The resolution of ground based astronomical telescopes is limited by the presence of atmospheric turbulence. The current Very Large Telescopes [VLTs] (10 meter-class) are now equipped with Adaptive Optics [AO] that provides a real time correction of turbulence. These systems are however limited in performance and sky coverage by anisoplanatism effects. Various ideas have emerged in the last few years to partially overcome such a limitation and to answer new astronomical challenges: Multi-Conjugate AO, per se, for diffraction limited imaging in a wide field of view [FoV], Ground-Layer AO [GLAO] for resolution enhancement in large FoV, Multi-Object AO [MOAO] for the simultaneous observation of several faint galaxies with 3D spectroscopy. These variations around the theme of Multi-Conjugate AO [MCAO] are based on two main concepts: the use of multi-guide-star wavefront sensing, and the potential use of several deformable mirrors.
The first MCAO demonstrators and systems are now under development. The interest of such techniques for VLTs is discussed and the associated key issues and design choices are presented. Besides, a new era of astronomical telescopes is about to start with diameters reaching 30 to 60 meters. The respective advantage of the various MCAO techniques is shown to evolve with the diameter size. The influence of the change in diameter on MCAO design rules is also studied: evolution of the deformable mirror characteristics (number, stroke, pitch...), of the temporal sampling frequency, choice of natural versus laser guide stars... Of course the effect of atmospheric parameters (outer-scale, distribution in altitude) will be discussed.