Adaptive optics on large telescopes

Observations with ground based telescopes suffer from atmospheric turbulence. Independent of the telescope size the angular resolution in the visible is equivalent to that of a telescope with a diameter of 10-20 cm. This effect is caused by the turbulent mixing of air with different temperatures in the atmosphere. Thus, the perfect plane wave from a star at infinity is aberrated before it enters the telescope. In the following, we will discuss the physical background of imaging through turbulence, using Kolmogorov statistics, and the different techniques to sense and to correct the wave-front aberrations with adaptive optics. The requirements for the control loop of an adaptive optics system are discussed including formulas for the limiting magnitude of the guide star as a function of the wave-front sensing method, of the quality of the wave-front sensor camera, and of the degree of correction. Finally, a short introduction to deformable mirror technology will be given followed by the presentation of a new method to measure and to distinguish individual turbulent layers in order to increase the isoplanatic angle.