Optical interferometry is a technique of interferometry combining light from multiple sources in an optical instrument in order to make various precise measurements. 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. ... See also: List of optical topics Optics (appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... In classical physics and engineering, measurement generally refers to the process of estimating or determining the ratio of a magnitude of a quantitative property or relation to a unit of the same type of quantitative property or relation. ...

The technique of optical interferometry can make use of white light, of monochromatic light (e.g., a sodium lamp) or of coherent monochromatic light (laser light). The main difference between these types of light is their coherence length: for white light, only a few wavelengths, but for laser light it can be decimeters or more. In order to see interference fringes at all, the optical path lengths travelled by the interfering beams or rays must differ by less than their correlation length. Alternate meanings: White (disambiguation) White is a color (more accurately it contains all the colors of the spectrum and is sometimes described as an achromatic color—black is the absence of color) that has high brightness but zero hue. ... Something which is monochromatic has a single color. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Coherence is from Latin cohaerere = stick together, to be connected with, logically consistent. ... The range of sizes in which lasers exist is immense, extending from microscopic diode lasers (top) to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion. ... In physics, coherence length is the propagation distance from a coherent source to a point where an electromagnetic wave maintains a specified degree of coherence. ... In probability theory and statistics, correlation, also called correlation coefficient, is a numeric measure of the strength of linear relationship between two random variables. ...

Astronomical optical interferometry

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. This method was extended to measurements using separated telescopes by Johnson, Betz and Towns (1974) in the infrared and by Labeyrie (1975) in the visible. The red giant star Betelgeuse was among the first to have its diameter determined in this way. 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. In the 1980s the aperture synthesis 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 techniques have now been applied at a number of other astronomical telescope arrays, including the Keck Interferometer, the Palomar Testbed Interferometer, Navy Prototype Optical Interferometer, the VLTI, the CHARA array, the IOTA array and the MRO Interferometer. 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. The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California. ... Hubble UV picture of Betelgeuse. ... Schematic diagram illustrating how optical wavefronts from a distant star may be perturbed by a turbulent layer in the atmosphere. ... 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. ... 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. ... 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... The Magdalena Ridge Observatory Interferometer will be an optical array composed of ten telescopes, each approximately 1. ... 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. ...

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 compartively bright stars. However, they have proven useful for making very high precision measurements of simple stellar parameters such as size and position (astrometry). Angular resolution describes the resolving power of a telescope. ... This article does not cite its references or sources. ...

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.

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 is: a part of the eye an optical device that may be used in a camera or in a telescope; see lens (optics) or Category:Lenses. ... 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. ...

For further information see

• 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)
• J. 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)
• J. D. Monnier, Optical interferometry in astronomy, Reports on Progress in Physics, 66, 789-857, 2003 IoP. (download PDF file)

See also: History of astronomical interferometry Prof. ... The Institute of Physics (IOP) is the United Kingdoms professional body for physicists. ... 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. ...

The Michelson-Morley experiment

Of the uses of optical interferometry mention should be made of the Michelson-Morley experiment, a test of the Special Theory of Relativity. Light travels along two alternative, mutually perpendicular paths, and meets again to form interference fringes. If the classical theory of absolute space and time and propagation of electromagnetic waves in a world aether are correct, rotating the whole device with respect to the direction of motion of the Earth should produce a shift in the interference fringes. No such shift was seen. The Michelson-Morley experiment, one of the most important and famous experiments in the history of physics, was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University, and is considered to be the first strong evidence against the theory of a luminiferous... A simple introduction to this subject is provided in Special relativity for beginners Special relativity (SR) or the special theory of relativity is the physical theory published in 1905 by Albert Einstein in his article On the Electrodynamics of Moving Bodies. SR theory is based on the previous works of... Electromagnetic radiation is a propagating wave in space with electric and magnetic components. ... The aether (also spelled ether) is a substance concept, historically used in science and philosophy. ...

Geodetic standard baseline measurements

A famous use of white light interferometry is the precise measurement of geodetic standard baselines as invented by Yrjö Väisälä. Here, the light path is split in two, and one leg is "folded" between a mirror pair 1 m apart. The other leg bounces once off a mirror 6 m away. Only if the second path is precisely 6 times the first, will fringes be seen. Yrjö Väisälä (IPA: ) (September 6, 1891 - July 21, 1971) was a Finnish astronomer and physicist. ...

Starting from a standard quartz gauge of 1 m length, it is possible to measure distances up to 864 m by repeated multiplication. Baselines thus established are used to calibrate geodetic distance measurement equipment on, leading to a metrologically traceable scale for geodetic networks measured by these instruments. Quartz is the most abundant mineral in the Earths continental crust. ... It has been suggested that geodetic system be merged into this article or section. ... Metrology is variously described as the science of measurement; the science of accuracy and precision; the history of measures; the history of measurement and other definitions. ... A geodetic network is a network of triangles which are measured exactly by techniques of terrestrial surveying or by satellite geodesy. ...

More modern geodetic applications of laser interferometry are in calibrating the divisions on levelling staffs, and in monitoring the free fall of a reflective prism within a ballistic or absolute gravimeter, allowing determination of gravity, i.e., the acceleration of free fall, directly from the physical definition at a few parts in a billion accuracy. In construction, level (as an adjective) is to horizontal what plumb is to vertical. ... Free-fall or free fall in the strict sense is the condition of acceleration which is due only to gravity. ...

Holography

A special application of optical interferometry using coherent light is holography, a technique for photographically recording and re-displaying three-dimensional scenes. The technique also lends itself to monitoring small deformations. Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. ...

Inertial navigation

In inertial navigation, ring laser gyroscopes are used that can detect rotation through optical interferometry of laser beams travelling around a circumference in opposite directions (Sagnac interferometer). The effect is amplified by using optic fibres wound around thousands of times. An inertial navigation system measures the position and altitude of a vehicle by measuring the accelerations and rotations applied to the systems inertial frame. ... Note: This page does not refer to a fiber optic gyroscope, even though the user is directed here when seaching for that term. ... The Sagnac effect manifests itself in an experimental setup called ring interferometry. ...

See also

• 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. ...

External links

• List of papers which chart the historical development of astronomical interferometry
• Description of one absolute gravimeter
• Optical Long Baseline Interferometry News
• Description of astronomical interferometry.