Radio telescope
Encyclopedia
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
used in radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
. The same types of antennas are also used in tracking and collecting data from satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s and space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...
s. In their astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
role they differ from optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
s in that they operate in the radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
portion of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
where they can detect and collect data on radio sources. Radio telescopes are typically large parabolic
Parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish...
("dish") antennas used singly or in an array. Radio observatories
Observatory
An observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
are preferentially located far from major centers of population to avoid electromagnetic interference
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
(EMI) from radio, TV, radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, and other EMI emitting devices. This is similar to the locating of optical telescopes to avoid light pollution
Light pollution
Light pollution, also known as photopollution or luminous pollution, is excessive or obtrusive artificial light.The International Dark-Sky Association defines light pollution as:...
, with the difference being that radio observatories are often placed in valleys to further shield them from EMI as opposed to clear air mountain tops for optical observatories.
Early radio telescopes
Karl Guthe Jansky
Karl Guthe Jansky was an American physicist and radio engineer who in August 1931 first discovered radio waves emanating from the Milky Way. He is considered one of the founding figures of radio astronomy.- Early life :...
, an engineer with Bell Telephone Laboratories
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
, in 1931. Jansky was assigned the job of identifying
sources of static that might interfere with radio telephone service. Jansky's antenna was an array of dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
s and reflectors
Reflector (antenna)
An antenna reflector is a device that reflects electromagnetic waves.It is often a part of an antenna assembly.The most common reflector types are...
designed to receive short wave radio signals at a frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of 20.5 MHz (wavelength about 14.6 metres). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of approximately 100 ft (30.5 m). and stood 20 ft (6 m). tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of the received interfering radio source (static) could be pinpointed. A small shed to the side of the antenna housed an analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
pen-and-paper recording system. After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. Jansky finally determined that the "faint hiss" repeated on a cycle of 23 hours and 56 minutes. This period is the length of an astronomical sidereal day, the time it takes any "fixed" object located on the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...
to come back to the same location in the sky. Thus Jansky suspected that the hiss originated well beyond the Earth's atmosphere, and by comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way Galaxy and was strongest in the direction of the center of the galaxy, in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Sagittarius
Sagittarius (constellation)
Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...
.
Grote Reber
Grote Reber was an amateur astronomer and pioneer of radio astronomy. He was instrumental in investigating and extending Karl Jansky's pioneering work, and conducted the first sky survey in the radio frequencies...
, was one of the pioneers of what became known as radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
when he built the first parabolic "dish" radio telescope (9 metres (30 ft) in diameter) in his back yard in Illinois in 1937. He was instrumental in repeating Karl Guthe Jansky's pioneering but somewhat simple work at higher frequencies, and he went on to conduct the first sky survey at very high radio frequencies. The rapid development of radar technology
History of radar
The history of radar starts with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. This possibility was suggested in James Clerk Maxwell's seminal work on electromagnetism...
during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
was easily translated into radio astronomy technology after the war, and the field of radio astronomy began to blossom.
Types
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
that makes up the radio spectrum
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
is very large. This means that the types of antennas that are used as radio telescopes vary widely in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10 MHz - 100 MHz), they are generally either directional antenna
Directional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
arrays similar to "TV antennas" or large stationary reflectors with moveable focal points. Since the wavelengths being observed with these types of antennas are so long, the "reflector" surfaces can be constructed from coarse wire mesh
Mesh
Mesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to web or net in that it has many attached or woven strands.-Types of mesh:...
such as chicken wire
Chicken wire
Chicken wire, or poultry netting, is a mesh of wire commonly used to fence poultry livestock. It is made of thin, flexible galvanized wire, with hexagonal gaps...
. At shorter wavelengths “dish” style radio telescopes predominate. The angular resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...
of a dish style antenna is determined by the diameter of the dish expressed as a number of wavelengths of the electromagnetic radiation being observed. This dictates the dish size a radio telescope needs for a useful resolution. Radio telescopes that operate at wavelengths of 3 meters to 30 cm (100 MHz to 1 GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths shorter than 30 cm (above 1 GHz) range in size from 3 to 90 meters in diameter.
Frequencies
Negotiations to defend the frequency allocation
Frequency allocation
Use of radio frequency bands of the electromagnetic spectrum is regulated by governments in most countries, in a Spectrum management process known as frequency allocation or spectrum allocation. Radio propagation does not stop at national boundaries...
for parts of the spectrum most useful for observing the Universe are coordinated in the Scientific Committee on Frequency Allocations for Radio Astronomy and Space Science.
Some of the more notable frequency bands used by radio telescopes include:
- every frequency: in the United States National Radio Quiet ZoneUnited States National Radio Quiet ZoneThe United States National Radio Quiet Zone is a large area of land surrounding the National Radio Astronomy Observatory at Green Bank, West Virginia, and especially the large Green Bank Telescope radio telescope. The Radio Quiet Zone is a rectangle of land approximately in size that straddles...
- Channel 37Channel 37Channel 37 is an unused television channel in countries using the M and N broadcast television system standards. Channel 37 occupies a band of UHF frequencies from 608 to 614 MHz, frequencies that are particularly important to radio astronomy...
: 608 to 614 MHz - the "hydrogen lineHydrogen lineThe hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...
", aka the "21 centimeter line": 1420.40575177 MHz, is used by many radio telescopes including- The Big EarThe Big EarThe Ohio State University Radio Observatory was a Kraus-type radio telescope located on the grounds of the Perkins Observatory at Ohio Wesleyan University from 1963 to 1998. Known as "Big Ear", the observatory was part of The Ohio State University's Search for Extraterrestrial Intelligence project...
in its discovery of the Wow! signalWow! signalThe Wow! signal was a strong narrowband radio signal detected by Dr. Jerry R. Ehman on August 15, 1977, while working on a SETI project at the Big Ear radio telescope of The Ohio State University then located at Ohio Wesleyan University's Perkins Observatory, Delaware, Ohio. The signal bore...
- The Big Ear
- 1406 MHz and 430 MHz http://www.jb.man.ac.uk/~pulsar/Education/Tutorial/tut/node115.html
- the waterholeWaterhole (radio)The waterhole, or water hole, refers to an especially quiet band of the electromagnetic spectrum between 1,420 and 1,666 megahertz, corresponding to wavelengths of 21 and 18 centimeters respectively. The term was coined by Bernard Oliver. The strongest hydroxyl radical spectral line radiates at 18...
: 1,420 to 1,666 MHz - the Arecibo ObservatoryArecibo ObservatoryThe Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...
can has several receivers that together cover the whole 1–10 GHz range - the Wilkinson Microwave Anisotropy ProbeWilkinson Microwave Anisotropy ProbeThe Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...
mapped the cosmic microwave background radiationCosmic microwave background radiationIn cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
in 5 different frequency bands, centered on 23 GHz, 33 GHz, 41 GHs, 61 GHz, and 94 GHz.
Big dishes
Arecibo, Puerto Rico
Arecibo is a municipality in the northern midwest coast of Puerto Rico and located by the Atlantic Ocean, north of Utuado and Ciales; east of Hatillo; and west of Barceloneta, and Florida. Arecibo is spread over 18 wards and Arecibo Pueblo...
, whose 305 m (1,000.7 ft) dish is fixed in the ground. It was designed by engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
Bill Gordon (d. 2010)
Deaths in February 2010
Deaths in 2010 : ← - January - February - March - April - May - June - July - August - September - October - November - December - →The following is a list of notable deaths in February 2010.-28:*Martin Benson, 91, British stage actor....
. The suspension system was designed by George and Helias Doundoulakis
Helias Doundoulakis
Helias Doundoulakis is a Greek American scientific innovator who patented the suspension system for the largest radio telescope in the world, and served in the United States Army as a spy for the Office of Strategic Services.-War years:...
, for which Helias Doundoulakis
Helias Doundoulakis
Helias Doundoulakis is a Greek American scientific innovator who patented the suspension system for the largest radio telescope in the world, and served in the United States Army as a spy for the Office of Strategic Services.-War years:...
received a patent along with assignee William J. Casey
William J. Casey
William Joseph Casey was the Director of Central Intelligence from 1981 to 1987. In this capacity he oversaw the entire United States Intelligence Community and personally directed the Central Intelligence Agency ....
, ex-Central Intelligence Agency
Central Intelligence Agency
The Central Intelligence Agency is a civilian intelligence agency of the United States government. It is an executive agency and reports directly to the Director of National Intelligence, responsible for providing national security intelligence assessment to senior United States policymakers...
Director under President Ronald Reagan
Ronald Reagan
Ronald Wilson Reagan was the 40th President of the United States , the 33rd Governor of California and, prior to that, a radio, film and television actor....
http://www.pat2pdf.org/pat2pdf/foo.pl?number=3273156. The antenna beam is steerable (by means of a moving receiver) within about 20° of the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
. The largest individual radio telescope of any kind is the RATAN-600 located near Nizhny Arkhyz, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, which consists of a 576-meter circle of rectangular radio reflectors, each of which can be pointed towards a central conical receiver.
The largest radio telescope in Europe is the 100-meter diameter antenna in Effelsberg
Effelsberg
The Effelsberg 100-m Radio Telescope is a radio telescope in the Ahrgebirge in Bad Münstereifel, district of Euskirchen, North Rhine-Westphalia, Germany.- Geography :...
, Germany, which also was the world's largest fully steerable telescope for 30 years until the slightly larger Green Bank Telescope
Green Bank Telescope
The Robert C. Byrd Green Bank Telescope is the world's largest fully steerable radio telescope and the world's largest land-based movable structure. It is part of the National Radio Astronomy Observatory site at Green Bank, West Virginia, USA. The telescope honors the name of the late Senator...
was opened in West Virginia
West Virginia
West Virginia is a state in the Appalachian and Southeastern regions of the United States, bordered by Virginia to the southeast, Kentucky to the southwest, Ohio to the northwest, Pennsylvania to the northeast and Maryland to the east...
, United States, in 2000. The third largest fully steerable radio telescope is the 76-metre Lovell Telescope
Lovell Telescope
The Lovell Telescope is a radio telescope at Jodrell Bank Observatory, near Goostrey, Cheshire in the north-west of England. When it was constructed in 1955, the telescope was the largest steerable dish radio telescope in the world at 76.2 m in diameter;it is now the third largest, after the...
at Jodrell Bank Observatory in Cheshire
Cheshire
Cheshire is a ceremonial county in North West England. Cheshire's county town is the city of Chester, although its largest town is Warrington. Other major towns include Widnes, Congleton, Crewe, Ellesmere Port, Runcorn, Macclesfield, Winsford, Northwich, and Wilmslow...
, England. The fourth largest fully steerable radio telescopes is four the 70-metre radio telescopes: RT-70
RT-70
There are three radio telescopes designated RT-70, all in countries that were once part of the former Soviet Union, all with similar specifications: 70m dishes and an operating range of 5–300 GHz...
, Goldstone
Goldstone Deep Space Communications Complex
The Goldstone Deep Space Communications Complex — commonly called the Goldstone Observatory — is located in California's Mojave Desert. Operated by ITT Corporation for the Jet Propulsion Laboratory, its main purpose is to track and communicate with space missions. It includes the Pioneer...
. Feature of these telescopes is that it is the world's largest planetary radars (except Suffa RT-70).
A typical size of the single antenna of a radio telescope is 25 meters. Dozens of radio telescopes with comparable sizes are operated in radio observatories all over the world.
China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...
officially started construction of the world's largest single-aperture radio telescope in 2009, the FAST
Five hundred meter Aperture Spherical Telescope
FAST, or the Five hundred meter Aperture Spherical Telescope, first proposed in 1994, is a radio telescope to be built by 2013 in a natural basin （大窝凼洼地）, Pingtang County 平塘县 near Duyun, capital of the Qiannan Buyei and Miao Autonomous Prefecture 黔南布依族苗族自治州, Guizhou Province 贵州, southwest China ...
. The FAST, with a dish area as large as 30 football fields, will stand in a region of typical Karst depressions in Guizhou
Guizhou
' is a province of the People's Republic of China located in the southwestern part of the country. Its provincial capital city is Guiyang.- History :...
, and will be finished by 2013.
Radio interferometry
Astronomical interferometer
An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution by means of interferometry....
. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g., the One-Mile Telescope
One-Mile Telescope
The One-Mile Telescope at the Mullard Radio Astronomy Observatory is an array of radio telescopes designed to perform aperture synthesis interferometry.- History :The One Mile Telescope was completed by the Radio Astronomy Group of Cambridge...
), arrays of one-dimensional antennas (e.g., the Molonglo Observatory Synthesis Telescope
Molonglo Observatory Synthesis Telescope
The Molonglo Observatory Synthesis Telescope is a radio telescope operating at 843 MHz. It is operated by the School of Physics of the University of Sydney...
) or two-dimensional arrays of omni-directional dipoles
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
(e.g., Tony Hewish's
Antony Hewish
Antony Hewish FRS is a British radio astronomer who won the Nobel Prize for Physics in 1974 for his work on the development of radio aperture synthesis and its role in the discovery of pulsars...
Pulsar Array
Interplanetary Scintillation Array
The Interplanetary Scintillation Array was built at the Mullard Radio Astronomy Observatory in 1967 and originally covered four acres . It was extended in 1978 to nine, and re-furbished in 1989. It operates at 81.5 MHz , and is made up of 4096 dipoles in a phased array. 14 beams can map the...
). All of the telescopes in the array are widely separated and are usually connected together using coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
, optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
, or other type of transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
. Recent advances in the stability of electronic oscillators also now permit interferometry to be carried out by independent recording of the signals at the various antennas, and then later correlating the recordings at some central processing facility. This process is known as VLBI (Very Long Baseline Interferometry). Interferometry does increase the total signal collected, but its primary purpose is to vastly increase the resolution through a process called Aperture synthesis
Aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...
. This technique works by superposing (interfering) the signal wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
s from the different telescopes on the principle that wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
s that coincide with the same phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is equivalent in resolution (though not in sensitivity) to a single antenna whose diameter is equal to the spacing of the antennas furthest apart in the array.
A high quality image requires a large number of different separations between telescopes. Projected separation between any two telescopes, as seen from the radio source, is called a baseline. For example, the Very Large Array
Very Large Array
The Very Large Array is a radio astronomy observatory located on the Plains of San Agustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA...
(VLA) near Socorro, New Mexico
Socorro, New Mexico
Socorro is a city in Socorro County in the U.S. state of New Mexico. It stands in the Rio Grande Valley at an elevation of . The population was 9,051 at the 2010 census...
has 27 telescopes with 351 independent baselines at once, which achieves a resolution of 0.2 arc seconds at 3 cm wavelengths). Martin Ryle
Martin Ryle
Sir Martin Ryle was an English radio astronomer who developed revolutionary radio telescope systems and used them for accurate location and imaging of weak radio sources...
's group in Cambridge
Cavendish Astrophysics Group
The Cavendish Astrophysics Group is based at the Cavendish Laboratory at the University of Cambridge. The group operates all of the telescopes at the Mullard Radio Astronomy Observatory except for the 32m MERLIN telescope, which is operated by Jodrell Bank.The group is the second largest of three...
obtained a Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
for interferometry and aperture synthesis. The Lloyd's mirror
Lloyd's mirror
Lloyd's mirror is a classic optics experiment and was first described in 1834 and again in 1837 by Humphry Lloyd in the proceedings of the Royal Irish Academy of Science. In the experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come...
interferometer was also developed independently in 1946 by Joseph Pawsey's group at the University of Sydney
University of Sydney
The University of Sydney is a public university located in Sydney, New South Wales. The main campus spreads across the suburbs of Camperdown and Darlington on the southwestern outskirts of the Sydney CBD. Founded in 1850, it is the oldest university in Australia and Oceania...
. In the early 1950s the Cambridge Interferometer
Cambridge Interferometer
The Cambridge Interferometer was a radio telescope interferometer built by Martin Ryle and Antony Hewish in the early 1950s to the west of Cambridge . The interferometer consisted of an array of 4 fixed elements to survey the sky...
mapped the radio sky to produce the famous 2C and 3C
Third Cambridge Catalogue of Radio Sources
The Third Cambridge Catalogue of Radio Sources is an astronomical catalogue of celestial radio sources detected originally at 159 MHz, and subsequently at 178 MHz. It was published in 1959 by members of the Radio Astronomy Group of the University of Cambridge...
surveys of radio sources. A large physically connected radio telescope array is the Giant Metrewave Radio Telescope
Giant Metrewave Radio Telescope
Giant Metrewave Radio Telescope , located near Pune in India, is the world's largest, array of radio telescopes at metre wavelengths. It is operated by the National Centre for Radio Astrophysics, a part of the Tata Institute of Fundamental Research, Mumbai.-Location:The GMRT is located around...
, located in Pune
Pune
Pune , is the eighth largest metropolis in India, the second largest in the state of Maharashtra after Mumbai, and the largest city in the Western Ghats. Once the centre of power of the Maratha Empire, it is situated 560 metres above sea level on the Deccan plateau at the confluence of the Mula ...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. The largest array, LOFAR
LOFAR
LOFAR is the Low Frequency Array for radio astronomy, built by the Netherlands astronomical foundation ASTRON and operated by ASTRON's radio observatory....
(the 'LOw Frequency ARray'), is currently being constructed in western Europe, consisting of about 20 000 small antennas in 48 stations distributed over an area several hundreds of kilometres in diameter, and operates between 1.25 and 30 m wavelengths. VLBI systems using post-observation processing have been constructed with antennas thousands of miles apart. Radio interferometers have also been used to obtain detailed images of the anisotropies and the polarization of the Cosmic Microwave Background, like the CBI
Cosmic Background Imager
The Cosmic Background Imager was a 13-element interferometer perched at an elevation of 5,080 metres at Llano de Chajnantor Observatory in the Chilean Andes...
interferometer in 2004.
The world's largest physically connected telescopes, the SKA (Square Kilometre Array
Square Kilometre Array
The Square Kilometre Array is a radio telescope in development which will have a total collecting area of approximately one square kilometre. It will operate over a wide range of frequencies and its size will make it 50 times more sensitive than any other radio instrument...
), is planned to start operation in 2020.
Astronomical observations
Many astronomical objects are not only observable in visible lightVisible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
but also emit radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
at radio wavelengths
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
. Besides observing energetic objects such as pulsar
Pulsar
A pulsar is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name...
s and quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s, radio telescopes are able to "image" most astronomical objects such as galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
e, and even radio emissions from planets.
See also
- Aperture synthesisAperture synthesisAperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...
- AstropulseAstropulseAstropulse is a distributed computing project that uses volunteers across the globe to lend their unused computing power to search for primordial black holes, pulsars, and ETI. Volunteer resources are harnessed through Berkeley Open Infrastructure for Network Computing platform...
Distributed computing to search data tapes for primordial black holes, pulsars, and ETI - List of astronomical observatories
- List of radio telescopes
- List of telescope types
- Radio astronomyRadio astronomyRadio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
- SETISETIThe search for extraterrestrial intelligence is the collective name for a number of activities people undertake to search for intelligent extraterrestrial life. Some of the most well known projects are run by the SETI Institute. SETI projects use scientific methods to search for intelligent life...
- Search for Extra-Terrestrial Intelligence using radio telescopes (among others) - TelescopeTelescopeA 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...