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Laser Guide Star

 
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Laser guide star
 
 
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Laser guide stars are a form of artificial star created for use in astronomical adaptive optics
Adaptive optics

Adaptive optics is a technology used to improve the performance of optics by reducing the effects of rapidly changing optical distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the impact of ocular aberrations....
 imaging.

Adaptive optics
Adaptive optics

Adaptive optics is a technology used to improve the performance of optics by reducing the effects of rapidly changing optical distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the impact of ocular aberrations....
 (AO) systems require a wavefront
Wavefront

In optics and physics, a wavefront is the Locus of Point s having the same phase . Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described....
 reference source in order to correct atmospheric distortion of light (called "astronomical seeing
Astronomical seeing

Astronomical seeing refers to the blurring and scintillation of astronomical objects such as stars caused by turbulence in the Earth's atmosphere....
"). Sufficiently bright star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
s are not available in all parts of the sky, which greatly limits the usefulness of natural guide star
Guide star

In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth....
 adaptive optics. Instead, one can create an artificial guide star by shining a laser
Laser

A laser is a device that emits light through a process called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation....
 into the atmosphere.






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Laser guide stars are a form of artificial star created for use in astronomical adaptive optics
Adaptive optics

Adaptive optics is a technology used to improve the performance of optics by reducing the effects of rapidly changing optical distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the impact of ocular aberrations....
 imaging.

Adaptive optics
Adaptive optics

Adaptive optics is a technology used to improve the performance of optics by reducing the effects of rapidly changing optical distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the impact of ocular aberrations....
 (AO) systems require a wavefront
Wavefront

In optics and physics, a wavefront is the Locus of Point s having the same phase . Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described....
 reference source in order to correct atmospheric distortion of light (called "astronomical seeing
Astronomical seeing

Astronomical seeing refers to the blurring and scintillation of astronomical objects such as stars caused by turbulence in the Earth's atmosphere....
"). Sufficiently bright star
Star

A star is a massive, luminous ball of Plasma that is held together by its own gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth....
s are not available in all parts of the sky, which greatly limits the usefulness of natural guide star
Guide star

In astronomy, a guide star is a reference star used to accurately maintain the tracking by a telescope of a heavenly body, whose motion across the sky is primarily due to the rotation of the Earth....
 adaptive optics. Instead, one can create an artificial guide star by shining a laser
Laser

A laser is a device that emits light through a process called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation....
 into the atmosphere. This star can be positioned anywhere the telescope
Telescope

A telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century....
 desires to point, opening up much greater amounts of the sky to adaptive optics. Because the laser beam is deflected by astronomical seeing
Astronomical seeing

Astronomical seeing refers to the blurring and scintillation of astronomical objects such as stars caused by turbulence in the Earth's atmosphere....
 on the way up, the laser light moves around in the sky in a random
Randomness

Randomness is a lack of order, purpose, Causality, or predictability. Randomness as defined by Aristotle is the situation, when a choice is to be made which has no logical component by which to determine or make the choice ....
 fashion. 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, which means 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
Sodium

Sodium is an element which has the symbol Na , atomic number 11, atomic mass 23 amu , and a common oxidation number +1. Sodium is a soft, silvery white, highly reactive element and is a member of the alkali metals within "group 1" ....
 atoms which is naturally present in the mesosphere
Mesosphere

The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located from about 50 km to 80-90 km altitude above the Earth's surface....
 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 light
Street light

A street light, lamppost, street lamp, light standard, or lamp standard is a raised source of light on the edge of a road, which is turned on or lit at a certain time every night....
s in many cities. Rayleigh beacons rely on the scattering
Rayleigh scattering

Rayleigh scattering is the elastic scattering of light or other electromagnetism radiation by particles much smaller than the wavelength of the light....
 of light by the molecules which make up the lower atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
. In contrast to sodium beacons, Rayleigh beacons are a much simpler and less costly technology, but do not provide as good a wavefront reference as 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 a few microseconds after the pulse has been launched so that scattered light at ground level is ignored and only light which has travelled 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
Lick Observatory

The Lick Observatory is an astronomy observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton , in the Diablo Range just east of San Jose, California, USA....
 and Palomar
Palomar Observatory

Palomar Observatory is a privately owned observatory located in San Diego County, California, 90 miles southeast of Mount Wilson Observatory, on Palomar Mountain in the Palomar Mountain Range....
 Observatories in California
California

California is a U.S. state on the West Coast of the United States of the United States, along the Pacific Ocean. It is bordered by Oregon to the north, Nevada to the east, Arizona to the southeast, and to the south the Mexico state of Baja California....
, and the Keck Observatory in Hawaii
Hawaii

File:Pahoehoe and Aa flows at Hawaii.jpgThe State of Hawaii is a U.S. state in the United States, located on an archipelago in the central Pacific Ocean southwest of the continental United States, southeast of Japan, and northeast of Australia....
. However, laser guide star systems were under development at most major telescopes, with the William Herschel Telescope
William Herschel Telescope

The William Herschel Telescope or WHT was first conceived in the late 1960s, when the Anglo-Australian Observatory was being designed. The British astronomical community saw the need for telescopes of comparable power in the Northern Hemisphere....
, Very Large Telescope
Very Large Telescope

The Very Large Telescope is a system of four separate optical telescopes organized in an array formation, built and operated by the European Southern Observatory at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile....
 and Gemini North
Gemini Observatory

The Gemini Observatory is an astronomical observatory consisting of two telescopes at different sites. The Northern Operations Center is located in Hilo, Hawaii, and the Southern Operations Center is in La Serena, Chile....
 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
Large Binocular Telescope

The Large Binocular Telescope is located on 10,700-foot Mount Graham in the Pinaleno Mountains of southeastern Arizona and is a part of the Mount Graham International Observatory....
 and Gran Telescopio Canarias
Gran Telescopio Canarias

The Gran Telescopio Canarias , sometimes called Gran TeCan, is a 10.4 m reflecting telescope and is undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, Spain....
. The laser guide star system at the Very Large Telescope
Very Large Telescope

The Very Large Telescope is a system of four separate optical telescopes organized in an array formation, built and operated by the European Southern Observatory at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile....
 started regular science operations in June 2007.

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