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11 July 2018 Prototyping of large deformable mirrors for TMT: test results
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Stack Array Mirrors (SAM) technology offers high order correction, with up to several thousands of actuators, controllable at high frequency, up to several kHz. A new generation of piezo-electric actuators with high reliability has been developed during the last years. This technology is well-adapted for large deformable mirrors (DMs) with thousands of actuators for future needs for Extremely Large Telescopes. We present the design and the modelling of the two large DMs for NFIRAOS, the multi-conjugate adaptive optics system of the Thirty Meter Telescope (TMT): DM0 which shows 3125 actuators and DM11 which shows 4548 actuators. A DM prototype with 616 actuators has been manufactured to validate the manufacturing steps and the specifications of the future large DMs, including their behavior at both ambient and low temperature (-30°C). The prototype includes the new generation of piezo actuators with improved reliability thanks to an optimization of the fabrication processes. Experimental results of accelerated ageing tests and mechanical fatigue are presented. After complete assembly, the prototype is qualified in a specific cool chamber with interferometric measurements. The results are the following: operational stroke higher than 10 μm PV at both ambient and -30°C with uniformity better than 5%, overall non-linearity lower than 5%, resonance frequency of the actuators higher than 10 kHz. Based on the measurements done on the overall temperature range (+20°C to -30°C), the best flat is lower than the goal specification of 10 nm RMS mechanical. An enhanced protected silver coating done by magnetron sputtering allows a high level of reflectivity especially in the near infrared range and long-life durability.
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