An
astronomical interferometer 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 astronomyInfrared astronomy is the branch of astronomy and astrophysics which deals with objects visible in infrared radiation. Visible radiation ranges from 380 nm to 750 nm...
,
submillimetre astronomySubmillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a...
and
radio 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, but subsequent advances have identified a number of different sources of radio emission...
.
Aperture 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...
can be used to perform high-resolution imaging using astronomical interferometers.
Very Long Baseline InterferometryVery Long Baseline Interferometry is a type of astronomical interferometry used in radio astronomy. It allows observations of an object that are made simultaneously by many telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.Data...
uses a technique related to the
closure phaseThe closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...
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 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...
allows the
atmospheric seeingAstronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulence in the Earth's atmosphere. It is the equivalent of looking at the bottom of a lake on a windy day. The astronomical seeing conditions on a given night at a given location describe how...
resolution limit to be overcome, allowing the angular resolution to reach the diffraction-limit of the array.
Astronomical interferometers can produce higher
resolutionAngular resolution or 'spatial resolution' describes the resolving power of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye.- Definition of terms :...
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
milliarcsecondsA minute of arc or arcminute is a unit of angular measurement, equal to one sixtieth of one degree. Since one degree is defined as one three hundred sixtieth of a circle, 1 minute of arc is 1/21,600 of the amount of arc in a closed circle...
can be achieved at visible and infrared wavelengths.
One simple layout of an astronomical interferometer is a parabolic arrangement of mirrors, giving a partially complete
reflecting telescopeA reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from...
(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
LabeyrieAntoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....
's hypertelescope will use a spherical geometry, for example.
History of astronomical interferometers
One of the first uses of optical interferometry was the construction of a
Michelson stellar interferometerThe Michelson stellar interferometer is one of the earliest astronomical interferometers built and used. The interferometer was proposed by Albert Michelson in 1890, following a suggestion by Hippolyte Fizeau....
on the
Mount Wilson ObservatoryThe Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles....
's reflector telescope in order to measure the diameters of stars. The red giant star
BetelgeuseBetelgeuse is a semiregular variable star located approximately 640 light-years from the Earth. It is Alpha Orionis , but the second brightest star in the constellation Orion and the ninth brightest star in the night sky...
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 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, but subsequent advances have identified a number of different sources of radio emission...
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 ArrayThe Very Large Array is a radio astronomy observatory located on the Plains of San Augustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA. U.S. Route 60 passes through the complex, which is adjacent to the Boy Scout Double H High Adventure Base. The...
and the
Atacama Large Millimeter ArrayThe Atacama Large Millimeter/submillimeter Array is an international astronomy project that consists of an astronomical interferometer formed from an array of radio telescopes, located at Llano de Chajnantor Observatory 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
LabeyrieAntoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....
(1975) in the visible. In the late 1970s improvements in computer processing allowed for the first "fringe-tracking" interferometer, which operates fast enough to follow the blurring effects of
astronomical seeingAstronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulence in the Earth's atmosphere. It is the equivalent of looking at the bottom of a lake on a windy day. The astronomical seeing conditions on a given night at a given location describe how...
, 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 InterferometerThe Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. It was built by Caltech/JPL and is intended to serve as a testbed for developing interferometric techniques to be used at the Keck Interferometer...
.
In the 1980s the
aperture 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...
interferometric imaging technique was extended to visible light and infrared astronomy by the
Cavendish Astrophysics GroupThe 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...
, providing the first very high resolution images of nearby stars. In 1995 this technique was demonstrated on
an array of separate optical telescopesCOAST, the Cambridge Optical Aperture Synthesis Telescope, is a multi-element optical astronomical interferometer with baselines of up to 100 metres, which uses aperture synthesis to observe stars with angular resolution as high as one thousandth of one arcsecond COAST, the Cambridge Optical...
for the first time, allowing a further improvement in resolution, and allowing even higher resolution
imaging of stellar surfaces. Software packages such as BSMEM or MIRA are used to convert the measured visibility amplitudes and
closure phaseThe closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...
s into astronomical images. The same techniques have now been applied at a number of other astronomical telescope arrays, including the
Navy Prototype Optical InterferometerThe Navy Prototype Optical Interferometer is an astronomical interferometer operated by the US Naval Observatory , the Naval Research Laboratory and The Lowell Observatory. The facility is located on Anderson Mesa about 15 miles southeast of Flagstaff, Arizona...
, the
Infrared Spatial InterferometerThe Infrared Spatial Interferometer is an astronomical interferometer array of three 65 inch telescopes operating in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for placements as far as 70 m apart, giving the resolution of a telescope of that...
and the
IOTAThe Infrared Optical Telescope Array 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...
array. A number of other interferometers have made
closure phaseThe closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...
measurements and are expected to produce their first images soon, including the
VLTVLT may stand for:* Very Large Telescope, a system of four large optical telescopes organized in an array formation, located in northern Chile...
I, the
CHARA arrayThe CHARA Array is an optical astronomical interferometer operated by The Center for High Angular Resolution Astronomy of the Georgia State University . CHARA is the World's highest angular resolution telescope at near-infrared wavelengths...
and
LabeyrieAntoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....
's Hypertelescope prototype. When completed, the MRO Interferometer with its ten moveable telescopes will produce the first high fidelity images from a long baseline interferometer.
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 InterferometerThe Palomar Testbed Interferometer is a near-IR, long-baseline stellar interferometer located at Palomar Observatory in north San Diego County. It was built by Caltech/JPL and is intended to serve as a testbed for developing interferometric techniques to be used at the Keck Interferometer...
and the
VLTVLT may stand for:* Very Large Telescope, a system of four large optical telescopes organized in an array formation, located in northern Chile...
I), through the use of nulling (as will be used by the Keck Interferometer and
DarwinDarwin is a European Space Agency program designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. The launch date will not be before 2016. The current design envisions three free-flying space telescopes, each at least 3 meters in...
) or through direct imaging (as proposed for
LabeyrieAntoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....
's Hypertelescope).
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
NPOIThe Navy Prototype Optical Interferometer is an astronomical interferometer operated by the US Naval Observatory , the Naval Research Laboratory and The Lowell Observatory. The facility is located on Anderson Mesa about 15 miles southeast of Flagstaff, Arizona...
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.
Optical interferometersOne of the first astronomical interferometers was built on the Mount Wilson Observatory's reflector telescope in 1920 in order to measure the diameters of stars. The red giant star Betelgeuse was among the first to have its diameter determined in this way...
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 resolutionAngular resolution or 'spatial resolution' describes the resolving power of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye.- Definition of terms :...
. The amount of light gathered—and hence the dimmest object that can be seen—depends on the real aperture size, so an interferometer would offer little improvement. 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 (
astrometryAstrometry is the branch of astronomy that relates to precise measurements and explanations of the positions and movements of stars and other celestial bodies...
), for imaging the nearest
giant starA giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
s and probing the cores of nearby active galaxies.
For details of individual instruments, see the
list of astronomical interferometers at visible and infrared wavelengths.
 |
 |
| A simple two-element optical interferometer. Light from two small telescopes (shown as lenses A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens is a lens consisting of a single optical element...
) 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 Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...
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 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...
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 wavelengths, interferometers such as the
Very Large ArrayThe Very Large Array is a radio astronomy observatory located on the Plains of San Augustin, between the towns of Magdalena and Datil, some fifty miles west of Socorro, New Mexico, USA. U.S. Route 60 passes through the complex, which is adjacent to the Boy Scout Double H High Adventure Base. The...
and
MERLINThe Multi-Element Radio Linked Interferometer Network is an interferometer array of radio telescopes spread across England. The array is run from Jodrell Bank Observatory in Cheshire by the University of Manchester on behalf of STFC as a National Facility.The array consists of up to seven radio...
have been in operation for many years. The distances between telescopes are typically 10-100 km although arrays with much longer baselines utilize the techniques of
Very Long Baseline InterferometryVery Long Baseline Interferometry is a type of astronomical interferometry used in radio astronomy. It allows observations of an object that are made simultaneously by many telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.Data...
. In the (sub)-millimetre, existing arrays include the
Submillimeter ArrayThe Submillimeter Array consists of eight 6 m diameter radio telescopes arranged as an interferometer for submillimeter wavelength observations...
and the IRAM Plateau de Bure facility. Currently under construction is the
Atacama Large Millimeter ArrayThe Atacama Large Millimeter/submillimeter Array is an international astronomy project that consists of an astronomical interferometer formed from an array of radio telescopes, located at Llano de Chajnantor Observatory in the Atacama desert in northern Chile...
.
Antoine LabeyrieAntoine Émile Henry Labeyrie is a French astronomer and holds since 1991 the "Observational Astrophysics" chair at the Collège de France....
has proposed the idea of an astronomical interferometer where the individual telescopes seen as parts of a
fractionated spacecraftA fractionated spacecraft is a satellite architecture where the functional capabilities of a conventional monolithic spacecraft are distributed across multiple modules which interact through wireless links...
or a
satellite constellationA group of electronic satellites working in concert is known as a satellite constellation. Such a constellation can be considered to be a number of satellites with coordinated ground coverage, operating together under shared control, synchronised so that they overlap well in coverage and...
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
DarwinDarwin is a European Space Agency program designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. The launch date will not be before 2016. The current design envisions three free-flying space telescopes, each at least 3 meters in...
and
TPFThe Terrestrial Planet Finder is a proposed project by the National Aeronautics and Space Administration of the United States for a telescope system which is intended to detect extrasolar terrestrial planets.-History:...
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 FridlundMalcolm Fridlund is a Swedish Astronomer. He made his doctor thesis 1987 in astronomy in Stockholm and works since 1988 on ESA in Noordwijk in the Netherlands as scientific project manager. Since 1996 is Malcolm Fridlund the scientific manager of the Darwin project. From the spring of 2006 is he...
, project scientist for ESA's Darwin mission, the cost of the Hypertelescope "would be really prohibitive".
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