KEYWORDS: Mirrors, James Webb Space Telescope, Optical testing, Adaptive optics, Image segmentation, Space telescopes, Telescopes, Optical components, Wavefronts, Temperature metrology
In 2017, the James Webb Space Telescope Optical Telescope Element and Integrated Science Instrument Module (OTIS) underwent cryogenic optical testing at the Johnson Space Center. In this paper, we summarize the successful execution and results of this 100-day test, which was a major program milestone. We summarize the as-run test configuration and provide a top-level as-run timeline. We also provide the top-level functional, optical, thermal, and operational results from the test. We summarize the key technical issues encountered and the resolution of those issues. The results of the OTIS test indicate that the payload should be fully capable of delivering on JWST’s exciting scientific potential.
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.
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.
The James Webb Space Telescope Optical Telescope Element (OTE) and Optical Telescope Element/Integrated Science
Instrument Module (OTIS) will be tested at the same time in the final and only cryogenic optical test of the observatory.
Due to the size and temperature of JWST, this is a complex test which has undergone changes in the last year aimed at
reducing test execution risk. We will summarize the test plan changes, architecture changes, and predicted timeline
changes for this test. We will also explain the checkout plans for assuring the test will go smoothly.
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 James Webb Space Telescope (JWST) requires testing of the full optical system in a cryogenic vacuum environment
before launch. Challenges with the telescope architecture and the test environment lead to placing removable optical test
sources at the Cassegrain intermediate focus of the Telescope. The Science Instrument suite will be used to align the
telescope and to verify the wavefront error. The Science Instruments capture test images that are analyzed using focus
diverse phase retrieval. The wavefront sensing algorithms have the large dynamic range required to measure the
relatively small wavefronts of interest in the presence of the large aberrations resulting from the off-axis source locations
at the intermediate focus. These inherent aberrations of the off-axis design are removed analytically from the measured
data. The test design and in-situ wavefront sensing process enables a number of tests to verify the alignment and optical
quality of the system.
It is imperative that we have high confidence that the optical performance capability of JWST is well-understood
before launch. With the telescope operating at cryogenic temperatures and sporting a 6.6 meter primary mirror
diameter, the optical metrology equipment required to measure the optical performance can be quite complex. The
JWST Test team undertook an effort to greatly simplify the optical metrology approach, while retaining the key
measurements and verification methodology. The result is a cryogenic optical test configuration and
implementation using Chamber A at NASA's Johnson Space Center that uses the science instruments to help
understand JWST's optical performance.
The James Webb Space Telescope (JWST) is a large aperture (6.6 m primary mirror) cryogenic telescope with active
control of the segmented primary and secondary mirror optical elements. The architecture of the telescope makes full
end-to-end testing on the ground prohibitive due to both cost and technical considerations. Additionally, because the
telescope will be launched in a folded configuration to fit in the Ariane V launch fairing and aligned during flight using
image-based Wavefront Sensing and Control (WFS&C), the telescope cannot be tested in the classical "test-as-you-fly"
architecture. Due to these considerations, the primary optical performance requirements will be verified through analysis.
In order to have high confidence in this approach, a robust analysis validation program has been developed based on
testing from the component level through the integrated telescope level. This verification approach focuses on ground
testing at the telescope level to ensure there will be adequate range in the adjustable optics for alignment on orbit. In
addition to the incremental test program planned for optical verification, a double-pass sampled aperture test of the integrated
telescope and instruments is planned at flight-like temperatures as a crosscheck to the analytic verification for
flight. Error budgets have been developed to understand the uncertainty propagation through the test and analysis program.
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