Why use a laser guide star?
In order for a high-order adaptive optics system to perform up to its potential, a key requirement is having a bright star in the adaptive optics system’s field of view. This allows the wavefront sensor to sample the atmospheric distortions with enough accuracy to make good corrections. However, since the odds of bright stars naturally occuring very close to interesting objects are very small, a crucial technique is to generate an artificial star for the AO system’s wavefront sensor. The basic requirements for the artificial star are that it be bright for a better signal-to-noise ratio in the wavefront sensor and small in size for better accuracy in determining the wavefront distortions. Furthermore, to better sample the atmospheric turbulence, the artificial star should be put as high up in the atmosphere as possible. All of these requirements can be met by using a laser with its wavelength tuned to the sodium D2 line. With such a laser, a small, bright, artificial “sodium star” can b
