Astronomical interferometer 4293361 187056442 2008-01-26T17:16:02Z Srleffler 252195 change cat. [[Image:Interf_diagram.gif|thumb|right|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).]] [[Image:Very_Large_Telescope_Array.aerial_view.jpg|thumb|right|The [[VLTI]] infrared astronomical interferometer.]] 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 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 [[astronomical seeing|atmospheric seeing]] resolution limit to be overcome, allowing the angular resolution to reach the diffraction-limit of the array. Astronomical interferometers can produce higher [[Angular resolution|resolution]] astronomical images than any other type of telescope. At radio wavelengths image resolutions of a few micro-[[arcsecond]]s have been obtained, and image resolutions of a few [[milliarcsecond]]s 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 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 [[Antoine Émile Henry Labeyrie|Labeyrie]]'s hypertelescope will use a spherical geometry, for example. == History of astronomical interferometers == See main article [[History of astronomical interferometry]] [[Image:Hooker interferometer.jpg|left|thumb|280px|A 20-foot Michelson interferometer mounted on the frame of the 100-inch [[Hooker Telescope]], 1920.]] One of the first uses of optical interferometry was the construction of a [[Michelson stellar interferometer]] 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 interferometer|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]]. Optical/infrared interferometry was extended to measurements using separated telescopes by Johnson, Betz and Towns (1974) in the infrared and by [[Antoine Émile Henry Labeyrie|Labeyrie]] (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 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]]. 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 [[Cambridge Optical Aperture Synthesis Telescope|an array of separate optical telescopes]] for the first time, allowing a further improvement in resolution, and allowing even higher resolution [http://www.mrao.cam.ac.uk/telescopes/coast/astronomy.html#supergiants02-04 imaging of stellar surfaces]. Software packages such as BSMEM or MIRA are used to convert the measured visibility amplitudes and [[closure phase]]s into astronomical images. The same techniques have now been applied at a number of other astronomical telescope arrays, including the [[Navy Prototype Optical Interferometer]], the [[Infrared Spatial Interferometer]] and the [[Infrared Optical Telescope Array|IOTA]] array. A number of other interferometers have made [[closure phase]] measurements and are expected to produce their first images soon, including the [[VLT|VLTI]], the [[CHARA array]] and [[Antoine Émile Henry Labeyrie|Labeyrie]]'s Hypertelescope prototype. When completed the [[Magdalena Ridge Observatory Interferometer|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 Interferometer]] and the [[VLT|VLTI]]), through the use of nulling (as will be used by the [[Keck Interferometer]] and [[Darwin (ESA)|Darwin]]) or through direct imaging (as proposed for [[Antoine Émile Henry Labeyrie|Labeyrie]]'s Hypertelescope). A detailed description of the development of astronomical optical interferometry can be found [http://www.geocities.com/CapeCanaveral/2309/page1.html here]. Impressive results were obtained in the 1990s, with the [[Mark III Interferometer|Mark III]] measuring diameters of 100 stars and many accurate stellar positions, [[Cambridge Optical Aperture Synthesis Telescope (COAST)|COAST]] and [[Navy Prototype Optical Interferometer|NPOI]] producing many very high resolution images, and [[Infrared Stellar Interferometer|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 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 galaxy|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 star]]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]]. {| |----- | align="center" valign="bottom" | [[Image:Ast_opt_int_lba.gif|250px]] | align="center" valign="bottom" | [[Image:Ast_opt_int_mask.gif|250px]] |----- | valign="top" | A simple two-element optical interferometer. Light from two small [[telescopes]] (shown as [[lens (optics)|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. | valign="top" | A single large telescope with an [[Aperture Masking Interferometry|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 wavelengths, interferometers such as the [[Very Large Array]] and [[MERLIN]] 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 Interferometry]]. In the (sub)-millimetre, existing arrays include the [[Submillimeter Array]] and the [[Institut_de_Radio_Astronomie_Millim%C3%A9trique|IRAM]] Plateau de Bure facility. Currently under construction is the [[Atacama Large Millimeter Array]]. [[Antoine Émile Henry Labeyrie|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 (ESA)|Darwin]] and [[Terrestrial Planet Finder|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". ==References== *[[John E. Baldwin]] and [[Chris A. Haniff]]. The application of interferometry to optical astronomical imaging. Phil. Trans. A, 360, 969-986, 2001. ([http://www.mrao.cam.ac.uk/telescopes/coast/papers/tyoung.ps download PostScript file]) *[[John E. Baldwin]] et al, Astronomy and Astrophysics, v.306, L13, [[1996]] [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1996A%26A...306L..13B&db_key=AST 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. ([ftp://ftp.mrao.cam.ac.uk/pub/coast/spie4838-01-letter.ps download PostScript file]) * M. Johnson, A. Betz, C. Townes, 1974 Physical Review Letters 33, 1617 * [[Antoine Émile Henry Labeyrie|A. Labeyrie]], [[1975]] Astrophys. J. 196, L71 *J. D. Monnier, Optical interferometry in astronomy, Reports on Progress in Physics, 66, 789-857, [[2003]] [[Institute of Physics|IoP]]. ([http://www.astro.lsa.umich.edu/~monnier/Publications/ROP2003_final.pdf download PDF file]) *[[Martin Ryle|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 ==Books== * Basics of Interferometry, 2E by P. Hariharan - Outstanding introduction to the world of optical interferometry with summaries at the beginning and end of each chapter, several appendices with essential information, and worked numerical problems / Practical details enrich understanding for readers new to this material / New chapters on white-light microscopy for medical imaging and interference with single photons(quantum optics) See also: [[History of astronomical interferometry]] ==External links== * [http://www.space.com/scienceastronomy/astronomy/interferometry_101.html How to combine the light from multiple telescopes] for astrometric measurements * [http://www.astronomycafe.net/anthol/remote.html Remote Sensing] the potential and limits of astronomical interferometry [[Category:Observational astronomy]] [[Category:Interferometers]] [[Category:Astronomical instruments]] [[pt:Interferômetro de Michelson]]