The SUSI (Sydney University Stellar Interferometer), Narrabri, Australia. 10 

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... Because a full computer control of the siderostats and delay lines allowed automated acqui- sition of stars and data acquisition, it could observe up to 200 stars in a single night, until 1993, when the interferometer stopped the operation. The COAST (Cambridge Optical Aperture Synthesis Telescope) operated from 1991 until 2005 at Cambridge University, England. It was planned as a coherent array of four telescopes operating in the red and near infra-red, using Michelson interfer- ometry on baselines of up to 100m to give images with a resolution down to 1mas. The number of telescopes (40cm apertures) has been brought up to five, and light from four of them could be interfered simultaneously. Switch- ing between two four-telescope arrays allowed taking data on nine baselines and seven closure triangles during a single night. Observations could be carried out in the visible (R,I) or near-infrared (J,H bands). It was the first instrument of its kind to exploit the techniques of aperture synthesis and closure phase at optical or infra-red wavelengths, producing the first images from an optical aperture synthesis array (Baldwin et al. 1996). The IOTA (Infrared Optical Telescope Array) operated from 1993 to 2006 and was an in- terferometer of Smithsonian Astrophysical Observa- tory on Mt. Hopkins, Arizona, US (Fig. 10), with three 45cm telescopes (enabling closure phase observations) and baselines from 5m up to 38m. Observa- tions have been carried out in the visible or near-IR (J,H,K bands). Visibilities with excellent calibration could be obtained with the single-mode fiber system FLUOR (Fiber Linked Unit for Optical Recombina- tion, Coudé Du Foresto et al. 1997) which accounts for a large fraction of the astronomical results ob- tained with IOTA. The GI2T (Grand Interféromètre à 2 Télescopes), Observatoire C ote D’Azur, Plateau de Calern, France (Mourard et al. 1994, see Fig. 11) was a successor of the I2T since 1985 and used two ”boule” telescopes with 1.5m apertures on a north- south baseline that could be reconfigured from 10m to 65m. The GI2T had the capability of performing spectrally resolved interferometry. After cloture in 2006, it’s beam combination table, including a ver- satile visible spectrograph and an IR focus has been moved to CHARA array (Mourard et al. 2009). The MIRA (Mitake Infrared Array), National Astronomical Observatory, Japan, was an ambitious plan to build a series of interferometers with in- creasing capabilities. The first phase of this project (MIRA-I), which consisted of two 25cm telescopes with coudé optics on a 4m baseline in Tokyo has suc- cessfully been completed in 1998, and has acquired stellar fringes. The next step was to built an instru- ment with a slightly larger siderostats and a 30m baseline (MIRA-I.2). The interferometer ceased op- eration in 2007. The PTI (Palomar Testbed Interferometer), NASA JPL, Mt Palomar, CA, US (Colavita et al. 1999, see Fig. 12) was built in 1995 and it consisted of three 40cm siderostats, up to 110m baselines which provided 3mas resolution in the near-infrared (H and K bands). It was developed primarily to demonstrate the utility of ground-based differential astrometry in the search for planets around nearby stars, and to develop key technologies for the Keck Interferometer and space-based missions. PTI was notable for being equipped with a ”dual-star” tracking system, the first of its kind, which simultaneously tracked interference fringes from a target star and a reference star against which the target was measured. This allowed to cancel some of the atmospheric effects of astronomical see- ing and to make very high precision measurements possible. Aside from its role for the technical develop- ment of high precision ”dual-star” astrometry, the PTI was used mainly for stellar diameter measure- ments, stellar masses and binary star work. The in- strument concluded operations in 2009. The NPOI (Navy Prototype Optical Interferometer), located on Anderson Mesa near Flagstaff, Arizona, US (see Fig. 13) operates since 1995 and combines an imaging array and an astrometric fa- cility (Armstrong et al. 1998). It can observe in 9 the visible with 32 spectral channels covering the wavelength range from 450nm to 850nm. The imag- ing subarray consists of six movable siderostats with baseline lengths from 2.0m to 437m. The array ge- ometry has been optimized for baseline bootstrap- ping to facilitate the imaging of stellar surface struc- ture. Like the Mark III, the NPOI uses vacuum delay lines for pathlength compensation. The four-element astrometric subarray of the NPOI includes an exten- sive site metrology system that monitors the motions of the siderostats with respect to one another in order to perform wide-angle astrometry with more than 2mas precision. The SUSI (Sydney University Stellar Interferometer) is an array of 13 telescopes with 14cm apertures, operating since 1991 at Sydney University Narrabri, Australia (Davis et al. 1999, see Fig. 14). The SUSI has two beam-combining systems: The original ”blue” system was designed to operate in the wavelength range 400-540 nm and employs nar- row bandwidths (typically a few nanometers). The newer ”red” system is designed to work in the range 500-950 nm. SUSI makes observations on a single baseline selected from a set of fixed north-south base- lines with lengths ranging from 5m to 640m and it can achieve angular resolutions from 20mas to 50 μ as. The 640m baseline length, the longest of all instruments currently operational or under construc- tion, has been chosen to resolve a sample of O stars at a wavelength of 450 nm. The focus of SUSI’s obser- vations is on improving our understanding of stellar astrophysics including: single stars (measuring ef- fective temperatures, radii and luminosities), binary stars, (as for single stars, plus measuring distances and masses), variable stars (e.g. Cepheids and Mi- ras), and emission line stars (e.g. Be and Wolf-Rayet stars). The ISI (Infrared Spatial Interferometer), University of California at Berkeley, US is located close to the CHARA array and the former site of the Mark III on Mt. Wilson. It started operation in 1990 as two-telescope interferometer but presently consists of three 1.65m telescopes observing in the mid-infrared. The telescopes are fully mobile and their current site on Mount Wilson allows for place- ments as far as 70m apart providing a resolution of 3mas at 11 μ m. On July 2003, the ISI recorded it’s first closure phase aperture synthesis measurements. The Fig. 15 shows three ISI’s telescopes all in a line which were used for initial testing purposes, with 4m, 8m, and 12m baselines. However, they can be moved in such a way that they form a triangle and three baselines at three different angles can be measured simultaneously providing the closure phase measurments. The interferometer operates at wavelengths between 9 μ m and 12 μ m and the stellar radiation is mixed with the output of a CO 2 laser, which acts as the local oscillator. Observations have been carried out with baseline lengths up to 56m. One of many advantages of the ISI’s narrow heterodyne detection bandwidth is that one can tune the detection wave- length to be deliberately in or out of known spectral lines, allowing for interferometry on spectral lines to be carried out. Interest of the ISI science has mainly been fo- cused on the study of evolved stars and dust shells. However, an interferometer like the ISI is well suited for making very precise measurements of positions of stars and particularly to measure positions of in- frared stars which are hardly detectable in visible light. Investigations in the field of astrometry should help to tie the astronomical reference frames together with that established in the radio, infrared and visi- ble. The CHARA array, named after Center for High Angular Resolution Astronomy, Georgia State University is located on Mt Wilson, US (see Fig. 16). It consists of 6 telescopes of 1m in diameter arranged in a Y-shaped configuration with baselines ranging from 30m to 330m. First fringes on a sin- gle baseline have been obtained in November 1999 and commissioning of the full array continues. Light from the individual telescopes is conveyed through vacuum tubes to the central Beam Synthesis Facil- ity in which the six beams can be combined together. When the paths of the individual beams are matched to an accuracy of less than one micron, after the light traverses distances of hundreds of meters, the Array then acts like a single coherent telescope for the pur- poses of achieving an exceptionally high angular res- olution. The Array is capable of resolving details as small as 200 μ as. CHARA has entered into several collabora- tions with groups offering unique instruments or technologies for enhanced performance. These in- ternational collaborations have brought a significant added value to the science capabilities of the CHARA Array. At present these collaborations include a joint observing collaboration with the Observatoire de Paris through the FLUOR instrument which has been moved from the IOTA interferometer (Section 5.4) and upgraded for the CHARA Beam Synthe- sis Facility. A collaboration with the University of Michigan has led to the development of an imaging beam combiner (MIRC) which has already produced the first images of stellar surfaces and close binary stars. A joint project with the University of Syd- ney has led to the development of a second beam combiner with significantly improved sensitivity to fainter objects while also providing measurements of very high precision. Finally, an agreement with Ob- servatoire de la C ote d’Azur has brought the third new beam combiner (VEGA) to the Array capable of providing spectroscopic and polarimetric channels for high resolution work. The first four-telescope fringes with VEGA Beam Combiner at the CHARA Array were obtained with CHARA (MIRC used as the infrared fringe ...