A compact design of a WFS for a natural guide star-based ELT adaptive optics system

We describe the opto-mechanical implementation of a group of wavefront sensors able to drive an MCAO system in order to cover a Field of View of the order of 1-2 arc-minutes, but getting advantages from the starlight coming from a Field of View as large as 10 arc-minutes in diameter. This involves a number of arms with a miniaturized, very small Field of View, single reference Adaptive Optics systems. A pyramid wavefront sensor working in close loop is fed through a small Deformable Mirror that is continuously monitored by an extremely high dynamic range wavefront sensor, whose signal has similar modality than the Pyramid one, namely YAW. In this way, a very compact wavefront sensor with a dynamic range limited by the stroke of the Deformable Mirror is achieved. Such a sensor is characterized by a limiting magnitude performances typical of a closed loop coherent wavefront sensor. This concept, in addition with an architecture of a Wavefront Computer that allows the implementation of a number of virtual Deformable Mirrors, allows for the development of a NGS based concept described elsewhere. Emphasis is given in this talk to the practical implementation and to the opto-mechanical details, including an overview of the required components, especially the detectors and the deformable mirrors and we show that the goal is attainable with today existing components.