Diagram showing a possible layout for an astronomical interferometer, with the mirrors laid out in a parabolic arrangement (similar to the shape of a conventional telescope mirror).

The VLTI infrared astronomical interferometer.

An astronomical interferometer or hypertelescope is an array of telescopes or mirror segments acting together to probe structures with higher resolution. Astronomical interferometers are widely used for optical astronomy, infrared astronomy, submillimetre astronomy and radio astronomy. Aperture synthesis can be used to perform high-resolution imaging using astronomical interferometers. Very Long Baseline Interferometry uses a technique related to the closure phase to combine telescopes separated by thousands of kilometers to form a radio interferometer with the resolution which would be given by a single dish which was thousands of kilometers in diameter. At optical wavelengths, aperture synthesis allows the atmospheric seeing resolution limit to be overcome, allowing the angular resolution to reach the diffraction-limit of the array. Image File history File links Interf_diagram. ... Image File history File links Interf_diagram. ... Download high resolution version (703x651, 64 KB) Aerial view of the Very Large Telescope Array (VLT) in the Paranal Mountain, Chile. ... Download high resolution version (703x651, 64 KB) Aerial view of the Very Large Telescope Array (VLT) in the Paranal Mountain, Chile. ... The four telescopes of the European Southern Observatory Paranal site. ... Optical astronomy encompasses a wide variety of observations via telescopes that are sensitive in the range of visible light. ... Infrared astronomy is the branch of astronomy and astrophysics which deals with objects visible in infrared (IR) radiation. ... Submillimetre Astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths. ... Microwave image of 3C353 galaxy at 8. ... Aperture synthesis is a type of interferometry that mixes signals from a collection instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. ... Very Long Baseline Interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy, in which the data received at each antenna in the array is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. ... The closure phase is an observable quantity in imaging interferometry, which allowed the use of interferometry with very long baselines. ... Aperture synthesis is a type of interferometry that mixes signals from a collection instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. ... Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a turbulent layer in the atmosphere. ...

Astronomical interferometers can produce higher resolution astronomical images than any other type of telescope. At radio wavelengths image resolutions of a few micro-arcseconds have been obtained, and image resolutions of a few milliarcseconds can be achieved at visible and infrared wavelengths. Angular resolution describes the resolving power of a telescope. ... A second of arc or arcsecond is a unit of angular measurement which comprises one-sixtieth of an arcminute, or 1/3600 of a degree of arc or 1/1296000 ≈ 7. ... A milliarcsecond (m, mas) , or a thoundsanth of an arcsecond. ...

One simple layout of astronomical interferometer is a parabolic arrangement of mirrors, giving a partially complete reflecting telescope (with a "sparse" or "dilute" aperture). In fact the parabolic arrangement of the mirrors is not important, as long as the optical path lengths from the astronomical object to the beam combiner or focus are the same as given by the parabolic case. Most existing arrays use a planar geometry instead, and Labeyrie's hypertelescope will use a spherical geometry, for example. Ritchey 24 reflecting telescope A reflecting telescope (reflector) is an optical telescope which uses mirrors to reflect light, rather than lenses to pass light. ... French astronomer, graduated from the grande école SupOptique (École supérieure doptique), he is the inventor of the interferometric astronomy. ...

History of astronomical interferometers

See main article History of astronomical interferometry William Herschel knew as early as 1779 (Herschel 1805) that stars appeared much larger in telescopes than they really were but he did not know why. ...

One of the first uses of optical interferometry was on the Mount Wilson Observatory's reflector telescope in order to measure the diameters of stars. The red giant star Betelgeuse was the first to have its diameter determined in this way between 1920 and 1921. In the 1940s radio interferometry was used to perform the first high resolution radio astronomy observations. For the next three decades astronomical interferometry research was dominated by research at radio wavelengths, leading to the development of large instruments such as the Very Large Array and the Atacama Large Millimeter Array. The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California. ... Hubble UV picture of Betelgeuse. ... 1920 (MCMXX) was a leap year starting on Thursday (link will take you to calendar) // Events January January 7 - Forces of Russian White admiral Kolchak surrender in Krasnoyarsk. ... 1921 (MCMXXI) was a common year starting on Saturday (see link for calendar). ... 1940 (MCMXL) was a leap year starting on Monday (the link is to a full 1940 calendar). ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ... Microwave image of 3C353 galaxy at 8. ... The Very Large Array (VLA) is a radio astronomy observatory located on the Plains of San Augustin, between the towns of Magdalena and Datil, some fifty miles (80 km) west of Socorro, New Mexico, USA. The VLA stands at , , at an altitude of 6970 ft (2124 m) above sea level. ... ALMAs logo The Atacama Large Millimeter Array (ALMA) is an international astronomy project that consists of a system of radio telescopes in an array formation, located in the Atacama desert in northern Chile. ...

Optical/infrared interferometry was extended to measurements using separated telescopes by Johnson, Betz and Towns (1974) in the infrared and by Labeyrie (1975) in the visible. In the late 1970's improvments in computer processing allowed for the first "fringe-tracking" interferometer, which operates fast enough to follow the blurring effects of astronomical seeing, leading to the Mk I,II and III series of interferometers. Similar techniques have now been applied at other astronomical telescope arrays, including the Keck Interferometer and the Palomar Testbed Interferometer. 1974 (MCMLXXIV) was a common year starting on Tuesday (the link is to a full 1974 calendar). ... French astronomer, graduated from the grande école SupOptique (École supérieure doptique), he is the inventor of the interferometric astronomy. ... 1975 (MCMLXXV) was a common year starting on Wednesday (the link is to a full 1975 calendar). ... Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a turbulent layer in the atmosphere. ... This article needs to be wikified. ... The Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. ...

In the 1980s the aperture synthesis interferometric imaging technique was extended to visible light and infrared astronomy by the Cavendish Astrophysics Group, providing the first very high resolution images of nearby stars. In 1995 this technique was demonstrated on an array of separate optical telescopes for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same technique has now been applied at a number of other astronomical telescope arrays, including the Navy Prototype Optical Interferometer, the VLTI, the CHARA array, the IOTA array, Labeyrie's Hypertelescope prototype and soon the MRO Interferometer. Aperture synthesis is a type of interferometry that mixes signals from a collection instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. ... The Cavendish Astrophysics Group (formerly the Radio Astronomy Group) is based at the Cavendish Laboratory at Cambridge University. ... 1995 (MCMXCV) was a common year starting on Sunday of the Gregorian calendar. ... COAST, the Cambridge Optical Aperture Synthesis Telescope, is a multi-element optical interferometer with baselines of up to 100 metres, designed to observe stars with angular resolution as high as one thousandth of one arcsecond (much higher resolution than can be obtained with individual telescopes such as the Hubble Space... The Navy Prototype Optical Interferometer (NPOI) is an interferometer operated by the US Naval Observatory, the Naval Research Laboratory and The Lowell Observatory. ... The four telescopes of the European Southern Observatory Paranal site. ... The CHARA Array is an astronomical optical interferometer operated by The Center for High Angular Resolution Astronomy (CHARA) at Georgia State University. ... The Infrared Optical Telescope Array (IOTA) began with an agreement in 1988 among five Institutions, the Smithsonian Astrophysical Observatory, Harvard University, the University of Massachusetts, the University of Wyoming, and MIT/Lincoln Laboratory, to build a two-telescope stellar interferometer for the purpose of making fundamental astrophysical observations, and also... French astronomer, graduated from the grande école SupOptique (École supérieure doptique), he is the inventor of the interferometric astronomy. ... The Magdalena Ridge Observatory Interferometer will be an optical array composed of ten telescopes, each approximately 1. ...

Modern astronomical interferometry

Projects are now beginning that will use interferometers to search for extrasolar planets, either by astrometric measurements of the reciprocal motion of the star (as used by the Palomar Testbed Interferometer and the VLTI), through the use of nulling (as will be used by the Keck Interferometer and Darwin) or through direct imaging (as proposed for Labeyrie's Hypertelescope). Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ... The Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. ... The four telescopes of the European Southern Observatory Paranal site. ... This article needs to be wikified. ... Darwin is a proposed European Space Agency (ESA) mission designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. ... French astronomer, graduated from the grande école SupOptique (École supérieure doptique), he is the inventor of the interferometric astronomy. ...

A detailed description of the development of astronomical optical interferometry can be found here. Impressive results were obtained in the 1990s, with the Mark III measuring diameters of 100 stars and many accurate stellar positions, COAST and NPOI producing many very high resolution images, and ISI measuring stars in the mid-infrared for the first time. Additional results include direct measurements of the sizes of and distances to Cepheid variable stars, and young stellar objects. COAST. the Cambridge Optical Aperture Synthesis Telescope, is a multi-element optical interferometer with baselines of up to 100 metres, designed to observe stars with angular resolution as high as one thousandth of one arcsecond (much higher resolution than can be obtained with individual telescopes such as the Hubble Space... The Navy Prototype Optical Interferometer (NPOI) is an interferometer operated by the US Naval Observatory, the Naval Research Laboratory and The Lowell Observatory. ... A Cepheid variable is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute stellar luminosity. ...

Optical interferometers are mostly seen by astronomers as very specialized instruments, capable of a very limited range of observations. It is often said that an interferometer achieves the effect of a telescope the size of the distance between the apertures; this is only true in the limited sense of angular resolution. The combined effects of limited aperture area and atmospheric turbulence generally limit interferometers to observations of comparatively bright stars and active galactic nuclei. However, they have proven useful for making very high precision measurements of simple stellar parameters such as size and position (astrometry), for imaging the nearest giant stars and probing the cores of nearby active galaxies. Angular resolution describes the resolving power of a telescope. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... This article does not cite its references or sources. ... Giant star is a star that has stopped fusing hydrogen in its core. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ...

For details of individual instruments, see the list of astronomical interferometers at visible and infrared wavelengths. Current Performance of Ground-Based Interferometers Here is a list of currently existing astronomical interferometers, and some parameters describing their performance. ...

A simple two-element optical interferometer. Light from two small telescopes (shown as lenses) is combined using beam splitters at detectors 1, 2, 3 and 4. The elements creating a 1/4 wave delay in the light allow the phase and amplitude of the interference visibility to be measured, which give information about the shape of the light source.

A single large telescope with an aperture mask over it (labelled Mask), only allowing light through two small holes. The optical paths to detectors 1, 2, 3 and 4 are the same as in the left-hand figure, so this setup will give identical results. By moving the holes in the aperture mask and taking repeated measurements, images can be created using aperture synthesis which would have the same quality as would have been given by the right-hand telescope without the aperture mask. In an analogous way, the same image quality can be achieved by moving the small telescopes around in the left-hand figure - this is the basis of aperture synthesis, using widely separated small telescopes to simulate a giant telescope.

At radio and submillimetre wavelengths, large interferometers such as the Very Large Array and the Atacama Large Millimeter Array are the mainstays of astronomical research. Download high resolution version (382x759, 13 KB)A two-element optical interferometer which is optically identical to this telescope with an aperture mask Picture from my project. ... Download high resolution version (306x637, 9 KB) A telescope with a mask covering it allowing light through two holes. ... 50 cm refracting telescope at Nice Observatory. ... A lens. ... a) shows a simple experiment using an aperture mask in a re-imaged aperture plane. ... Aperture synthesis is a type of interferometry that mixes signals from a collection instruments to produce measurements having the same angular resolution as an instrument the size of the entire collection. ... The Very Large Array (VLA) is a radio astronomy observatory located on the Plains of San Augustin, between the towns of Magdalena and Datil, some fifty miles (80 km) west of Socorro, New Mexico, USA. The VLA stands at , , at an altitude of 6970 ft (2124 m) above sea level. ... ALMAs logo The Atacama Large Millimeter Array (ALMA) is an international astronomy project that consists of a system of radio telescopes in an array formation, located in the Atacama desert in northern Chile. ...

Antoine Labeyrie has proposed the idea of an astronomical interferometer where the individual telescopes are positioned in a spherical arrangement. This geometry reduces the amount of pathlength compensation required in re-pointing the interferometer array (in fact a Mertz corrector can be used rather than delay lines), but otherwise is little different from other existing instruments. He has suggested a space-based interferometer array much larger than the Darwin and TPF projects using this spherical geometry of array elements and using a densified pupil beam combiner, and calls this his "Hypertelescope" project. As pointed out by Malcolm Fridlund, project scientist for ESA's Darwin mission, the cost of the Hypertelescope "would be really prohibitive". French astronomer, graduated from the grande école SupOptique (École supérieure doptique), he is the inventor of the interferometric astronomy. ... Darwin is a proposed European Space Agency (ESA) mission designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. ... Terrestrial Planet Finder - Infrared interferometer concept The Terrestrial Planet Finder (TPF) is a mothballed NASA telescope system that would be capable of detecting extrasolar terrestrial planets. ...

References

• John E. Baldwin and Chris A. Haniff. The application of interferometry to optical astronomical imaging. Phil. Trans. A, 360, 969-986, 2001. (download PostScript file)
• John E. Baldwin et al, Astronomy and Astrophysics, v.306, L13, 1996 The first images from an optical aperture synthesis array: mapping of Capella with COAST at two epochs. -- the first imaging with optical astronomical interferometers
• John E. Baldwin, Ground-based interferometry - the past decade and the one to come, in Interferometry for Optical Astronomy II, volume 4838 of Proc. SPIE, page 1, 22-28 August 2002, Kona, Hawaii, SPIE Press, 2003. (download PostScript file)
• M. Johnson, A. Betz, C. Townes, 1974 Physical Review Letters 33, 1617
• A. Labeyrie, 1975 Astrophys. J. 196, L71
• J. D. Monnier, Optical interferometry in astronomy, Reports on Progress in Physics, 66, 789-857, 2003 IoP. (download PDF file)
• M. Ryle & D. Vonberg, 1946 Solar radiation on 175Mc/s, Nature 158 pp 339
• Govert Schilling, New Scientist, 23 February 2006 The hypertelescope: a zoom with a view

See also: History of astronomical interferometry Prof. ... Prof. ... 1996 (MCMXCVI) was a leap year starting on Monday of the Gregorian calendar, and was designated the International Year for the Eradication of Poverty. ... Prof. ... For the Cusco album, see 2002 (album). ... French astronomer, graduated from the grande école SupOptique (École supérieure doptique), he is the inventor of the interferometric astronomy. ... 1975 (MCMLXXV) was a common year starting on Wednesday (the link is to a full 1975 calendar). ... 2003 (MMIII) was a common year starting on Wednesday of the Gregorian calendar. ... The Institute of Physics (IOP) is the United Kingdoms professional body for physicists. ... Sir Martin Ryle (September 27, 1918 – October 14, 1984) was a British radio astronomer who developed revolutionary radio telescope systems (see e. ... 1946 (MCMXLVI) was a common year starting on Tuesday. ... 2006 (MMVI) is a common year starting on Sunday of the Gregorian calendar. ... William Herschel knew as early as 1779 (Herschel 1805) that stars appeared much larger in telescopes than they really were but he did not know why. ...

External link

• How to combine the light from multiple telescopes