IRAM is an international institute for research in millimeter astronomy founded in 1979 by the Centre National de la Recherche Scientifique (CNRS), France, and the Max Planck Gesellschaft (MPG), Germany. The organization was joined by the Instituto Geografico Nacional (IGN), Spain, in September 1990. IRAM operates two major facilities: a 30 m radio telescope on Pico Veleta in the Sierra Nevada (southern Spain), and an array of six 15m radio telescopes on the Plateau de Bure in the French Alps. IRAM telescopes observe in microwave "windows" at 0.85 mm, 1.3 mm, 2 mm, and 3 mm.
The Pico Veleta observatory is located at an altitude of 2,920 m on the second highest mountain of the Iberian peninsula. Its high altitude, southern location, and dry climate are extremely favorable for millimetric observations due to the low column abundances of water vapor which, on cold, dry winter days, can drop to as low as 1 precipitable-mm. The 30 m Cassegrain telescope at Pico Veleta is currently the world's largest telescope operating at wavelengths between 0.8 mm and 3.5 mm (350 to 80 GHz), with a collection area of 700 m2, and received its first millimetric "light" in May 1984. The angular resolution of the Pico Veleta observatory is purely diffraction-limited and thus depends inversely on wavelength, with an angular resolution of 10" at a wavelength of 1.3 mm. Radio telescopes have historically operated with a single receiver (or pixel), and thus an image was built up by scanning the entire telescope across the sky. More recently, multiple detectors are located in imaging arrays at the focal plane of these telescopes to provide instant imaging of objects.
The Plateau de Bure observatory is located at an altitude of 2,552m in the French Alps. Atmospheric conditions are good for millimetric observations and the column amount of water vapor can drop below 2 precipitable-mm on dry winter days. Work on the construction of the site began in 1985, and the first interferometric fringes were obtained in 1988. The site consists of six 15 m antennas operating between 2.6 mm and 3.7 mm (81 to 115 GHz) and also between 1.2mm and 1.4mm (205 to 245 GHz), which can move on rail tracks up to a maximum separation of 408 m in the east-west direction and 232 m in the north-south direction (Figure 7.19). Each dish has a collecting area of 175 m2 and thus the combined collecting area of the six telescopes is 1,050 m2, which make this one of the most sensitive, and highly resolving interferometers in the world. As mentioned earlier, for an interferometer the field of view is defined by the ratio of wavelength to diameter for each antenna, while the resolution is determined by the ratio of the wavelength to the maximum separation between antennas. Hence, for example, at 1.3 mm the field of view of the interferometer is 20" while the best angular resolution is 0.5".
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