Motivation for using laser beacons

The goal of diffraction-limited correction of large telescopes

In Chapters 8–10, we have seen that adaptive optics (AO) is a powerful tool to enhance the resolution and contrast of astronomical images. Several 2- to 4-m class astronomical telescopes now have AO user instruments. These include the ESO and Canada–France–Hawaii 3.6-m telescope, and the Mt Wilson 100- inch telescope. All are being used for scientific observations, and are producing dramatic results, fulfilling the promise of AO to overcome the problem of seeing which has plagued astronomers for centuries.

The AO systems on the above telescopes, as well as several other systems which will be operational in the near future, use the light from a field star to sense the wave-front aberrations, as originally envisaged by Babcock (1953). As discussed in Section 3.5, to produce diffraction-limited correction a bright source must be available within the isoplanatic patch. (We will quantify this requirement in the following section.) There are many applications where a natural star can be utilized, notably in applications of AO for stellar astronomy and the search for faint companions around bright stars. In addition, many extended objects will contain a bright stellar component suitable for wave-front sensing. However, the brightness requirement for field stars is quite severe, resulting in a low probability for finding a sufficiently bright “guide” star for diffraction-limited correction (within ∼ 10″ at an imaging wavelength λi=1 μm) for arbitrary program objects.