Modal tomography for adaptive optics

Multiple Laser Guide Stars can be used to retrieve the three-dimensional di stribution of the perturbing layers on the coming wavefront in Adaptive Opt ics system and to derive corrections for conical anisoplanatism. We outline the basic analytical details of a modal approach to this problem. The adva ntages of this approach with respect to the traditional zonal one are point ed out along with a preliminary discussion of the way the involved matrices are to be treated in order to minimize noise propagation problems.