16 September 2011 Cryogenic focal plane flatness measurement with optical zone slope tracking
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We describe a non-contact optical measurement method used to determine the surface flatness of a cryogenic sensor array developed for the JDEM mission. Large focal planes envisioned for future visible to near infra-red astronomical large area point-source surveys such as JDEM, WFIRST, or EUCLID must operate at cryogenic temperatures while maintaining focal plane flatness within a few 10's of μm over half-meter scales. These constraints are imposed by sensitivity conditions that demand low noise observations from the sensors and the large-field, fast optical telescopes necessary to obtain the science yield. Verifying cryogenic focal plane flatness is challenging because μm level excursions need to be measured within and across many multi-cm sized sensors using no physical contact and while situated within a high-vacuum chamber. We have used an optical metrology Shack-Hartmann scheme to measure the 36x18 cm focal plane developed for the JDEM mission at the Lawrence Berkeley National Laboratory. The focal plane holds a 4x8 array of CCDs and HgCdTe detectors. The flatness measurement scheme uses a telescope-fed micro-lens array that samples the focal plane to determine slope changes of individual sensor zones.
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Jerry Edelstein, Martin Sirk, Patrick N. Jelinsky, Robert W. Besuner, Matthew Hoff, Paul Perry, Henry D. Heetderks, Christopher J. Bebek, Michael E. Levi, "Cryogenic focal plane flatness measurement with optical zone slope tracking", Proc. SPIE 8155, Infrared Sensors, Devices, and Applications; and Single Photon Imaging II, 81550N (16 September 2011); doi: 10.1117/12.892521; https://doi.org/10.1117/12.892521

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