ADAPTIVE OPTICS, A KEY-ELEMENT OF THE VLT

The principle and the components of an adaptive optics system are presented. The system scaling parameters are: the Fried diameter and the average speed of the turbulence. They determine the number of actuators of the deformable mirror and the bandwidth of the servoloop. In addition, two fondamental limitations must be taken into account: the photon noise and the anisoplanatism error. Therefore, an adaptive optics strategy in astronomy can be defined from the observing conditions and the required image quality. The application to the ESO VLT project is presented and the << COME-ON >> prototype system is described: recently it was successfully operated. The main obtained results are given in terms of Strehl ratios, image profiles and modulation transfer functions.