Cryogenic focal plane flatness measurement with optical zone slope tracking
2011
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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