- 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 astronomyand radio astronomy. Aperture synthesiscan be used to perform high-resolution imaging using astronomical interferometers. Very Long Baseline Interferometryuses a technique related to the closure phaseto 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 synthesisallows the atmospheric seeing resolution limit to be overcome, allowing the angular resolution to reach the diffraction-limit of the array.
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.
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 Labeyrie's hypertelescope will use a spherical geometry, for example.
History of astronomical interferometers
See main article
History of astronomical interferometry
One of the first uses of optical interferometry was the construction of a
Michelson stellar interferometeron the Mount Wilson Observatory's reflector telescope in order to measure the diameters of stars. The red giant star Betelgeusewas 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 astronomyobservations. 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 Arrayand 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 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 Interferometerand the Palomar Testbed Interferometer.
In the 1980s the
aperture synthesisinterferometric 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 [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 phases 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 Interferometerand the IOTA array. A number of other interferometers have made closure phasemeasurements and are expected to produce their first images soon, including the VLTI, the CHARA arrayand Labeyrie'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 Interferometerand the VLTI), through the use of nulling (as will be used by the Keck Interferometerand Darwin) or through direct imaging (as proposed for 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 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
Cepheidvariable 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 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.
For details of individual instruments, see the
list of astronomical interferometers at visible and infrared wavelengths.
At radio wavelengths, interferometers such as the
Very Large Arrayand MERLINhave 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 Arrayand the IRAM Plateau de Bure facility. Currently under construction is the Atacama Large Millimeter Array.
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".
John E. Baldwinand 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. Baldwinet 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
* A. Labeyrie,
1975Astrophys. J. 196, L71
*J. D. Monnier, Optical interferometry in astronomy, Reports on Progress in Physics, 66, 789-857,
2003IoP. ( [http://www.astro.lsa.umich.edu/~monnier/Publications/ROP2003_final.pdf download PDF file] )
*M. Ryle & D. Vonberg,
1946Solar radiation on 175Mc/s, Nature 158 pp 339
*Govert Schilling, New Scientist,
23 February 2006The hypertelescope: a zoom with a view
* 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)
History of astronomical interferometry
* [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
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