A NGSs based WFS for the E-ELT and the VLT

In the framework of the European ELT design, partially open-loop MCAO systems, coupled with virtual DMs, have been proposed to achieve AO correction using solely NGSs, to be selected in a FoV as wide as allowed by the Telescope optical design. The conceptual design of a very compact wavefront sensor, exploiting the just mentioned concept and characterized by a dynamic range limited by the stroke of the Deformable Mirror and by a limiting magnitude performance typical of a closed loop coherent wavefront sensor, have been presented in the past. This concept was based on the usage of a very linear wavefront sensor, like a YAW sensor, but a DM having the actual shape known “a-priori” could simplify a lot the design of such a compact WFS. We investigate here the realm of possible opto-mechanical realization of a probe, capable to co-exist with the currently foreseen E-ELT LGS probes and giving the possibility to exploit the open loop wavefront sensing operation with the aim of reaching a preliminary design of such a system. Furthermore, we devise a conceptual opto-mechanical design of a precursor of such a system, which could exploit at the VLT Global MCAO correction on the lower part of the atmosphere.