Sky Coverage with Laser Guide Star Systems on 8m Telescopes

Abstract

In order to work, an adaptive optics (AO) system needs a reference object in order to measure the wavefront perturbations created by atmospheric turbulence. This object has to be bright enough to provide a good signal to noise ratio on the wavefront sensor, so that a good correction can be achieved. If the science object is bright enough, it can be used as a reference. Otherwise, another reference (usually a star) has to be found. This star has to be close to the astronomical object (within the isoplanatic patch), so that the measurements made on the star also apply to the science object. These two constraints severely limit the use of AO systems, because a suitable star cannot usually be found. As will be seen, Natural Guide Star (NGS) systems sky coverage is usually only of order a few per-cent, at best.