The James Webb Space Telescope (JWST) cryogenic testing required measurement systems that both obtain a very high degree of accuracy and can function in that environment. Close-range photogrammetry was identified as meeting those criteria. Extensive modeling prior to installation verified that the design would meet the desired accuracy goals. Extensive validation work was done to ensure that the actual as-built system met accuracy and repeatability requirements. The simulated image data predicted the uncertainty in measurement to be within specification and this prediction was borne out experimentally. Uncertainty at all levels of a measurement volume that comprised 8 × 8 m was verified experimentally to be <0.1 mm with a repeatability of <0.03 mm at the primary mirror (PM) level, achieving measurement accuracies on par with a laser tracker or radar system. During a 3-month Optical Telescope Element and Integrated Science Instrument Module thermal vacuum test performed in chamber A at Johnson Space Center, the photogrammetric system provided key data to allow for verification of actuator ranges needed for on-orbit alignment of JWST, alignment of the PM and secondary mirror (SM), and positional information needed for thermal model verification.
Direct imaging of exoearths with high-contrast internal coronagraphs depends on ultra-stable opto-mechanical systems. Ultra-stable mirror assemblies enable decadal survey missions like LUVOIR and HabEx. To precisely define the necessary level of stability, the essential first step is to budget the maximum allowable disturbances for each optic in the system. Ideally, allocations are budgeted with respect to spatial- and time-domain frequencies. If allocations do not span these domains, the optic assembly designer cannot take advantage of frequency bands where requirements are looser because of assumptions about telescope control systems and internal coronagraph filtering. This paper explores how mirror assembly technologies and designs are predicted to impact stability, especially within the frequency bands that drive coronagraph contrast performance.
Coronographic missions require ultra-stable mirror systems to achieve 10 E-10 contrast. The LUVOIR ULTRA study is assessing technological capabilities for a 15-meter telescope requiring sub nanometer optical stability. For ULTRA individual mirror stabilities at the picometer level are required. Thermal sensitivities for a proposed mirror design have been incorporated into a stability budget that indicates the level of stability required is attainable. Key factors to meeting stability allocations are an athermal design, acceptable mirror CTE homogeneity and mirror mount pad design. This paper will present the sensitivities and error budget used to predict the on-orbit mirror stability.
The author requests that a paper given by Matt East from Harris come before this presentation if both are presented.
KEYWORDS: Adaptive optics, Mirrors, James Webb Space Telescope, Cryogenics, Telescopes, Optical alignment, Space telescopes, Photogrammetry, Monte Carlo methods, Actuators
The James Webb Space Telescope is a large space-based astronomical telescope that will operate at cryogenic temperatures. Because of its size, the telescope must be stowed in an inoperable configuration for launch and remotely reconfigured in space to meet the operational requirements using active Wave Front Sensing and Control (WFSC). Predicting optical performance for the flight system relies on a sequence of incremental tests and analyses that has culminated with the cryogenic vacuum test of the integrated Optical Telescope Element (OTE) and Integrated Science Instrument Module (ISIM) referred to as OTIS. The interplay between the optical budgeting process, test verification results at incrementally increasing levels of integration, use of test validated models, and the WFSC process to produce the final optical performance predictions for final verification by analysis will be presented.
The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point-to-point. Each segment has a 6 degree-of-freedom hexapod actuation system and a radius-of-curvature (ROC) actuation system. The full telescope was tested at its cryogenic operating temperature at Johnson Space Center (JSC) in 2017. This testing included center-of-curvature measurements of the PM wavefront error using the Center-of-Curvature Optical Assembly (COCOA), along with the Absolute Distance Meter Assembly (ADMA). The COCOA included an interferometer, a reflective null, an interferometer-null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple-wavelength interferometer was used to enable alignment and phasing of the PM segments. By combining measurements at two laser wavelengths, synthetic wavelengths up to 15 mm could be achieved, allowing mirror segments with millimeter-level piston errors to be phased to the nanometer level. The ADMA was used to measure and set the spacing between the PM and the focus of the COCOA null (i.e., the PM center-of-curvature) for determination of the ROC. This paper describes the COCOA, the PM test setup, the testing performed, the test results, and the performance of the COCOA in aligning and phasing the PM segments and measuring the final PM wavefront error.
KEYWORDS: Image segmentation, James Webb Space Telescope, Space telescopes, Wavefronts, Telescopes, Wavefront sensors, Control systems, Mirrors, Image processing, Sensors
A subset of the Wavefront Sensing and Controls (WFSC) operations for JWST were demonstrated during its recent cryo-vac testing using the flight telescope and instruments, and a functional simulation of the spacecraft and ground system. The demonstration had three goals: to confirm the operation of the flight data collection scripts, to check the WFSC optical components, and to verify the coordinates and influence functions that will be used for flight WFSC. In this paper, we present the results and lessons learned from this demonstration.
The James Webb Space Telescope (JWST) cryogenic testing requires measurement systems that both obtain a very high
degree of accuracy and can function in that environment. Close-range photogrammetry was identified as meeting those
criteria.
Testing the capability of a close-range photogrammetric system prior to its existence is a challenging problem. Computer
simulation was chosen over building a scaled mock-up to allow for increased flexibility in testing various configurations.
Extensive validation work was done to ensure that the actual as-built system meets accuracy and repeatability
requirements. The simulated image data predicted the uncertainty in measurement to be within specification and this
prediction was borne out experimentally. Uncertainty at all levels was verified experimentally to be <0.1 mm.
KEYWORDS: Mirrors, James Webb Space Telescope, Cryogenics, Interferometers, Space telescopes, Mirrors, James Webb Space Telescope, Cryogenics, Space telescopes, Image segmentation, Wavefronts, Optical fabrication, Aerospace engineering, Telescopes
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, 18
segment primary mirror, all-reflective, three-mirror anastigmat operating at cryogenic temperatures. To verify
performance of the primary mirror, a full aperture center of curvature optical null test is performed under cryogenic
conditions in Chamber A at the National Aeronautics and Space Administration (NASA) Johnson Space Center (JSC)
using an instantaneous phase measuring interferometer. After phasing the mirrors during the JWST Pathfinder
testing, the interferometer is utilized to characterize the mirror relative piston and tilt dynamics under different facility
configurations. The correlation between the motions seen on detectors at the focal plane and the interferometer validates
the use of the interferometer for dynamic investigations. The success of planned test hardware improvements will be
characterized by the multi-wavelength interferometer (MWIF) at the Center of Curvature Optical Assembly (CoCOA).
After integration of the Optical Telescope Element (OTE) to the Integrated Science Instrument Module (ISIM) to
become the OTIS, the James Webb Space Telescope OTIS is tested at NASA’s Johnson Space Center (JSC) in the
cryogenic vacuum Chamber A for alignment and optical performance.
The alignment of the mirrors comprises a sequence of steps as follows: The mirrors are coarsely aligned using
photogrammetry cameras with reflective targets attached to the sides of the mirrors. Then a multi-wavelength
interferometer is aligned to the 18-segment primary mirror using cameras at the center of curvature to align reflected
light from the segments and using fiducials at the edge of the primary mirror. Once the interferometer is aligned, the 18
primary mirror segments are then adjusted to optimize wavefront error of the aggregate mirror. This process phases the
piston and tilt positions of all the mirror segments. An optical fiber placed at the Cassegrain focus of the telescope then
emits light towards the secondary mirror to create a collimated beam emitting from the primary mirror. Portions of the
collimated beam are retro-reflected from flat mirrors at the top of the chamber to pass through the telescope to the
Science Instrument (SI) detector. The image on the detector is used for fine alignment of the secondary mirror and a
check of the primary mirror alignment using many of the same analysis techniques used in the on-orbit alignment.
The entire process was practiced and evaluated in 2015 at cryogenic temperature with the Pathfinder telescope.
The James Webb Space Telescope (JWST) primary mirror (PM) is 6.6 m in diameter and consists of 18 hexagonal segments, each 1.5 m point-to-point. Each segment has a six degree-of-freedom hexapod actuation system and a radius of-curvature (RoC) actuation system. The full telescope will be tested at its cryogenic operating temperature at Johnson Space Center. This testing will include center-of-curvature measurements of the PM, using the Center-of-Curvature Optical Assembly (COCOA) and the Absolute Distance Meter Assembly (ADMA). The COCOA includes an interferometer, a reflective null, an interferometer-null calibration system, coarse and fine alignment systems, and two displacement measuring interferometer systems. A multiple-wavelength interferometer (MWIF) is used for alignment and phasing of the PM segments. The ADMA is used to measure, and set, the spacing between the PM and the focus of the COCOA null (i.e. the PM center-of-curvature) for determination of the ROC. The performance of these metrology systems was assessed during two cryogenic tests at JSC. This testing was performed using the JWST Pathfinder telescope, consisting mostly of engineering development and spare hardware. The Pathfinder PM consists of two spare segments. These tests provided the opportunity to assess how well the center-of-curvature optical metrology hardware, along with the software and procedures, performed using real JWST telescope hardware. This paper will describe the test setup, the testing performed, and the resulting metrology system performance. The knowledge gained and the lessons learned during this testing will be of great benefit to the accurate and efficient cryogenic testing of the JWST flight telescope.
In 2014, the Optical Ground Support Equipment was integrated into the large cryo vacuum chamber at Johnson Space Center (JSC) and an initial Chamber Commissioning Test was completed. This insured that the support equipment was ready for the three Pathfinder telescope cryo tests. The Pathfinder telescope which consists of two primary mirror segment assemblies and the secondary mirror was delivered to JSC in February 2015 in support of this critical risk reduction test program prior to the flight hardware. This paper will detail the Chamber Commissioning and first optical test of the JWST Pathfinder telescope.
The light-weighted design of the Optical Telescope Element (OTE) of the James Webb Telescope (JWST) leads to
additional sensitivity to vibration from the ground – an important consideration to the measurement uncertainty of the
wavefront error (WFE) in the primary mirror. Furthermore, segmentation of the primary mirror leads to rigid-body
movements of segment areas in the WFE. The ground vibrations are minimized with modifications to the test facility,
and by the architecture of the equipment supporting the load. Additional special test equipment (including strategically
placed isolators, tunable mass dampers, and cryogenic magnetic dampers) mitigates the vibration and the response
sensitivity before reaching the telescope. A multi-wavelength interferometer is designed and operated to accommodate
the predicted residual vibration. Thermal drift also adds to the measurement variation. Test results of test equipment
components, measurement theory, and finite element analysis combine to predict the test uncertainty in the future
measurement of the primary mirror. The vibration input to the finite element model comes from accelerometer
measurements of the facility with the environmental control pumps operating. One of the isolators have been built and
tested to validate the dynamic performance. A preliminary model of the load support equipment and the OTE with the
Integrated Science Instrument Module (ISIM) is complete. The performance of the add-on dampers have been
established in previous applications. And operation of the multi-wavelength interferometer was demonstrated on a
scaled hardware version of the JWST in an environment with vibration and thermal drift.
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 meter clear aperture, all-reflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. Two wavefront calibration tests are utilized to verify the low and mid/high spatial frequency performance of the test system. In this paper the methods and results of the wavefront calibration tests are presented.
KEYWORDS: James Webb Space Telescope, Mirrors, Space telescopes, Image segmentation, Liquid crystal lasers, Interferometers, Telescopes, Phase modulation, Wavefronts, Space mirrors
The James Webb Space Telescope (JWST) integration includes a center of curvature test on its 18 primary mirror
segment assemblies (PMSAs). This important test is the only ground test that will demonstrate the ability to align all 18
PMSAs. Using a multi-wavelength interferometer (MWIF) integrated to the test bed telescope (TBT), a one-sixth scale
model of the JWST, we verify our ability to align and phase the 18 PMSAs. In this paper we will discuss data analysis
and test results when using the MWIF to align the segments of the TBT in preparation for alignment of the JWST.
The Optical Telescope Element (OTE) consists of a 6.6 m, all-reflective, three-mirror anastigmat. The 18-segment
primary mirror (PM) presents unique and challenging assembly, integration and alignment verification requirements.
Each mirror segment is mechanically integrated with the Primary Mirror Backplane Support Structure (PMBSS) using
compound angle shims to compensate for global alignment and local co-planarity errors. The processes used to
determine the mechanical shim prescription, primary mirror alignment and integration, and placement verification are
discussed. In an effort to reduce process uncertainty and program risk, the JWST program recently conducted a PMSA
Integration Demonstration at ITT. Through this activity, full scale demonstrations of the Ground Support Equipment
(GSE) and critical integration processes were successfully completed. The results of these demonstrations indicate that
the equipment, processes, and procedures developed by ITT meet the critical requirements for PMSA placement.
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective,
three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly,
integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic
vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature
Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA
contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration
capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA
distance. Two wave front calibration tests are utilized to verify the low and Mid/High spatial frequencies, overcoming
the limitations of the standard null/hologram configuration in its ability to resolve mid and high spatial frequencies. This
paper will introduce the systems level architecture and optical test layout for the CoCOA.
The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 m clear aperture, allreflective,
three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly,
integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic
vacuum conditions at the integrated observatory level to verify PM performance requirements. The Center of Curvature
Optical Assembly (CoCOA), designed and being built by ITT satisfies the requirements for this test. The CoCOA
contains a multi wave interferometer, patented reflective null lens, actuation for alignment, full in situ calibration
capability, coarse and fine alignment sensing systems, as well as a system for monitoring changes in the PM to CoCOA
distance. This paper will introduce the systems level architecture and optical layout of the CoCOA and its main
subsystems.
The Optical Telescope Element (OTE) consists of a 6.6 m, all-reflective, three-mirror anastigmat. The 18-segment
primary mirror (PM) presents unique and challenging assembly, integration and alignment requirements. To integrate
and verify each of the Primary Mirror Segment Assemblies (PMSAs), an integrated network of laser trackers will be
used in the Ambient Optical Assembly Stand (AOAS). The AOAS consists of an optical bench (OB) to support the
JWST Optical Telescope Element (OTE), a personnel access platform structure (PAPS), an optics integration gantry
system (OIGS), and a PMSA alignment and integration fixture (PAIF). The PAIF and OIGS are used to deliver; offload
and precision align each PMSA segment and the aft optical subsystem (AOS) to their integration locations. This paper
will introduce the functional design of this ground support equipment (GSE), illustrate the coordinate systems used for
integration, and detail the integration processes.
KEYWORDS: Observatories, James Webb Space Telescope, Image quality, Mirrors, Cryogenics, Space telescopes, Data modeling, Telescopes, Optical components, Optical testing
The optical system of the James Webb Space Telescope (JWST) is split between two of the Observatory's element, the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM). The OTE optical design consists of an 18-hexagonal segmented primary mirror (25m2 clear aperture), a secondary mirror, a tertiary mirror, and a flat fine steering mirror used for fine guidance control. All optical components are made of beryllium. The primary and secondary mirror elements have hexapod actuation that provides six degrees of freedom rigid body adjustment. The optical components are mounted to a very stable truss structure made of composite materials. The OTE structure also supports the ISIM. The ISIM contains the Science Instruments (SIs) and Fine Guidance Sensor (FGS) needed for acquiring mission science data and for Observatory pointing and control and provides mechanical support for the SIs and FGS. The optical performance of the telescope is a key performance metric for the success of JWST. To ensure proper performance, the JWST optical verification program is a comprehensive, incremental, end-to-end verification program which includes multiple, independent, cross checks of key optical performance metrics to reduce risk of an on-orbit telescope performance issues. This paper discusses the verification testing and analysis necessary to verify the Observatory's image quality and sensitivity requirements. This verification starts with component level verification and ends with the Observatory level verification at Johnson Space Flight Center. The optical verification of JWST is a comprehensive, incremental, end-to-end optical verification program which includes both test and analysis.
NASA’s James Webb Space Telescope (JWST) is a 6.6 m all-reflective on-axis three-mirror anastigmat. The 18-segment and fully actuated primary mirror (PM) presents unique and challenging assembly, integration and alignment requirements. In order to properly integrate the Primary Mirror Segment Assemblies (PMSAs) and align the completed PM, a variety of ground support equipment is used within a tower specifically built for the program. Handling fixtures, gravity offloaders, a variety of optical references, laser scanners, and interferometers are used. This paper outlines the hardware and techniques for integration and alignment of the JWST PM.
An imaging ellipsometer has been developed which employs phase shifting interferometry to characterize the ellipsometric parameters. A modified Michelson interferometer is used in conjunction with a Wollaston prism to generate two interferograms with orthogonal polarization states. Subtraction of the phases in the two interferograms yields the ellipsometric parameter (Delta) . The fringe modulation of the two interferograms is used to calculate the ellipsometric parameter (Psi) . The characterization of the average intensity of the interferogram is the largest contributor to the errors in the modulation. New algorithms for reducing the errors in modulation calculations for phase shifting interferometry are presented. The deign of the instrument, results of measurements and algorithms for modulation characterization will be presented.
An imaging ellipsometer has been developed which employs phase shifting interferometry to characterize the ellipsometric parameters. Polarized light from a laser or incoherent source is collimated and reflected off of the surface under test. A modified Michelson interferometer is used in conjunction with a Wollaston prism to generate two interferograms with orthogonal polarization states. Subtraction of the phases in the two interferograms yields the ellipsometric parameter (Delta) . The fringe modulation of the two interferograms is used to calculate the ellipsometric parameter (Psi) . The instrument uses imaging optics to image the surface under test to a CCd, yielding a truly two dimensional ellipsometric measurement. The deign of the instrument and result of measurement will be presented.
Polarization modeling of imaging systems has assumed more importance in recent earth remote sensing systems. Santa Barbara Research Center (SBRC) has developed computer programs that model the polarization characteristics of optical systems. This single ray simplified approach can be used when coating design information is not available or for modeling of an as built instrument using measured component data. For systems without exotic coating designs, the polarization ray trace capability of the optical design program CodeV is used to perform polarization modeling. Two polarization modeling case studies are reported. Measured component data is used for single ray modeling of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor currently being built at SBRC. CodeV was used to model the full bundle of a five mirror off axis unobstructed ground based calibrator.
A high precision polarimeter based on a dual-beam analyzer prism is capable of polarimetric accuracy's near +0.2% over the visible to near-IR range. Accuracy's were determined using computed Fresnel polarizances from fused silica at several incidence angles. A comparison of polarization measurements made at several different laboratories is presented.
The 2-percent polarization sensitivity specification in many of NASA's Earth Observing System sensors has motivated Santa Barbara Research Center (SBRC) to develop computer programs that model the polarization characteristics of optical systems. In support of the Moderate Resolution Imaging Spectroradiometer program, SBRC has developed an analytical tool which unites the completed 3D lens design and coating designs to accurately predict an optical system's polarization characterization compensators, nonnormal incidence reflection and transmission from dichroic beam splitters and protected silver mirrors are analyzed without the need for proprietary coating prescriptions. The phenomenon of diattenuation and phase change are treated using the Jones matrix formalism.
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