The wavefront sensor in active and adaptive telescopes is usually not in the optical path toward the scientific detector. It may generate additional wavefront aberrations, which have to be separated from the errors due to the telescope optics. The aberrations that are not rotationally symmetric can be disentangled from the telescope aberrations by a series of measurements taken in the center of the field, with the wavefront sensor at different orientation angles with respect to the focal plane. This method has been applied at the VLT Survey Telescope on the ESO Paranal observatory.

Optical astronomy is crucial to our understanding of the universe, but the capabilities of ground-based telescopes are severely limited by the effects of telescope errors and of the atmosphere on the passage of light. Recently, it has become possible to construct inbuilt corrective devices that can compensate for both types of degradations as observations are conducted. For full use of the newly emerged class of 8-meter telescopes, such active corrective capabilities, known as active and adaptive optics, are essential. Some physical limitations in the adaptive optics field can be overcome by artificially created reference stars, called laser guide stars. These new technologies have lately been applied with success to some medium and very large telescopes.

No abstract

International audienceWe present a formalism for performance analyses of adaptive optics systems that use a polychromatic laser guide star to measure the tilt of atmospherically distorted wavefronts. This formalism can be applied to feasibility and design studies of polychromatic laser guide star tip-tilt systems that are used to make the adaptive optics system of a telescope independent of natural guide stars. Using a few simplifying assumptions, the results are presented in analytical form such that a range of system parameters can be studied easily. Directions for the use of more detailed models necessitating numerical calculations are also presented in case more in-depth studies of certain system aspects are desired. Along with the theoretical development, we also present examples of possible solutions of the planned ELP-OA system as well as of an implementation of polychromatic laser guide star systems at large astronomical telescopes

No abstract