Laser guide star
Encyclopedia
Laser guide stars are a form of artificial star created for use in astronomical adaptive optics
imaging.
Adaptive optics
(AO) systems require a wavefront
reference source in order to correct atmospheric distortion of light (called astronomical seeing
). Sufficiently bright star
s 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 laser light moves around in the sky in a random
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
atoms which are 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 light
s in many cities. Rayleigh beacons rely on the scattering
of light by the molecules which make up 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 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
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. Also, new laser systems based on tunable diode lasers with subsequent narrow-band Raman fiber amplification and frequency-conversion are being developed as alternatives.
Important output features in the tunable laser
s used in this application are: diffraction limited beam divergence and narrow-linewidth emission.
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. 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...
imaging.
Adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. 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...
(AO) systems require a wavefront
Wavefront
In physics, a wavefront is the locus of points 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 twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
). Sufficiently bright star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. 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 of optical amplification based on the stimulated emission of photons. The term "laser" originated as 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 that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
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 twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...
on the way up, the laser light moves around in the sky in a random
Randomness
Randomness has somewhat differing meanings as used in various fields. It also has common meanings which are connected to the notion of predictability of events....
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 a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
atoms which are 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. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
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 or walkway, which is turned on or lit at a certain time every night. Modern lamps may also have light-sensitive photocells to turn them on at dusk, off at dawn, or activate...
s in many cities. Rayleigh beacons rely on the scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
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, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the 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 astronomical 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, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
Observatories in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, and the Keck Observatory in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
. However, laser guide star systems were under development at most major telescopes, with the William Herschel Telescope
William Herschel Telescope
The William Herschel Telescope is a optical/near-infrared reflecting telescope located at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, is part of the Isaac Newton Group of Telescopes...
, Very Large Telescope
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
and Gemini North
Gemini Observatory
The Gemini Observatory is an astronomical observatory consisting of two telescopes at sites in Hawai‘i and Chile. Together, the twin Gemini telescopes provide almost complete coverage of both the northern and southern skies...
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
Large Binocular Telescope is an optical telescope for astronomy located on 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 , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...
. The laser guide star system at the Very Large Telescope
Very Large Telescope
The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...
started regular science operations in June 2007.
Dye Lasers
Dye laser
A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and...
played, and continue to play, a significant role in the development of laser guide stars. Also, new laser systems based on tunable diode lasers with subsequent narrow-band Raman fiber amplification and frequency-conversion are being developed as alternatives.
Important output features in the tunable laser
Tunable laser
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range....
s used in this application are: diffraction limited beam divergence and narrow-linewidth emission.