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MOEMS Deformable Mirrors (DM) are key components for next generation optical instruments implementing innovative adaptive optics systems, both in existing telescopes and in the future ELTs. Characterizing these components well is critical for next generation instruments. This is done by interferometry, including surface quality measurement in static and dynamical modes, at ambient and in vacuum/cryo. We use a compact cryo-vacuum chamber designed for reaching 10–6 mbar and 160 K in front of our custom Michelson interferometer, which is able to measure performance of the DM at actuator/segment level and at the entire mirror level, with a lateral resolution of 2 μm and a sub-nanometer z-resolution. We tested the PTT 111 DM from Iris AO: an array of single crystalline silicon hexagonal mirrors with a pitch of 606 μm, able to move in tip, tilt, and piston (stroke 5–7 μm, tilt +/− 5 mrad). The device could be operated successfully from ambient to 160 K. At room temperature, we developed an improved best flat procedure and obtained a mirror surface deformation, as low as 10 nm RMS. An additional, 500 nm deformation of the entire mirror is measured at 160K; by analyzing at all spatial scales our measurements, we are able to discriminate the different effects at packaging, actuator and mirror segment level. We developed a strategy of weighted addition of the consecutive measurement residual errors to be applied to each actuator; in a single measurement step, we were able to recover the cryo best flat by correcting the focus and local tip-tilts on all segments, reaching a mirror surface deformation as low as 12 nm RMS at 160K. Finally, the goal of these studies is to test DMs in cryo and vacuum conditions as well as to improve their architecture for stable operation in a harsh environment. They are foreseen in a wide variety of applications, from astronomy as presented in this paper, but also in microscopy and in laser beam shaping.
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