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MOEMS Deformable Mirrors (DM) are key components for next generationinstruments with innovative adaptive optics systems, in existing telescopes andin the future ELTs. These DMs must perform at room temperature as well as incryogenic and vacuum environment. Ideally, the MOEMS-DMs must be designed tooperate in such environment. We present some major rules for designing /operating DMs in cryo and vacuum. We chose to use interferometry for the fullcharacterization of these devices, including surface quality measurement instatic and dynamical modes, at ambient and in vacuum/cryo. Thanks to ourprevious set-up developments, we placed a compact cryo-vacuum chamber designedfor reaching 10-6 mbar and 160K, in front of our custom Michelsoninterferometer, able to measure performances of the DM at actuator/segmentlevel as well as whole mirror level, with a lateral resolution of 2{\mu}m and asub-nanometric z-resolution. Using this interferometric bench, we tested theIris AO PTT111 DM: this unique and robust design uses an array of singlecrystalline silicon hexagonal mirrors with a pitch of 606{\mu}m, able to movein tip, tilt and piston with strokes from 5 to 7{\mu}m, and tilt angle in therange of +/-5mrad. They exhibit typically an open-loop flat surface figure asgood as <20nm rms. A specific mount including electronic and opto-mechanicalinterfaces has been designed for fitting in the test chamber. Segmentdeformation, mirror shaping, open-loop operation are tested at room and cryotemperature and results are compared. The device could be operated successfullyat 160K. An additional, mainly focus-like, 500 nm deformation is measured at160K; we were able to recover the best flat in cryo by correcting the focus andlocal tip-tilts on some segments. Tests on DM with different mirror thicknesses(25{\mu}m and 50{\mu}m) and different coatings (silver and gold) are currentlyunder way.

MOEMS deformable mirror testing in cryo for future optical instrumentation