Two prototype antennas for the world鈥檚 largest array of millimetre-wave telescopes have passed a key test, working together to track and image Saturn for more than an hour on 2 March. Ultimately, ALMA (Atacama Large Millimeter/submillimeter Array) is expected to resolve details 10 times finer than the Hubble Space Telescope when it is completed in 2012.
ALMA will use up to 64 antennas and will be located in the Atacama desert, 5 kilometres above sea level in the Chilean Andes. Designed to look through dust clouds to reveal star formation, image embryonic planets and probe the early universe, it will be the world鈥檚 most sensitive telescope at wavelengths of 0.3 to 9.6 millimetres 鈥� a regime obscured at lower altitudes by atmospheric moisture.
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But making all of the 12-metre dishes function like a single giant telescope will be a challenge. Fibre-optic cables will link each dish to every other dish in the array, and to a giant special-purpose electronic computer called a correlator.
鈥淚t collects the amplitude and phase information from each of the antennas, and knowing their distances from each other, it lines everything up to produce a coherent picture of the source,鈥� says Jeff Mangum, an ALMA project scientist at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, US.
鈥楩ringes of the universe鈥�
The 2 March test actually linked two prototype antennas at another observatory, the Very Large Array site in New Mexico, US, with each other and with a small-scale prototype of the correlator. But the test, called 鈥榝irst fringes鈥�, bodes well for ALMA鈥檚 future, Mangum told New 杏吧原创: 鈥淚t verifies that ALMA can make measurements not just as single telescopes, but as a collection of antennas, which is the primary mode of operation.鈥�
Millimetre waves are at the upper end of the radio spectrum, just below infrared radiation. They can reveal important organic molecules, but are obscured by atmospheric moisture.
Small arrays at lower elevations have probed the sky since the 1980s, but atmospheric moisture made observations difficult at wavelengths shorter than 3 millimetres. A 1990 report urged building a large array at high elevation, and the NRAO, the European Southern Observatory, and the National Astronomical Observatory of Japan later picked the Atacama site. The altitude puts the array above most of our atmosphere鈥檚 water, allowing observations down to 0.3 millimetres.
Although plans call for up to 64 antennas, present funding can pay for only 50 or so dishes. The dishes will be movable, allowing baselines from 150 metres to 18 kilometres, with the longest baseline and the shortest wavelength giving resolution as fine as 0.005 arcsecond, a factor of 10 better than Hubble.
Because millimeter waves can penetrate dust that blocks visible light, ALMA鈥檚 large collecting area 鈥渨ill make it much easier to detect very faint objects at the fringes of the universe鈥�, says Mangum.
