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: 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).
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 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.
The alignment between the Aft Optical Subsystem (AOS) and the Integrated Science Instruments Module (ISIM) is non-adjustable
in orbit, so the alignment must be carefully verified in a cryogenic vacuum environment prior to launch.
Optical point source locations calibrated by optical metrology instruments are imaged through the AOS onto the Science
Instruments to determine focal, lateral, and clock angle alignment. The pupil image of the AOS is overlaid onto the
pupil image of the NIRCam to determine the tip and tilt alignment. In addition, an image from fiducial lights at the
Primary Mirror checks the pupil alignment between the telescope entrance pupil, the telescope pupil mask, and the
NIRCam aperture stop. The image positions are combined to determine the relative alignment between the Optical
Telescope Element (OTE) and the ISIM in all six degrees of freedom with corresponding alignment uncertainties.
Uncertainties in the position of focused images of the test sources and images from the pupils are derived from
sensitivities of an optical model of the system and the Science Instrument sensing capability. Additional uncertainty in
the pupil alignment measurement is due to uncertainty in the analytical removal of gravity effects that simulate the on-orbit
alignment environment.
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 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.
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