane. This paper discusses characterization of the Electronic Speckle Pattern Interferometer (SPS-DSPI) developed for JWST to verify its capabilities to measure structural deformations in large composite structures at cryogenic temperature. Interferometer performance during the Backplane Stability Test Article (BSTA) test that completed the TRL-6 (Technology Readiness Level-6) demonstration of Large Precision Cryogenic Structures will also be discussed.

Electronic Speckle Pattern Interferom-etry for JWVST

Development of many new technologies is required to successfully produce the large, lightweight, deployable, cryogenic telescope with segmented primary mirror for the James Webb Space Telescope (JWST) mission. One of the technologies is interferometry to verify structural deformations in large, deployable, lightweight, cryogenic, precision structures to nanometer level accuracy. An instantaneous acquisition phase shifting speckle interferometer was designed and built to support the development of JWST optical telescope element (OTE) primary mirror backplane. This paper discusses characterization of the electronic speckle pattern interferometer (SPS-DSPI) developed for JWST to verify its capability to measure structural deformations in large composite structures at cryogenic temperature.