14 September 2011 In-process testing for cryo-figuring 1.5 meter diameter auto-collimating flats
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Three auto-collimating flats (ACFs) of 1.5 meter clear aperture are being manufactured for use in the JSC Cryo-Optical Metrology test of the James Webb Space Telescope. In-process interferometric testing of the ACFs is used to guide their surface-figure processing. The surface measurement is performed in a vacuum chamber at both room (+20 °C) and cryogenic (-240 °C) temperatures. With a 12-inch beam diameter FizCam interferometer, sub-aperture measurements are taken across the ACF diameter at multiple rotations. These measurements are stitched together to compute the surface figure. The figure change between room-temperature and cryogenic temperature is measured and used to enable cryo-figuring based on room-temperature measurements. The data analysis is calibrated to account for gravity sag on test-set optics and surface aberrations caused by vacuum pressure and temperature gradients on vacuum-chamber windows. The first ACF is complete and meets specification with surface error of less than 75 nm RMS.
© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David J. Fischer, David J. Fischer, Joseph Hayden, Joseph Hayden, James K. Lawton, James K. Lawton, Brandon Olson, Brandon Olson, Chris Brophy, Chris Brophy, Scott Kennard, Scott Kennard, } "In-process testing for cryo-figuring 1.5 meter diameter auto-collimating flats", Proc. SPIE 8146, UV/Optical/IR Space Telescopes and Instruments: Innovative Technologies and Concepts V, 81460A (14 September 2011); doi: 10.1117/12.896355; https://doi.org/10.1117/12.896355

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