Images furnished by ground-based telescopes are degraded by the turbulence of the terrestrial atmosphere. Nevertheless, using a technique called adaptive optics, developed independently by the Observatory and the U.S. Air Force, can free oneself from this effect, at least in the infrared domain. To do this, one analyzes the image of a star in the field of view, or the image of an artificial star created by a laser beam in the upper atmosphere of the Earth, and a thin mirror in the path of the light in the telescope is deformed so as to compensate for the atmospheric turbulence. This operation has to be done at least every hundredth of a second since the turbulence varies very rapidly. Adaptive optics is now a feature of all large telescopes.

caption : Creation of an artificial star at the VLT - credits : ESO/Y. Beletsky