Editing Laser guide star (section)

== Progress ==
[[File:Artificial stars over Paranal.webm|thumb|Example of an artificial reference star.]]
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 information|classified]] at the time.<ref>{{cite journal|title=Laser Guide Star Adaptive Optics: Present and Future|author=Olivier, S. S.|author2=Max, C. E.|journal=Very High Angular Resolution Imaging |year=1994 |volume=158 |page=283 |doi=10.1007/978-94-011-0880-5_48 |bibcode=1994IAUS..158..283O|isbn=978-0-7923-2633-5 |s2cid=115762227 |url=https://digital.library.unt.edu/ark:/67531/metadc1396405/ |url-access=subscription }}</ref>

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-review]]ed scientific literature:  those at the [[Lick Observatory|Lick]] and [[Palomar Observatory|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 Observatory|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.<ref>{{cite web|title=Free from the Atmosphere – Laser Guide Star System on ESO's VLT Starts Regular Science Operations|url=http://www.eso.org/public/news/eso0727/|work=ESO for the public|publisher=ESO|accessdate=2 June 2011|author=Markus Kasper|author2=Stefan Stroebele|author3=Richard Davies |author4=Domenico Bonaccini Calia |date=13 June 2007}}</ref>

Since April 2016,<ref>{{cite web|title=Four Lasers Over Paranal|url=http://www.eso.org/public/news/eso1613/|website=European Southern Observatory|accessdate=27 April 2016}}</ref> the 4 Laser Guide Star Facility (4LGSF) has been installed at the ESO's [[Very Large Telescope]] (VLT),<ref name="eso.org">{{cite web|title=Very Large Telescope – The world's most advanced visible-light astronomical observatory|url=http://www.eso.org/public/teles-instr/paranal/|website=European Southern Observatory}}</ref> as a new subsystem of the Adaptive Optics Facility (AOF).<ref>{{cite web|title=Adaptive Optics|url=http://www.eso.org/public/teles-instr/technology/adaptive_optics/|website=European Southern Observatory}}</ref> 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&nbsp;km altitude. These four stars enable getting a better correction in a specific direction, or widening the field of view corrected by an adaptive optics. Each laser delivers 22 watts in a diameter of {{convert|30|cm|abbr=on}}. The 4LGSF Laser System is based on a fiber Raman laser technology, developed at ESO and transferred to industry.<ref>{{cite web|title=ESO Signs Technology Transfer Deal|url=http://www.eso.org/public/announcements/ann1048/|website=ESO announcement}}</ref><ref>{{cite web|title=Laser Guide Star Units Accepted and Shipped to Chile|url=http://www.eso.org/public/announcements/ann15089/|website=ESO announcement}}</ref>

The upgrade to four lasers with fiber Raman laser technology is necessary to support the new instruments at Paranal Observatory,<ref name="eso.org"/> like HAWK-I (with GRAAL) <ref>{{cite web|title=HAWK-I – High Acuity Wide-field K-band Imager|url=http://www.eso.org/public/teles-instr/vlt/vlt-instr/hawk-i/|website=European Southern Observatory}}</ref> and  MUSE (with GALACSI).<ref>{{cite web|title=MUSE – Multi Unit Spectroscopic Explorer|url=http://www.eso.org/public/teles-instr/vlt/vlt-instr/muse/|website=European Southern Observatory}}</ref> Also with the 4LGSF the stability is increased, the amount of preventative maintenance support and the preparation of an observing run time will be considerably reduced compared to the LGSF, which currently still uses its original dye laser (planned to be replaced by a [[fiber laser]]).
The 4LGSF helps astronomers to test devices for the [[E-ELT]],<ref>{{cite web|title=The European Extremely Large Telescope – The world's biggest eye on the sky|url=http://www.eso.org/public/teles-instr/e-elt/|website=European Southern Observatory}}</ref> which will have a similar system to support the adaptive optics of the telescope.
Given its power, the 4LGSF operations follow a protocol to avoid any risk. The laser system is equipped with an automatic aircraft avoidance system that shuts down the lasers if an aircraft ventures too close to the beams.

For sodium laser guide stars, there are three main challenges to overcome: [[Larmor precession]], [[Recoil (laser physics)|recoil]], and [[Transition saturation (laser physics)|transition saturation]].<ref name="D. Bonaccini Calia D 2010">D. Bonaccini Calia D. Budker J. M. Higbie W. Hackenberg R. Holzlohner, S. M. Rochester. Optimization of CW sodium laser guide star efficiency. Astronomy and Astrophysics, 510, 2010.</ref> Larmor precession, which is the precession of the sodium atom in the geomagnetic field (precisely, it is the precession of the quantized total atomic angular momentum vector of the atom), decreases the atomic fluorescence of the laser guide star by changing the angular momentum of the atom before a two-level cycling transition can be established through optical pumping with circularly polarized light. Recoil from spontaneous emission, resulting in a momentum kick to the atom, causes a redshift in the laser light relative to the atom, rendering the atom unable to absorb the laser light and thus unable to fluoresce. Transition saturation is the depopulation of atoms from a state of higher angular momentum (F=2) to a state of lower angular momentum (F=1), resulting in a different absorption wavelength.<ref name="D. Bonaccini Calia D 2010"/>