Laser guide star
Adaptive optics (AO) systems require a wavefront reference source in order to correct atmospheric distortion of light (called astronomical seeing). Sufficiently bright stars are not available in all parts of the sky, which greatly limits the usefulness of natural guide star adaptive optics. Instead, one can create an artificial guide star by shining a laser into the atmosphere. This star can be positioned anywhere the telescope desires to point, opening up much greater amounts of the sky to adaptive optics. Because the laser beam is deflected by astronomical seeing on the way up, the returning laser light does not move around in the sky as astronomical sources do. In order to keep astronomical images steady, a natural star nearby in the sky must be monitored in order that the motion of the laser guide star can be subtracted using a tip–tilt mirror. However, this star can be much fainter than is required for natural guide star adaptive optics because it is used to measure only tip and tilt, and all higher-order distortions are measured with the laser guide star. This means that many more stars are suitable, and a correspondingly larger fraction of the sky is accessible.
There are two main types of laser guide star system, known as sodium and Rayleigh beacon guide stars.
Sodium beacons are created by using a laser specially tuned to 589.2 nanometers to energize a layer of sodium atoms that is naturally present in the mesosphere at an altitude of around 90 kilometers. The sodium atoms then re-emit the laser light, producing a glowing artificial star. The same atomic transition of sodium is used to create bright yellow street lights in many cities.
Rayleigh beacons rely on the scattering of light by the molecules in the lower atmosphere. In contrast to sodium beacons, Rayleigh beacons are a much simpler and less costly technology, but do not provide as good a wavefront reference, since the artificial beacon is generated much lower in the atmosphere. The lasers are often pulsed, with measurement of the atmosphere being time-gated (taking place several microseconds after the pulse has been launched, so that scattered 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 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 science operations in June 2007.
Dye lasers played, and continue to play, a significant role in the development of laser guide stars. However, the use of fluid gain media is considered by some as disadvantageous. Apart from that, these dye laser systems are said to be electrically relatively inefficient. The same is said to be true for the sum-frequency-mixed solid-state lasers that are widely used as the second generation of sodium guide star 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.
Allgäu Public Observatory in Ottobeuren, Germany
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