The world's largest radiotelescope, the Square Kilometre Array (SKA), will be built in two places at once, planners announced today. SKA's partners decided to capitalize on investments already made at the two candidate sites—in Australia/New Zealand and South Africa—by having each site focus on different parts of the electromagnetic spectrum.
The Australia/New Zealand array, with its focus in Murchison, Western Australia, will concentrate on low frequencies, below 500 megahertz (MHz). The South African array, centered in the Karoo desert in the Northern Cape, will observe in the middle and high frequencies above 500 MHz.
"The members agreed this was the best way forward," says Michiel van Haarlem, interim director general of the SKA Organization. "It's a scheme we've only really developed over the last month or so. It will build on developments already ongoing in South Africa and Australia."
Others welcomed the decision. "It keeps everyone on board and you don't fragment the community," says Albert Zijlstra, director of the Jodrell Bank Centre for Astrophysics in the United Kingdom. The two-site plan will modestly increase the cost of the €1.5 billion project, he notes, but says that is "a price worth paying."
The idea of SKA is to create a radiotelescope with a collecting area of 1 square kilometer. It will involve thousands of dishes and other antennas spread over an area thousands of kilometers across. South Africa has teamed with eight other countries in Africa and the Indian Ocean to get the required reach. Both of the bidders are already building so-called precursor arrays to demonstrate their capabilities and these will now be incorporated into the two SKA arrays.
South Africa's MeerKAT array will eventually comprise 64 dish antennas. Phase I of SKA, which is slated to begin in 2016, will add another 190 dishes, ultimately creating a 254-strong array that will focus on high-sensitivity deep observations. Australia's SKA Pathfinder will have 36 dishes; an additional 60 will be added in phase I. The Australian team has pioneered the use of a new kind of detector on its dishes called a phased-array feed, which is specially suited to carrying out high-speed surveys, so that will be the focus of the Australian dishes.
Dish antennas are best at high frequencies. For middle and low frequencies, however, SKA will use simple static antennas called aperture arrays. The middle-frequency-aperture arrays will be built in South Africa, and the low-frequency arrays will be sited in Murchison. A prototype called the Murchison Widefield Array is already under construction at the Australian site.
Van Haarlem says that the two arrays will be managed as one instrument, although they will be able to act independently and make separate observations. He says that SKA's member countries (Canada, China, Italy, the Netherlands, the United Kingdom, South Africa, Australia, and New Zealand) will spend the next 6 months working out how to incorporate the precursor arrays into SKA and fine-tuning the cost implications of having two sites. "The member states will have had to bite the bullet on the extra cost," says Zijlstra. But after a tense few months coming to decision, "everyone is just relieved," he says.