Compensation of Non Common Path Aberrations on a pyramid wavefront sensor with an Adaptive Lens.

Closed loop systems based on the Pyramid wavefront sensor (P-WFS) can reach very high sensitivity respect to other wavefront sensors. The sensitivity increases as the Point Spread Function (PSF) on the tip of the pyramid gets smaller. Therefore, it is very important that in closed loop systems operating with the P-WFS the Non common Path Aberrations (NCPA) between the scientific camera arm and the P-WFS arm are reduced as much as possible. This thesis aims to study the performances of the reduction of NCPA using a multi-actuator deformable lens. Starting from the analytic simulations of the sensitivity gain of the PWFS, mainly conducted by Ragazzoni, Verinuad and Viotto, I developed an AO system to show experimentally the magnitude gain due to the P-WFS with respect to a Shack-Hartmann wavefront sensor (SH-WFS) when the sensitivity of the P-WFS increases with the decreasing of the aberrations. In fact, the P-WFS has the intrinsic capability to increase the gain as the aberrations are reduced; differently the SH-WFS maintains a constant gain under the same conditions. The most remarkable consequence is an improvement of the magnitude limit of observable stars with closed loop adaptive optics system. This is the feature of the P-WFS, that has made it the most implemented wavefront sensor in astronomic field in the last decade. Moreover, the multi-actuator deformable lens, recently developed at CNRIFN of Padua, is a promising device already used, with great results, in microscopy and ophthalmic instruments field. This adaptive lens has the unique feature of correcting aberrations up to the 4th order such as a deformable mirror allowing the realization of very compact optical setup. In particular, I have used the deformable lens (DL) to correct the non common path aberrations in closed loop mode between a P-WFS and a SH-WFS. The results show that the multi-actuator adaptive lens can be used to reduce the NCPA very close to the diffraction limit and that the correction can be maintained over a medium term operation with good stability.