view corrected by an adaptive optics. Each laser delivers 22 watts in a diameter of . The 4LGSF Laser System is based on a fiber Raman laser technology, developed at ESO and transferred to industry. The upgrade to four lasers with fiber Raman laser technology is necessary to support the new instruments at Paranal Observatory, like HAWK-I (with GRAAL) and MUSE (with GALACSI). 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 uses still its original dye laser (planned to be replaced by a fiber laser). The 4LGSF helps astronomers to test devices for the E-ELT, 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, and transition saturation. 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. References External links ESOcast 34: How To Stop a Star's Twinkle ESO’s New Compact Laser Guide Star Unit Tested Gemini's Laser Vision Reveals Striking New Details in Orion Nebula Astronomical imaging Laser applications