An active wavefront sensor to make feasible adaptive optics on 100-m class telescopes

Layer Oriented wavefront sensors can be made with a reasonable compact detector by the adoption of several stars enlargers, increasing only locally the focal ratio on the reference stars. The main opto-mechanical requirement in this kind of device is represented by the tolerances in tip and tilt of these star enlargers, which have to be moved over the Field Of View and aligned with the reference stars. A differential tip-tilt among the star enlargers leads to a mismatch between the different pupil images related to the reference stars. This misalignment eventually translates into a blurring of the measured wavefront, reducing the sensing quality. We describe a conceptual layout for an active control of the wavefront sensor, in order to reach the best mechanical positioning of these stars enlargers. In particular we discuss an algorithm to determine the effective pupils positions by simple movements and apply the requested displacement through commercially available piezoelectric actuators, shown in a preliminary opto-mechancial design of such wavefront sensor.