Cryogenic focal plane flatness measurement with optical zone slope tracking Jerry Edelstein 1 , Martin Sirk 1 , Patrick N. Jelinsky 1 , Robert W. Besuner 1 , Matthew Hoff 2 , Paul Perry 2 , Henry D. Heetderks 1 , Christopher J. Bebek 2 , Michael E. Levi 2 1 U.C. Berkeley Space Sciences Laboratory 2 Lawrence Berkeley National Laboratory ABSTRACT 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. Keywords: Flatness, metrology, JDEM, WFIRST, CCD, HgCdTe, Silicon Carbide, Space Telescope 1 INTRODUCTION Large focal planes envisioned for future visible to near infra-red astronomical large area 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. We have developed an optical surface profilometer in order to verify the cryogenic flatness of a focal plane that contains a mixed array of 32 CCD and HgCdTe detectors mounted on a SiC structure and covering a 36x18 cm active area. The focal plane was developed 1 at the Lawrence Berkeley National Laboratory as a technology demonstrator for the JDEM mission. The cryogenic flatness of the Demonstrator constitutes important metrology verification for ultimate mission science performance based on requirements established for the former proposed SuperNova/Acceleration Probe 2 (SNAP) and JDEM space telescopes. Some of the key mechanical and thermal performance goals include detector focal surface flatness better than 40 μm P-V cold across 36 cm, at operating at temperatures as low as 120K. Verifying cryogenic focal plane flatness is challenging because excursions of a few ȝm need to be measured without physical contact both across half-meter scales and within numerous multi-cm sized sensors - all operating from room to cryogenic temperatures within a vacuum vessel. Our objective was to provide validation data on the mode and magnitude of numeric thermo-mechanical distortion models of the focal plane and sensors. We planned to first perform non-contact optical metrology over the focal plane at room temperature with a coordinate measuring machine and then to examine changes over the sensors and focal plane as a result of cooling. To accomplish this, we chose a measurement scheme that observes the change in slope of numerous individual regions over each sensor within a large fraction of the focal plane. The results have revealed useful information regarding the surface form of CCD and NIR detector packages as well as the entire focal plane. Infrared Sensors, Devices, and Applications; and Single Photon Imaging II, edited by Paul D. LeVan, Ashok K. Sood, Priyalal S. Wijewarnasuriya, Manijeh Razeghi, Jose Luis Pau Vizcaíno, Rengarajan Sudharsanan, Melville P. Ulmer, Tariq Manzur, Proc. of SPIE Vol. 8155, 81550N · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.892521 Proc. of SPIE Vol. 8155 81550N-1