Adaptive optics for 100m class telescopes: new challenges require new solutions.

For giant, 100m-class, telescopes, a completely new class of problems arises. Trying to solve them adopting the techniques used for the classical 4... 10m-class telescope systems is indeed a reductive approach. Because of the cone effect and of the difficulties in retrieving the tilt due to the use of LGSs, tomography and multiconjugation will play a central role in the development of these systems. We review the tomographic concept and we report about recent experiments to validate the concept using sky observations. The Field of View requirement is another crucial step in the design of the optical system of these telescopes and hence it needs to be attacked in a detailed and as wide as possible approach. The adoption of classical wavefront sensors looking to different stars is here overcome with wavefront sensors directly conjugated to different layers, with big advantages in term of intrinsic complication of the system, both from the optomechanical point of view (less complexity and saving of a huge amount of light) and from the computational point of view. The adoption of these new sensing techniques (that, gaining from the close loop situation, can be efficiently coupled to a number of very faint stars, whose complexity does not scale with the number of references but, rather, to the number of sensed layers) is here reviewed. This concept of multiple object wavefront sensing leads to an example of a system with a minimum number of independent adaptive optics loop, in which each layer represented in a wavefront sensor can be conjugated to a specifically conjugated deformable mirror.