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See Inside September 2008

Digital Upgrades for a Radio Astronomy Revolution

Using more sophisticated computers and electronics will vastly increase the resolution, sensitivity and data capacity of the Very Large Array telescope



Dave Finley NRAO/AUI

Bell Telephone Laboratories engineer Karl Guthe Jansky was only looking for ways to cut down on shortwave radio static when he found radio waves coming from outer space in 1932. Yet Jansky’s serendipitous discovery soon gave birth to radio astronomy, which has since delivered paradigm-shifting revelations ranging from the cosmic microwave background to the presence of dark matter in the universe. That science is now on the verge of a 21st-century renaissance that promises even greater discoveries, ushered in not by traditional huge radio dishes but by vast, powerful arrays of smaller dishes.

First developed by British radio astronomers in 1946, arrays make use of several radio telescopes spaced some distance apart, “synthesizing” a single telescope with an aperture equal to the spacing between the farthest elements. The most famous example, operating since 1980, is the Very Large Array (VLA) near Socorro, N.M., which has 27 active radio antennas mounted on railroad tracks in a Y configuration (another dish is kept as a spare). The instrument’s angular resolution is adjusted simply by moving the antennas closer together or farther apart. “The VLA has been and still remains the most powerful and flexible radio synthesis imaging telescope on earth,” says veteran VLA researcher Rick Perley. “But since that time there’s been enormous changes both in technology and in where science is headed.”

In particular, the VLA is going digital as the EVLA, the Expanded Very Large Array, using more sophisticated computers and electronics that will vastly increase the resolution, sensitivity and data capacity of the facility. The heart of the EVLA, as with any array, is the correlator, the supercomputer system that processes, compares and combines the signals from the antennas. “You just don’t go to Radio­Shack and buy a bunch of PCs and configure them for this kind of thing,” explains EVLA project manager Mark Mc­Kin­non of the correlator, designed and built by a team from the National Research Council of Canada Herzberg Institute of Astrophysics in British Columbia. It will handle up to 80 times the bandwidth of the old VLA correlator and crunch many more data channels simultaneously.

Engineers also upgraded the path by which signals get from the antenna dishes to the correlator, using all-digital fiber optics that replace the old analog waveguides. The dishes are getting new, exquisitely sensitive digital receivers, providing continuous band coverage from one to 50 gigahertz. All these upgrades will pump up the VLA’s capabilities at least 10-fold, making it able, in principle, to detect a signal as weak as a cell phone call from Jupiter.

With $100 million from the National Science Foundation and the VLA’s Canadian and Mexican partners, researchers have finished installing the digital data lines and upgrading, by this past May, 16 of the 28 antennas; by early 2010 the new correlator should be up and running. “We are on budget and on schedule, and there aren’t many astronomy projects that can make that claim,” McKinnon boasts. “For the most part, we’re going to have this thing wrapped up in 2012.”

Meanwhile the next generation of radio-astronomy observatories is taking shape. The Atacama Large Millimeter/submillimeter Array (ALMA) is under construction on an Andean plain in northern Chile’s Atacama Desert. The high-altitude locale 5,000 meters above sea level will enable the ALMA’s 12-meter-wide dishes, at least 50 of them, to probe the shorter radio wavelengths near the infrared that the atmosphere tends to filter out. Two enormous, custom-built, 28-wheel heavy transporter vehicles will be used to move the antennas to give the array some reconfigurability. Barring cost concerns (already approaching $1 billion), technical problems and political exigencies, the ALMA should be ready around 2012.

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