Astronomers are celebrating the formal acceptance of the first North American antenna by the Joint ALMA Observatory. The Atacama Large Millimeter/submillimeter Array (ALMA) is a gathering armada of shortwavelength radio telescopes. Their combined power will enable astronomers to probe phenomena and regions that are beyond the reach of visible-light telescopes with unprecedented sharpness. The observatory is being assembled high in the Chilean Andes by a global partnership (www.almaobservatory.org).

ALMA will operate at wavelengths of 0.3 to 9.6 mm. At these wavelengths, a high, dry site is needed for the telescope to be able to see through the Earth's atmosphere. ALMA is thus being built on the 5000-m-high plateau of Chajnantor in the Atacama region of Chile. ALMA will offer unprecedented sensitivity and resolution, as the 12-m antennas will have reconfigurable baselines ranging from 15 m to 16 km. Resolutions as fine as 0.005 arcseconds will be achieved at the shortest wavelengthsa factor of 10 better than the Hubble Space Telescope.

The 12-m-diameter antenna is the first of 25 being provided by North America's ALMA partners. Their efforts are led by the National Radio Astronomy Observatory or NRAO and supported by the US National Science Foundation or NSF in cooperation with the National Research Council of Canada and the National Science Council of Taiwan. The antenna was manufactured by General Dynamics SATCOM Technologies.

The acceptance comes just weeks after the first ALMA antenna was handed over to the observatory. That antenna was produced under the direction of the National Astronomical Observatory of Japan on behalf of ALMA's East Asian partners.

A single 12-m antenna's dish is bigger than the largest optical telescope's reflective mirror. To match the sharpness achieved by an optical telescope, however, a millimeter-wavelength dish would have to be miles across. ALMA will combine signals from dozens of antennas spread across miles of desert to synthesize the effective sharpness of such a single, gigantic antenna. The process, called interferometry, involves an analysis of the ways in which the signals coming from each antenna interfere constructively and destructively with one another.

When completed early next decade, ALMA will have a total of 66 antennas provided by partners in North America, Europe, and East Asia. The first European antennas, which are being produced under the auspices of the European Organization for Astronomical Research in the Southern Hemisphere (ESO), are scheduled to begin arriving at the observatory early this year.

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