Future space observatory missions require controlling wave front error and system alignment stability to picometer scale. Picometer stability performance demands precision knowledge of the mirror and metering structure materials to the same level. A high-speed electronic speckle pattern interferometer was designed and built to demonstrate measurements of both static and dynamic responses of picometer level amplitudes in mirror and structural materials subjected to very low energy disturbances. This paper summarizes the current status of tests to impart a dynamic disturbance of picometer scale and measure the response of specular and diffuse materials. The results show that subpicometer scale effects can be accurately measured in an open test environment outside a vacuum chamber.
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.
The James Webb Space Telescope (JWST) recently saw the completion of the assembly process for the Optical Telescope Element and Integrated Science Instrument Module (OTIS). This integration effort was performed at Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. In conjunction with this assembly process a series of vibration and acoustic tests were performed. To help assure the telescope’s primary mirror was not adversely impacted by this environmental testing an optical center of curvature (CoC) test was performed to measure changes in the mirror’s optical performance. The primary is a 6.5 meter diameter mirror consisting 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, an innovative alignment method, and both static and dynamic test results.
James Webb Space Telescope Optical Telescope Element (OTE) is a three mirror anastigmat consisting of a 6.5 m primary mirror (PM), a secondary mirror (SM) and a tertiary mirror. The primary mirror is made out of 18 segments. The telescope and instruments will be assembled at Goddard Space Flight Center (GSFC) to build the Optical Telescope Element-Integrated Science Instrument Module (OTIS). The OTIS will go through environmental testing at GSFC before being transported to Johnson Space Center for testing at cryogenic temperature. The objective of the primary mirror Center of Curvature test (CoC) is to characterize the PM before and after the environmental testing for workmanship. This paper discusses the CoC test including both a surface figure test and a new method for characterizing the state of the primary mirror using high speed dynamics interferometry.
The James Webb Space Telescope (JWST) Primary Mirror Segment Assembly (PMSA) was required to meet NASA
Technology Readiness Level (TRL) 06 requirements in the summer of 2006. These TRL06 requirements included
verifying all mirror technology systems level readiness in simulated end-to-end operating conditions. In order to support
the aggressive development and technology readiness schedule for the JWST Primary Mirror Segment Assembly
(PMSA), a novel approach was implemented to verify the nanometer surface figure distortion effects on an in-process
non-polished beryllium mirror surface. At the time that the TRL06 requirements needed to be met, a polished mirror
segment had not yet been produced that could have utilized the baselined interferometric optical test station. The only
JWST mirror segment available was a finished machined segment with an acid-etched optical surface. Therefore an
Electronic Speckle Pattern Interferometer (ESPI) was used in coordination with additional metrology techniques to
perform interferometric level optical testing on a non-optical surface. An accelerated, rigorous certification program was
quickly developed for the ESPI to be used with the unfinished optical surface of the primary mirror segment. The ESPI
was quickly implemented into the PMSA test program and optical testing was very successful in quantifying the
nanometer level surface figure deformation changes in the PMSA due to assembly, thermal cycling, vibration, and
acoustic testing. As a result of the successful testing, the PMSA passed all NASA TRL06 readiness requirements.
James Webb Space Telescope (JWST) has a segmented Primary Mirror (PM). PM is made of 18 beryllium hexagonal shaped segments. Flat-to-flat dimension of a segment is 1.315 meters. The PM is an ellipsoid of~ 6.5 meters in diameter with a conic constant of -0.99666 and a radius of curvature ~16 meters. After the PM, telescope are assembled and instruments are installed the observatory will go through environmental testing. The environmental test consist of acoustic and vibration test. The objective is to measure the change in the surface astigmatism of the Primary mirror segments at center of curvature before and after vibration and acoustic test. At the final stage of assembly the inner segments of the PM have no external fiducials. The challenge is to separate the alignment astigmatism from surface astigmatism without any external fiducials. This paper describes an alignment method that uses the print-through in the mirror segments as fiducials to separate the two astigmatisms.
NASA's 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.
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.
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.
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.
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
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.
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.
Digital Speckle Pattern Interferometry (DSPI) is a well-established method for the measurement of diffuse objects in experimental mechanics. DSPIs are phase shifting interferometers. Three or four bucket temporal phase shifting algorithms are commonly used to provide phase shifting. These algorithms are sensitive to vibrations and can not be used to measure large optical structures far away from the interferometer. In this research a simultaneous phase shifted interferometer, PhaseCam product of 4D Technology Corporation in Tucson Arizona, is modified to be a Simultaneous phase shifted Digital Speckle Pattern Interferometer (SDSPI). Repeatability, dynamic range, and accuracy of the SDSPI are characterized by measuring a 5 cm x 5 cm carbon fiber coupon.
An Integrated Product Team was formed to develop a detailed concept for optical test methodology for testing of the NGST individual primary, secondary and tertiary mirrors and the full telescope system on the ground. The large, lightweight, deployable primary mirror, and the cryogenic operating environment make optical testing of NGST OTA (Optical Testing Assembly) extremely challenging. A telescope of the complexity of NGST has never been built and tested on the ground in 1-g environment. A brief summary of the preliminary metrology test plan at the mirror component and telescope system level is presented.
Several designs of filters for use in vacuum UV imaging systems are discussed. These designs incorporate all reflective optics,and are characterized by comparatively high in-band throughout, very low out-of-band transmission and sub-arcsecond spatial resolution. In addition, they an be tuned over ranges useful for vacuum UV astronomical observations. Results from a simplified laboratory version of the filters intended to prove the concept are presented.
A compact, acousto-optic tunable filter (AOTF) imaging spectropolarimeter for ground based astronomy from 400-1100 nm has been constructed at NASA/GSFC. The key components of this instrument are a TeO2 non-collinear AOTF, CCD camera, and an all-reflective optical relay assembly which uses a single elliptical mirror to produce side-by-side orthogonally polarized spectral images. The instrument was used at the Lowell Observatory 42-inch telescope for 'first light' planetary imaging and measurements of photometric standard stars. Narrow-band images of Saturn near 700 nm appear to show polarization effects which result from multiple scattering by aerosols. The instrument has recently been upgraded in order to integrate the RF drive electronics and eliminate contamination by scattered light. Design of the instrument and some initial results are presented.
The spatial pointing angle and far field beamwidth of a high-power semiconductor laser are characterized as a function of CW power and also as a function of temperature. The time-averaged spatial pointing angle and spatial lobe width were measured under intensity-modulated conditions. The measured pointing deviations are determined to be well within the pointing requirements of the NASA Laser Communications Transceiver (LCT) program. A computer-controlled Mach-Zehnder phase-shifter interferometer is used to characterize the wavefront quality of the laser. The rms phase error over the entire pupil was measured as a function of CW output power. Time-averaged measurements of the wavefront quality are also made under intensity-modulated conditions. The measured rms phase errors are determined to be well within the wavefront quality requirements of the LCT program.