Adaptive optics is concerned with the correction of phase distortions in wavefronts which degrade the quality of images produced by optical systems. It was originally developed for both astronomy and the military, where the Earth's atmosphere causes distortions, although other uses are now being developed. As ground based telescopes become increasingly large the size and complexity of adaptive optics systems also increase, creating "extreme adaptive optics”. This thesis deals with such an adaptive optics system. A novel self referenced phase shifting interferometer based on a liquid crystal (LC) waveplate is presented which can measure high spatial frequency phase distortions. This is then coupled to a LC spatial light modulator wavefront corrector. The geometry is matched such that there is no need for a wavefront reconstructor. The performance is measured in two stages. Firstly, spatially where static phase distortions are measured by the interferometer and corrected. Secondly, temporally where a simple analogue feedback is implemented to show correction over a single corrector pixel for fast time varying phase distortions. This work builds on other published research on using point diffraction interferometry in adaptive optics. The novelty lies in the development of a new implementation of a point diffraction interferometer, and in the demonstration of a high-speed closed loop single channel system. This work therefore contributes to the groundwork required to build an extreme adaptive optics system whose complexity scales linearly with the size (area) of the telescope aperture.

L’objectif principal de ce travail est d’améliorer notre compréhension de la rhéologie terrestre et du mouvement de Chandler qui en dépend. Nous calculons le facteur gravimétrique complexe à la période de Chandler par une analyse conjointe de séries temporelles gravimétriques et du mouvement du pôle. Nous montrons l’importance et les limites des corrections gravimétriques dans les analyses locales et l’intérêt d’une combinaison des données de différentes stations. Suivant une approche plus théorique, nous menons une étude analytique puis numérique afin de déterminer les solutions du problème des petites déformations d’une Terre homogène anélastique en rotation. L’influence de l’anélasticité sur la période et le facteur de qualité de Chandler est étudiée, et nous montrons que ce dernier est plus sensible que la période aux paramètres rhéologiques, notamment à la viscosité de Kelvin. Enfin, nous étudions, en domaine de Laplace, l’influence de la fréquence de forçage par une surcharge. / The main goal of this work is to improve our understanding of the Earth’s rheology and of the Chandler wobble which depends on it. We compute the complex gravimetric factor at the Chandler frequency by a joint analysis of gravimetric and polar motion time series. We show the importance and the limits of gravimetric corrections for local analyses and the effectiveness of the combination of data from different stations. Using a more theoretical approach, we carry out an analytical and numerical study to determine the solutions of the small deformations of an anelastic homogeneous rotating Earth model. The influence of anelasticity on the Chandler wobble period and quality factoris studied and we show that the latter is more sensitive than the period to the rheological parameters, especially to the Kelvin’s viscosity. Finally, we study in Laplace domain the influence of the forcing frequency for the loading problem.