ABSTRACT
This chapter addresses the problem astronomers face when using adaptive optics (AO): there are not enough bright stars in the sky. In summary, the spatial resolution at the focus of astronomical telescopes is limited by the poor optical quality of Earth’s atmosphere. This is due to turbulence, which is always and everywhere present, even at the quietest locations such as high altitude mountains where modern observatories are built. Adaptive optics is the most efficient tool for sharpening images to the ultimate diffraction-limited resolution of the telescope. It requires relatively bright reference sources close to the programme object to measure phase disturbances on the incoming wavefront. Such sources are so scarce in the sky that the probability is very low that one will be close enough to the faint programme object, e.g. a quasar. The remedy to bright star scarcity is to create artificial ones close to the programme objects. It is achieved by pointing a laser beam in the sky at the target. Backscattering of the beam in the upper atmosphere, above the turbulent layers, can produce the required reference source. This is the so-called laser guide star (hereinafter referred to as an LGS).
