light at ground level is ignored and only light that has traveled for several microseconds high up into the atmosphere and back is actually detected). Laser development Dye lasers were the first laser sources used in laser guide star applications. These tunable lasers have continued to play a significant role in this field. However, the use of fluid gain media has been considered by some researchers as disadvantageous. Second generation laser sources for sodium guide star applications include sum-frequency-mixed solid-state lasers. New third generation laser systems based on tunable diode lasers with subsequent narrow-band Raman fiber amplification and resonant frequency conversion have been under development since 2005. Since 2014 fully engineered systems are commercially available. Important output features of the tunable lasers mentioned here include diffraction-limited beam divergence and narrow-linewidth emission. Progress The sodium laser guide star for use in adaptive optics to correct for atmospheric distortions is believed to have been invented by Princeton physicist Will Happer in 1982, as part of the Strategic Defense Initiative, but it was classified at the time. Laser guide star adaptive optics is still a very young field, with much effort currently invested in technology development. As of 2006, only two laser guide star AO systems were regularly used for science observations and have contributed to published results in peer-reviewed scientific literature: those at the Lick and Palomar Observatories in California, and the Keck Observatory in Hawaii. However, laser guide star systems were under development at most major telescopes, with the William Herschel Telescope, Very Large Telescope and Gemini North having tested lasers on the sky but not yet achieved regular operations. Other observatories developing laser AO systems as of 2006 include the Large Binocular Telescope and Gran Telescopio Canarias. The laser guide star system at the Very Large Telescope started regular scientific operations in June 2007. Since April 2016, the 4 Laser Guide Star Facility (4LGSF) has been installed at the ESO's Very Large Telescope (VLT), as a new subsystem of the Adaptive Optics Facility (AOF). The 4LGSF is a complement of the VLT Laser Guide Star Facility (LGSF). Instead of a single laser beam, the 4LGSF propagates four laser beams into the skies of Paranal, in northern Chile, producing four artificial stars by illuminating sodium atoms located in the atmosphere at 90 km altitude. These four stars enable getting a better correction in a specific direction, or widening the field of