The crystal ball, the spider and other stories: a journey around  the test tower of the M4 adaptive mirror

Runa Briguglio; Giorgio Pariani; Marco Xompero; Beatrice Tofani; Marco Rossetti; Paolo Calabretto; Armando Riccardi

doi:10.1117/12.2311334

11 July 2018 The crystal ball, the spider and other stories: a journey around the test tower of the M4 adaptive mirror

Runa Briguglio, Giorgio Pariani, Marco Xompero, Beatrice Tofani, Marco Rossetti, Paolo Calabretto, Armando Riccardi

Proceedings Volume 10703, Adaptive Optics Systems VI; 1070373 (2018) https://doi.org/10.1117/12.2311334
Event: SPIE Astronomical Telescopes + Instrumentation, 2018, Austin, Texas, United States

Abstract

M4 is the adaptive mirror of ELT, currently at its FDR. It is composed by 6 thin shell mirror segments, controlled by 5136 voice coil actuators. Before its installation at the telescope, it will be optically calibrated on a test facility (OTT) in Italy. The calibration includes the computation of the flattening command and the segments co-phasing, i.e. the correction of the differential piston amongst them. Given the large complexity of the deformable mirror and the very tight requests on the measurement accuracy, we set-up a risk-mitigation activity based on the laboratory demonstration of some key elements within the test tower. In this paper we present the results of the experimentation. We measured at nanometer level the interferometric cavity; we investigated how the interferometer reacts in presence of spider arms dividing the test mirror into separated islands; we integrated and tested a multi wavelength sensor to measure the inter-segment absolute differential piston; we aligned and tested for stability the pupil relaying optical system to be installed on the OTT. Such activity is performed in the AO laboratory at INAF-Arcetri in Italy, in preparation of the M4 optical calibration on the OTT, scheduled to start in 2020. The M4 project is led by the Italian consortium AdOptica under an ESO contract.

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Runa Briguglio, Giorgio Pariani, Marco Xompero, Beatrice Tofani, Marco Rossetti, Paolo Calabretto, and Armando Riccardi "The crystal ball, the spider and other stories: a journey around the test tower of the M4 adaptive mirror", Proc. SPIE 10703, Adaptive Optics Systems VI, 1070373 (11 July 2018); https://doi.org/10.1117/12.2311334

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