Radio astronomy drives technology

Astronomy fosters knowledge that places humankind in a cosmic context, inspires a technically and scientifically literate population, and furthers innovative technological developments. A new international initiative, the Square Kilometer Array¹ (SKA), is working to build the world's largest radio telescope and address some of the biggest questions in contemporary physics. The technology is still being developed, and groups around the world are investigating the various approaches to enable such an ambitious project.

We have been investigating techniques to enable fast and deep surveys of the radio sky since the early days of radio astronomy. The Australia Telescope National Facility (ATNF), part of the Commonwealth Scientific and Industrial Research Organization (CSIRO), operates several radio-astronomy facilities and is embarking upon a new telescope-development project, the Australian SKA Pathfinder (ASKAP).² ASKAP will be one of the fastest survey instruments in the world. One of the project's objectives is to develop a new radio-quiet observatory in the Western Australian outback, the Murchison Radio-astronomy Observatory (MRO), which will provide the best environment on the planet for such sensitive surveys (see Figure 1).

Figure 1. Map showing the Murchison Radio-astronomy Observatory in relation to Geraldton (315km to the southwest), Perth (600km to the south), and the Australia Telescope National Facility headquarters in Sydney (3400km to the east). (inset) A rendering of the Australian Square Kilometer Array Pathfinder dishes on site. (Courtesy: University of Western Australia.)

ASKAP will be an interferometer consisting of 36 12m antennas, each with a phased-array feed—essentially a radio camera—that can observe over a very large field of view (FoV). It consists of many receiving elements, combined with a large digital beam former. The signals from each antenna are then transported back to a central processor for correlation to produce a wide-field image. The key issue is the need to develop large numbers of inexpensive receivers that are sensitive enough for radio astronomy and then to handle the extremely large data flows from the feed to the correlator, which will eventually be made available to astronomers across the world.

The group at CSIRO, which includes staff from the ATNF and the Information and Communications Technology Center, has been awarded approximately AU$100M ($65M) to design, develop, and build ASKAP on the MRO site over the next four years. A support facility will be built in Geraldton (Western Australia), about 315km southwest of the site on the coast. MRO will also host additional instruments aimed at taking advantage of the excellent radio-quiet characteristics of the site: a joint US–Australian–Indian project—the Murchison Wide-field Array³ (MWA)—and a US project, the Precision Array Probing the Epoch of Reionization⁴ (PAPER), will be colocated along with ASKAP and other smaller experiments.

The novel phased-array feed technology is currently being trialled with an early prototype at a testbed facility at our Parkes Observatory, about 5h west of the ATNF Sydney headquarters (see Figure 2). This followed a continuing program of in-depth electromagnetic and system modeling. Initial results have been very promising,⁵ with a system temperature over efficiency (T_sys/ε) of about 180K, consistent with theoretical modeling. New experiments using the prototype supplemented with additional elements in the beam former (to increase the efficiency) are ongoing. A detailed model of the full system indicates that a T_sys/ε ratio of 55K or better should be achievable, which is comparable with the performance of the best single receivers. The advantage is the roughly 30-fold increase in the FoV afforded by this technology.

Figure 2. Prototype ‘checkerboard’ phased-array feed installed at the 12m testbed antenna at Parkes Observatory.

Teams are working on the large data-handling problem and addressing the many issues in making the wide-FoV high-dynamic-range imaging work as required for the scientific program. The antenna has a third axis to allow it to follow the plane of the celestial sky as it tracks along, an arrangement we call a sky mount. The beam former needed for the antennas must each handle about 2Tb/s, while the correlator must deal with about 20Tb/s. The output from the correlator (roughly 16Gb/s) will be transported to the support facility over a dedicated fiber-optic cable and processed to produce calibrated images. Data products will then be generated and archived.

ASKAP provides a powerful testbed for the technologies and techniques required for SKA operation, in addition to being a world-class instrument in its own right. The first antenna is expected to arrive at MRO by December 2009, and initial operations will commence by the end of 2012. MRO is Australia's candidate site for the core of SKA, one of two short-listed sites (along with a South African location). SKA is projected to begin construction by the middle of the next decade. A large international consortium is working on its technology development, funding, and policies.