The resolution of large telescopes without AO is limited by atmospheric turbulence (seeing-limited). The telescope includes wavefront sensors that either image the Rayleigh scattering of those lasers (Rayleigh LGS) or else the resonant scattering from alkali atoms in the mesosphere, typically sodium (sodium LGS). Therefore, one has to resort to artificial beacons that can be generated by powerful lasers. There is a lack of sufficiently bright stars in the sky that can serve as natural guide stars (NGS) for AO. A correction signal is sent to a deformable mirror to correct and flatten the wavefront on atmospheric timescales that are on the order of one to several milliseconds. The emitted wavefront from a sufficiently bright guide star within the field of view of the telescope, whose light passes through the same turbulence as the science object, can be measured with wavefront sensors and analyzed by an adaptive optics (AO) system. Optical turbulence in the lower 15–20 km of the atmosphere distorts incoming light from a science object. Laser guide star (LGS) adaptive optics can enable large ground-based telescopes to observe near the diffraction limit in the optical and infrared wavelength regimes anywhere in the sky.
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