Babak Saif

ACOUSTO-OPTIC IMAGING SPECTROPOLARIMETRY FOR REMOTE SENSING

We review the operating principles of noncollinear acousto-optic tunable filters (AOTFs), emphasizing the use of two orthogonally polarized beams for narrow-band imaging. Spectral characterization and spectral broadening measurements of commercially available AOTFs agree with theoretical predictions and reveal difficulties associated with imaging noncollimated light. An AOTF imaging spectropolarimeter for ground-based astronomy that uses CCDs has been constructed at NASA Goddard Space Flight Center. It uses a TeO(2) noncollinear AOTF and a simple optical relay assembly to produce side-by-side orthogonally polarized spectral images. We summarize the instrument design and initial performance tests. We include sample spectral images acquired at the Goddard Geophysical and Astronomical Observatory.

Dynamic phase-shifting electronic speckle pattern interferometer

The technique for measuring changes in diffuse surfaces using Electronic Speckle Pattern Interferometry (ESPI) is well known. We present a new electronic speckle pattern interferometer that takes advantage of a single-frame spatial phase-shifting technique to significantly reduce sensitivity to vibration and enable complete data acquisition in a single laser pulse. The interferometer was specifically designed to measure the stability of the James Webb Space Telescope (JWST) backplane. During each measurement the laser is pulsed once and four phase-shifted interferograms are captured in a single image. The signal is integrated over the 9ns pulse which is over six orders of magnitude shorter than the acquisition time for conventional interferometers. Consequently, the measurements do not suffer from the fringe contrast reduction and measurement errors that plague temporal phase-shifting interferometers in the presence of vibration. This paper will discuss the basic operating principle of the interferometer, analyze its performance and show some interesting measurements.

Heterodyning scheme employing quantum interference

We propose a fundamentally new method of heterodyne detection of FIR signals using quantum interference between one- and two-photon absorption. We suggest different implementations of the scheme using multiple QW detectors and evaluate its sensitivity.

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.

Trades for ambient non-contact metrology

NASAs James Webb Space Telescope (JWST) will be a premier space science program for astrophysics following launch scheduled for 2014. JWST will observe the early universe, with emphasis on the time period during which the first stars and galaxies began to form. JWST has a 6.5 m diameter (25 square meters of collecting area), deployable, active primary mirror operating at cryogenic temperatures.

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.

JWST Optical Telescope Element Center of Curvature Test

The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) and Integrated Science Instrument Module (ISIM) completed their element level integration and test programs and were integrated to the next level of assembly called OTE/ISIM (OTIS) at Goddard Space Flight Center (GSFC) in Greenbelt, Maryland in 2016. Before shipping the OTIS to Johnson Space Center (JSC) for optical test at cryogenic temperature a series of vibration and acoustic tests were performed. To help ensure that the OTIS was ready to be shipped to JSC an optical center of curvature (CoC) test was performed to measure changes in the mirror’s optical performance to verify that the telescope’s primary mirror was not adversely impacted by the environmental testing and also help us in understanding potential anomalies identified during the JSC tests. The 6.5 meter diameter primary mirror consists of 18 individual hexagonal segments. Each segment is an off-axis asphere. There are a total of three prescriptions repeated six times each. As part of the CoC test each segment was individually measured using a high-speed interferometer (HSI) designed and built specifically for this test. This interferometer is capable of characterizing both static and dynamic characteristics of the mirrors. The latter capability was used, with the aid of a vibration stinger applying a low-level input force, to measure the dynamic characteristic changes of the PM backplane structure. This paper describes the CoC test setup and both static and dynamic test results.