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.
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).
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