The ALMA project is a joint European/American/Canadian/Japanese effort to build a huge millimeter array with a collecting area of approximately 7,000 m2. This will be roughly ten times the collecting area of the largest currently operating millimeter array in the world, the Institut de RadioAstronomie Millimetrique (IRAM) array of six 15m dishes at the Plateau de Bure Observatory in France.
ALMA will be able to image in all atmospheric windows between 0.3 mm and 9.6 mm (31-950 GHz) and will have a very large baseline extending up to
15km-18km, providing an angular resolution of 0.005 arcsec at the highest frequencies observed. To achieve these requirements ALMA will consist of an array of fifty 12 m dishes, which can be moved to form different array configurations (e.g., Schilling, 1999c). In addition, there will also be a more compact array (the Atacama Compact Array or ACA) consisting of twelve 7m dishes and four 12m dishes, which will be used in fixed configuration to image large-scale structures that are not well sampled by the main ALMA array.
To capitalize fully on such a massive investment, ALMA needs to be placed at as high an altitude as possible and in one of the driest regions of the world in order to minimize obscuration by terrestrial water vapor. The proposed site for the instrument is thus in Cerro Chajnantor, Chile which is a vast plain at an altitude of 5,000 m in the Chilean Andes, perfectly suited for the purpose. Unlike most interferometers, the dishes of the interferometer will not be limited to a fixed set of configurations, but can be moved to a continuous range of baselines between 200 m and 18 km. While the 18 km baseline gives the best angular resolution, smaller baselines will be better suited for the detection of weak signals. The Atacama Compact Array (ACA) will have essentially one configuration. Because the site is so high, altitude sickness is likely to be a real problem and thus there will probably be no scientists at the site. Instead, day-to-day maintenance will be conducted by local Chilean workers, acclimatized to the altitude.
Construction of ALMA at the observatory site started in 2005. When it is completed, hopefully by 2012, ALMA will provide unprecedented access to the millimeter spectra of astronomical bodies and in particular it will investigate high-redshifted galaxies and planetary formation in circumstellar disks. Its high sensitivity and angular resolution will also allow it to investigate, as never before, the microwave spectra of the giant planets, allowing it to probe the temperature and composition of these planets well below the visible cloud decks. It should also be able to image extrasolar planets directly, allowing for investigation of their composition and temperature (Schilling, 1999a).
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