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. When Thomas Young demonstrated interference and the wave nature of light unambiguously this was explained. As he stated in his Bakerian Lecture of 1803: "The proposition on which I mean to insist at present, is simply this, that fringes of colours are produced by the interference of two portions of light", and later: "that homogeneous light, at certain equal distances in the direction of its motion, is possessed of opposite qualities, capable of neutralizing or destroying each other, and of extinguishing the light, where they happen to be united" (Young 1804). Sir Wilhelm Friedrich Herschel (Hanover, November 15, 1738 – August 25, 1822 Slough, then in Buckinghamshire now in Berkshire) was a German-born British astronomer and composer who became famous for discovering the planet Uranus, and made many other astronomical discoveries. ... 1779 was a common year starting on Friday (see link for calendar). ... The Pleiades star cluster A star is a massive gaseous body in outer space which is currently or has in the past produced energy through nuclear fusion. ... There have been several well-known people named Thomas Young, including: Thomas Young, 16th century archbishop of York Thomas Young, M.A., Master of Jesus College, Cambridge 1644-50 Thomas Young (1773-1829), scientist Thomas Young VC, the recipient of the Victoria Cross Thomas Young, the Baptist Evangelist from Piedmont... 1803 was a common year starting on Saturday (see link for calendar). ...

But it was not Young's researches that prompted Herschel to investigate the origin of the spurious diameters of stars. Instead it was the exactly contemporaneous discovery of the ­first minor planets: Ceres in 1801, Pallas in 1802 and Juno in 1803. Were their apparent diameters as real as those of planets or spurious as for stars? To address this question Herschel conducted an extensive series of experiments in his garden in Slough, examining through his telescope small globules of differing sizes and materials placed in a tree some 800 ft (ca. 244 m) away (Herschel 1805). His observations showed that for the smallest globules the diameters were all spurious and all of the same size. Furthermore, he found that, if just the inner part of the aperture of the telescope were used, the spurious diameters, whether of globules or of stars, were larger. If the whole aperture was employed, the diameters were smaller, and if only an outer annular aperture was used the diameters were smaller still. This experimental discovery that unfi­lled apertures can be used to obtain high angular resolution remains today the essential basis for interferometric imaging in astronomy (in particular Aperture Masking Interferometry. The theoretical justifi­cation of this result came with Airy's analysis of the diffraction pattern of a circular aperture 30 years later (Airy 1835), and it took a further 30 years before the idea of using multiple apertures was developed. In an early study the Reverend W. R. Dawes noted that he had `frequently found great advantage from the use of a perforated whole aperture' and that when observing Venus this produced `a central image of the planet perfectly colourless, and very sharply de­ned' (Dawes 1866). But it was left to Fizeau, in his submission to the Commission for the Prix Bordin the following year, to remark on `une relation remarquable et n´ecessaire entre la dimension des franges et celle de la source lumineuse' and suggest that by using an interferometric combination of light from two separated slits `il deviendra possible d'obtenir quelques donn´ees nouvelles sur les diametres angulaires de ces astres' (Fizeau 1868). Minor planets, or planetoids are minor bodies of the solar system orbiting the sun that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a diameter of about 4880 km). ... For the first asteroid to be discovered, see 1 Ceres For the Melbourne community environment park, see the Centre for Education and Research in Environmental Strategies CERES (Clouds and the Earths Radiant Energy System) is an on-going NASA metereological experiment in Earth orbit. ... 1801 was a common year starting on Thursday (see link for calendar). ... Pallas Athena. ... 1802 was a common year starting on Friday (see link for calendar). ... Juno can refer to: Juno, the Roman equivalent of the Greek goddess Hera A guardian spirit for Roman women (equivalent of the male Genius) Jupiter IRBM rocket (Juno II) the Jupiter-C IRBM rocket (Juno or Juno I) the Juno Awards, a Canadian music award festival Juno Beach, one of... 1803 was a common year starting on Saturday (see link for calendar). ... Slough (pronounced ) is a town and unitary authority in the county of Berkshire in the south of England. ... a) shows a simple experiment using an aperture mask in a re-imaged aperture plane. ... Look up Airy in Wiktionary, the free dictionary Airy is the surname of Sir George Biddell Airy who is the eponym of craters located on the Moon and Mars. ... Genitive Venusian or Venetian (*min temperature refers to cloud tops only) Atmospheric characteristics Atmospheric pressure 9. ...

Steps towards the practical implementation of these techniques for optical astronomy were taken by Michelson, who de­ned the `visibility' of interference fringes obtained from a source of ­nite angular size (Michelson 1890) and followed this a year later with the measurement of the angular diameters of Jupiter's satellites (Michelson 1891). Finally, 30 years later, Fizeau's predictions became a reality when the direct interferometric measurement of a stellar diameter was realized by Michelson & Pease (1921) with their 20 ft (ca. 6.1 m) interferometer mounted on the 100 inch Mount Wilson Telescope. Albert Abraham Michelson. ... The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California. ...

Interferometric imaging in astronomy

diagram of a radio interferometer

Interferometry provides access to very-high-angular-resolution observations. It also importantly separates the issues of angular resolution and limiting sensitivity. A single mirror of diameter D has an angular resolution λ / D, and a collecting area for the flux of photons D², so that there is a well-de­ned relation between resolution and sensitivity. Since astronomers necessarily study objects beyond their control, it is unlikely that this ­fixed relationship will be a good match to all but a subset of their observational requirements. It was for this reason that the growth of radio astronomy after 1945 depended so dramatically on the use of interferometric methods. The extreme imbalance between the excellent sensitivity of small ­filled apertures at long radio wavelengths and their poor angular resolution, which could be many tens of degrees, led naturally to the development of sparse arrays of widely separated telescopes. Microwave image of 3C353 galaxy at 8. ... 1945 was a common year starting on Monday (link will take you to calendar). ...

In 1946 Ryle and Vonberg (Ryle and Vonberg 1946) constructed a radio analogue of the Michelson interferometer and soon located a number of new cosmic radio sources. The signals from two radio antennas were added electronically to produce interference. Ryle and Vonberg's telescope used the rotation of the Earth to scan the sky in one dimension. Fringe visibilities could be calculated from the variation of intensity with time. Later interferometers included a variable delay between one of the antennas and the detector as shown in the figure at the right. 1946 was a common year starting on Tuesday. ... Sir Martin Ryle (September 27, 1918 – October 14, 1984) was a British radio astronomer who developed revolutionary radio telescope systems (see e. ... The Michelson interferometer is the classic setup for optical interferometry and was invented by Albert Abraham Michelson and used for the famous Michelson-Morley experiment. ...

In the figure radio waves from a source at an angle θ to the vertical must travel a distance δl further in order to reach the left-hand antenna. These signals are thus delayed relative to the signals received at the right hand antenna by a time cδl = casin[θ] where c is the speed of the radio waves. The signal from the right hand antenna must be delayed artificially by the same length of time for constructive interference to occur. Interference fringes will be produced by sources with angles in a small range either side of determined by the coherence time of the radio source. Altering the delay time δt varies the angle at which a source will produce interference fringes. It should be noted that the effective baseline of this interferometer will be given by the projection of the telescope positions onto a plane perpendicular to the source direction. The length of the effective baseline, shown at the bottom of the figure, will be x = acos(θ) where a is the actual telescope separation.

Technical difficulties delayed the growth of optical interferometry. The human eye is a sensitive detector but is not capable of quantitative photometric assessment of interferometric fringe patterns. When coupled with the need to record data at kHz rates, progress in optical synthesis imaging had to wait for the development of sensitive photon-counting detectors. Furthermore, the limits of mechanical stability had been reached with the 50 ft (ca. 15.2 m) beam interferometer constructed by Michelson and Pease in 1930 (Pease 1931). This method was extended to short-wavelength measurements using separated telescopes by (Johnson, Betz and Towns 1974) in the infrared and by (Labeyrie 1975) in the visible. This demanded micrometre-level metrology of variable optical delay lines, which was not feasible until access to stabilized lasers had become routine. 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 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. Some scientists exaggerated the benefits of combining large diameter (adaptive optics corrected) telescopes for near-infrared interferometry, and this left many astronomers disappointed with new arrays utilizing small numbers of large telescopes which came online in the early 2000s. For details of individual instruments, see the list of astronomical interferometers at visible and infrared wavelengths. 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. ... 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. ... 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 as a prototype instrument on... 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. ... It has been suggested that Tip-tilt mirror be merged into this article or section. ... 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.

Once these technical considerations had been addressed, all of the principles used at radio wavelengths could be taken over with almost no modi­cation. This included the use of imaging software developed for VLBI at radio wavelengths, which was used to reconstruct the ­first image from an array of optical telescopes, that of the 50 milliarcsecond binary star Capella (Baldwin et al . 1996). The only signi­cant differences between the two wavelength regimes are the small temporal and spatial scales of the atmospheric fluctuations at optical wavelengths. For example, the characteristic time-scale for these fluctuations is measured in milliseconds at optical wavelengths rather than minutes, and the spatial scale is typically smaller than the telescope mirror diameter, whereas at centimetric radio wavelengths this scale can be as large as 20 km. An important consequence of this small spatial scale is that the area of sky over which the atmospheric phase path is constant, the isoplanatic patch, is at most a few arcseconds at visual wavelengths. 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). ... 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. ... Very Long Baseline Interferometry (VLBI) is a type of interferometry in which the data received at each antenna in the array is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. ... The term capella can refer to: A cappella, a music term referring to vocal music or singing without instrumental accompaniment. ...

References

• Airy, G. B. 1835 Trans. Camb. Phil. Soc. 5, 283
• Armstrong, J. T., Mozurkewich, D., Pauls, T. A. & Hajian, A. R. 1998 Proc. SPIE 3350, 461
• Baldwin, J. E. (and 15 others) 1996 Astron. Astrophys. 306, L13
• Boden, A. F. & Lane, B. F. 2001 Astrophys. J. 547, 1071
• Burns, D. (and 10 others) 1997 Mon. Not. R. Astron. Soc. 290, L11
• Buscher, D. F. 1988 Mon. Not. R. Astron. Soc. 235, 1203
• Capetti, A., Schreier, E. J., Axon, D., Young, S., Hough, J. H., Clark, S., Marconi, A., Macchetto, D. & Packham, C. 2000 Astrophys. J. 544, 269
• Colavita, M. M. (and 16 others) 1999 Astrophys. J. 510, 505
• Dawes, W. R. 1866 Mem. R. Astron. Soc. 35, 137
• Fizeau, H. 1868 C. R. Hebd. Seanc. Acad. Sci. Paris 66, 932
• Haniff, C. A., Mackay, C. D., Titterington, D. J., Sivia, D. & Baldwin, J. E. 1987 Nature 328, 694
• Herschel, W. 1805 Phil. Trans. R. Soc. Lond. 95, 31
• Horton, A. J., Haniff, C. A. & Buscher, D. F. 2001 Mon. Not. R. Astron. Soc. 327, 217
• Hummel, C. A., Carquillat, J.-M., Ginestet, N., Griffin, R. F., Boden, A. F., Hajian, A. R., Mozurkewich, D. & Nordgren, T. E. 2001 Astron. J. 121, 1623
• Jennison, R. C. 1958 Mon. Not. R. Astron. Soc. 118, 276
• Johnson, M. A., Betz, A. L., Townes, C. H., 1974 Physical Review Letters 33, 1617
• Labeyrie, A. 1975 Astrophys. J. 196, L71
• Lane, B. F., Kuchner, M. J., Boden, A. F., Creech-Eakman, M. & Kulkarni, S. R. 2000 Nature 407, 485
• Lane, B. F., Boden, A. F. & Kulkarni, S. R. 2001 Astrophys. J. 551, L81
• Mackay, C. D., Tubbs, R. N., Bell, R., Burt, D. & Moody, I. 2001 Proc. SPIE 4306, 289
• Marcy, G. W. & Butler, R. P. 1998 A. Rev. Astron. Astrophys. 36, 57
• Michelson, A. A. 1890 Phil. Mag. 30, 1
• Michelson, A. A. 1891 Publ. Astron. Soc. Pac. 3, 274
• Michelson, A. A. & Pease, F. G. 1921 Astrophys. J. 53, 249
• Nordgren, T. E. (and 13 others) 1999 Astron. J. 118, 3032
• Pearson, T. J. & Readhead, A. C. S. 1984 A. Rev. Astron. Astrophys. 22, 97
• Pease, F. G. 1931 Ergeb. Exakt. Naturwiss. 10, 84
• Perrin, G., Coude du Foresto, V., Ridgway, S. T., Mariotti, J.-M., Traub, W. A., Carleton, N. P. & Lacasse, M. G. 1998 Astron. Astrophys. 331, 619
• Peterson, B. M. 2001 Variability of active galactic nuclei. In Advanced lectures on the starburst AGN connection (ed. I. Aretxaga, D. Knuth & R. Mujica), pp. 3. World Scientific.
• Quirrenbach, A. 2001 A. Rev. Astron. Astrophys. 39, 353
• Rogers, A. E. E. (and 10 others) 1974 Astrophys. J. 193, 293
• Ryle, M. & Vonberg, D., 1946 Solar radiation on 175Mc/s, Nature 158 pp 339
• Thompson, A. R., Moran, J. M. & Swenson Jr, G. W. 1991 Interferometry and synthesis in radio astronomy, 2nd edn. Malabar: Krieger.
• Tuthill, P. G., Monnier, J. D., Danchi, W. C., Wishnow, E. H. & Haniff, C. A. 2000 Publ. Astron. Soc. Pac. 112, 555
• Tuthill, P. G., Monnier, J. D. & Danchi, W. C. 2001 Nature 409, 1012
• Wittkowski, M., Hummel, C. A., Johnston, K. J., Mozurkewich, D., Hajian, A. R. & White, N. M. 2001 Astron. Astrophys. 377, 981
• Woolf, N. & Angel, J. R. P. 1998 A. Rev. Astron. Astrophys. 36, 507
• Young, J. S. (and 11 others) 2000a Mon. Not. R. Astron. Soc. 315, 635
• Young, J. S., Baldwin, J. E., Boysen, R. C., Haniff, C. A., Pearson, D., Rogers, J., St-Jacques, D., Warner, P. J. & Wilson, D. M. A. 2000b Mon. Not. R. Astron. Soc. 318, 381
• Young, T. 1804 Phil. Trans. R. Soc. Lond. 94, 1