Optical calibration of large format adaptive mirrors

Abstract: Adaptive (or deformable) mirrors are widely used as wavefront correctors in adaptive optics systems. The optical calibration of an adaptive mirror is a fundamental step during its life-cycle: the process is in facts required to compute a set of known commands to operate the adaptive optics system, to compensate alignment and non common-path aberrations, to run chopped or field-stabilized acquisitions. In this work we present the sequence of operations for the optical calibration of adaptive mirrors, with a specific focus on large aperture systems such as the adaptive secondaries. Such systems will be one of the core components of the extremely large telescopes. Beyond presenting the optical procedures, we discuss in detail the actors, their functional requirements and the mutual interactions. A specific emphasys is put on automation, through a clear identification of inputs, outputs and quality indicators for each step: due to a high degrees-of-freedom count (thousands of actuators), an automated approach is preferable to constraint the cost and schedule. In the end we present some algorithms for the evaluation of the measurement noise; this point is particularly important since the calibration setup is typically a large facility in an industrial environment, where the noise level may be a major show-stopper.