Anastronomical interferometer ortelescope array is a set of separatetelescopes, mirror segments, orradio telescopeantennas that work together as a single telescope to provide higher resolution images of astronomical objects such asstars,nebulas andgalaxies by means ofinterferometry. The advantage of this technique is that it can theoretically produce images with theangular resolution of a huge telescope with anaperture equal to the separation, calledbaseline, between the component telescopes. The main drawback is that it does not collect as much light as the complete instrument's mirror. Thus it is mainly useful for fine resolution of more luminous astronomical objects, such as closebinary stars. Another drawback is that the maximum angular size of a detectable emission source is limited by the minimum gap between detectors in the collector array.[1]
Interferometry is most widely used inradio astronomy, in which signals from separateradio telescopes are combined. A mathematicalsignal processing technique calledaperture synthesis is used to combine the separate signals to create high-resolution images. InVery Long Baseline Interferometry (VLBI) radio telescopes separated by thousands of kilometers are combined to form a radio interferometer with a resolution which would be given by a hypothetical single dish with an aperture thousands of kilometers in diameter. At the shorterwavelengths used ininfrared astronomy andoptical astronomy it is more difficult to combine the light from separate telescopes, because the light must be keptcoherent within a fraction of a wavelength over long optical paths, requiring very precise optics. Practical infrared and optical astronomical interferometers have only recently been developed, and are at the cutting edge of astronomical research. At optical wavelengths, aperture synthesis allows theatmospheric seeing resolution limit to be overcome, allowing the angular resolution to reach thediffraction limit of the optics.
Astronomical interferometers can produce higher resolution astronomical images than any other type of telescope. At radio wavelengths, image resolutions of a few microarcseconds (a dozen picoradians) have been obtained, and image resolutions of hundreds of microarcseconds (a couple nanoradians) have been achieved at visible and infrared wavelengths.
One simple layout of an astronomical interferometer is a parabolic arrangement of mirror pieces, giving a partially completereflecting telescope but 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 (focus) are the same as would be given by the complete mirror case. Instead, most existing arrays use a planar geometry, andLabeyrie's hypertelescope will use a spherical geometry.
One of the first uses of optical interferometry was applied by theMichelson stellar interferometer on theMount Wilson Observatory's reflector telescope to measure the diameters of stars. The red giant starBetelgeuse was the first to have its diameter determined in this way on December 13, 1920.[3] In the 1940sradio interferometry was used to perform the first high resolutionradio 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 theVery Large Array and theAtacama Large Millimeter Array.
Optical/infrared interferometry was extended to measurements using separated telescopes by Johnson, Betz and Townes (1974) in the infrared and byLabeyrie (1975) in the visible.[4][5] In the late 1970s improvements in computer processing allowed for the first "fringe-tracking" interferometer, which operates fast enough to follow the blurring effects ofastronomical seeing, leading to the Mk I, II and III series of interferometers. Similar techniques have now been applied at other astronomical telescope arrays, including theKeck Interferometer and thePalomar Testbed Interferometer.
In the 1980s the aperture synthesis interferometric imaging technique was extended to visible light and infrared astronomy by theCavendish Astrophysics Group, providing the first very high resolution images of nearby stars.[6][7][8] In 1995 this technique was demonstrated onan array of separate optical telescopes for the first time, allowing a further improvement in resolution, and allowing even higher resolutionimaging of stellar surfaces. Software packages such as BSMEM or MIRA are used to convert the measured visibility amplitudes andclosure phases into astronomical images. The same techniques have now been applied at a number of other astronomical telescope arrays, including theNavy Precision Optical Interferometer, theInfrared Spatial Interferometer and theIOTA array. A number of other interferometers have madeclosure phase measurements and are expected to produce their first images soon, including theVLTI, theCHARA array andLe Coroller and Dejonghe'sHypertelescope prototype. If completed, theMRO Interferometer with up to ten movable telescopes will produce among the first higher fidelity images from a long baseline interferometer. The Navy Optical Interferometer took the first step in this direction in 1996, achieving 3-way synthesis of an image ofMizar;[9] then a first-ever six-way synthesis ofEta Virginis in 2002;[10] and most recently "closure phase" as a step to the first synthesized images produced bygeostationary satellites.[11]
Astronomical interferometry is principally conducted using Michelson (and sometimes other type) interferometers.[12] The principal operational interferometric observatories which use this type of instrumentation includeVLTI,NPOI, andCHARA.
Current projects will use interferometers to search forextrasolar planets, either by astrometric measurements of the reciprocal motion of the star (as used by thePalomar Testbed Interferometer and theVLTI), through the use of nulling (as will be used by theKeck Interferometer andDarwin) or through direct imaging (as proposed forLabeyrie's Hypertelescope).
Engineers at the European Southern ObservatoryESO designed the Very Large Telescope VLT so that it can also be used as an interferometer. Along with the four 8.2-metre (320 in) unit telescopes, four mobile 1.8-metre auxiliary telescopes (ATs) were included in the overall VLT concept to form the Very Large Telescope Interferometer (VLTI). The ATs can move between 30 different stations, and at present, the telescopes can form groups of two or three for interferometry.
When using interferometry, a complex system of mirrors brings the light from the different telescopes to the astronomical instruments where it is combined and processed. This is technically demanding as the light paths must be kept equal to within 1/1000 mm (the same order as the wavelength of light) over distances of a few hundred metres. For the Unit Telescopes, this gives an equivalent mirror diameter of up to 130 metres (430 ft), and when combining the auxiliary telescopes, equivalent mirror diameters of up to 200 metres (660 ft) can be achieved. This is up to 25 times better than the resolution of a single VLT unit telescope.
The VLTI gives astronomers the ability to study celestial objects in unprecedented detail. It is possible to see details on the surfaces of stars and even to study the environment close to a black hole. With a spatial resolution of 4 milliarcseconds, the VLTI has allowed astronomers to obtain one of the sharpest images ever of a star. This is equivalent to resolving the head of a screw at a distance of 300 km (190 mi).
Notable 1990s results included theMark III measurement of diameters of 100 stars and many accurate stellar positions,COAST andNPOI producing many very high resolution images, andInfrared Stellar Interferometer measurements of stars in the mid-infrared for the first time. Additional results include direct measurements of the sizes of and distances toCepheid variable stars, andyoung stellar objects.
High on the Chajnantor plateau in the Chilean Andes, the European Southern Observatory (ESO), together with its international partners, is building ALMA, which will gather radiation from some of the coldest objects in the Universe. ALMA will be a single telescope of a new design, composed initially of 66 high-precision antennas and operating at wavelengths of 0.3 to 9.6 mm. Its main 12-meter array will have fifty antennas, 12 metres in diameter, acting together as a single telescope – an interferometer. An additional compact array of four 12-metre and twelve 7-meter antennas will complement this. The antennas can be spread across the desert plateau over distances from 150 metres to 16 kilometres, which will give ALMA a powerful variable "zoom". It will be able to probe the Universe at millimetre and submillimetre wavelengths with unprecedented sensitivity and resolution, with a resolution up to ten times greater than the Hubble Space Telescope, and complementing images made with the VLT interferometer.
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 ofangular resolution. 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 as the image is dim (thethinned-array curse). The combined effects of limited aperture area and atmospheric turbulence generally limits interferometers to observations of comparatively bright stars andactive 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 nearestgiant stars and probing the cores of nearbyactive galaxies.
For details of individual instruments, see thelist of astronomical interferometers at visible and infrared wavelengths.
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| A simple two-element optical interferometer. Light from two smalltelescopes (shown aslenses) 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 interferencevisibility to be measured, which give information about the shape of the light source. | A single large telescope with anaperture mask over it (labelledMask), 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 usingaperture synthesis which would have the same quality aswould have been given by the right-hand telescopewithout 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 theVery Large Array andMERLIN have been in operation for many years. The distances between telescopes are typically 10–100 km (6.2–62.1 mi), although arrays with much longer baselines utilize the techniques ofVery Long Baseline Interferometry. In the (sub)-millimetre, existing arrays include theSubmillimeter Array and theIRAM Plateau de Bure facility. TheAtacama Large Millimeter Array has been fully operational since March 2013.
Max Tegmark andMatias Zaldarriaga have proposed the Fast Fourier Transform Telescope which would rely on extensive computer power rather than standard lenses and mirrors.[14] IfMoore's law continues, such designs may become practical and cheap in a few years.
Progressingquantum computing might eventually allow more extensive use of interferometry, as newer proposals suggest.[15]
