An optical system that enables rapid fluctuations in a telescope's image quality, caused by atmospheric turbulence, to be corrected in fractions of a second. These fluctuations are measured by a wavefront sensor that uses a reference star to measure the distortions that are taking place. The reference star is typically a reasonably bright star close in the sky to the object under study but may also be the object itself or a reflected laser beam that serves as an artificial reference star. The measured distortions are then removed with a phase corrector – typically a very thin mirror in the light path of the telescope that can be rapidly deformed by actuators to the equivalent shape of the wavefront, which must be subtracted to produce a sharp image.