Global wavefront sensing for extremely large telescopes

In ELTs the larger size of the aperture will translates into different categories of problems and different kind of solutions. The concept of Global Multi Conjugated Adaptive Optics is here introduced. In this, the wavefront sensing is extended to a much larger Field of View, practically limited by the telescope optics or optomechanics and by the limit given by the coverage of the metapupil at the highest altitude of interest. The correction of these layers is employed in a numerical fashion and the information is retrieved in order to compensate for a much limited Field of View. All this, being done in a layer oriented fashion, does allow for a simplified treatment of the Signal to Noise Ratio and to an estimate of the performances in the plot h vs. spatial scales where layers and the related Kolmogorov distributed turbulence are plotted. Once this information is retrieved it is fed back into the existing Deformable Mirror with a back-projection that allow for the most efficient way in terms of coverage of the spatial frequencies. The nature of the closed- vs open-loop of such an approach is also briefly discussed. The aim of a sky coverage and of performances getting closer or exceeding the ones provided by Laser Guide Stars can be at hands.