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On the Way to the SKA

It may look like just dots on a page, but an image of distant galaxies represents a huge step forward for CSIRO’s Australia SKA Pathfinder (ASKAP) radio telescope in Western Australia.

ASKAP is developing and proving technologies for the international Square Kilometre Array (SKA) telescope, which will start construction in Australia and South Africa in 2018. The image shows that ASKAP is now working as a fully fledged radio telescope after just a few months since six of the telescope’s 36 antennas were commissioned.

The image of a region of sky near the south celestial pole is the equivalent of a black and white photo, but made from radio waves. The image covers 10 square degrees on the sky – 50 times larger than the full Moon – and was made from nine overlapping regions captured simultaneously.

The quality of the image vindicates ASKAP’s phased array feeds, which allow the telescope to see large areas of sky at once. “This image shows that the phased array feeds are stable over the 12 hours it takes to make an observation like this,” said Dr David McConnell, who leads the ASKAP Commissioning and Early Science team.

As the telescope tracks radio sources, the phased array feed is kept in a fixed orientation to the sky, thanks to a special axis of rotation built into each ASKAP antenna. “With a conventional telescope we would have expected artefacts from bright sources at the edges of each beam,” McConnell said. “With ASKAP we don’t get that because the phased array feed is held at a constant angle to the stars.”

ASKAP was able to make the new image twice as fast as any comparable telescope in the Southern Hemisphere. When completed, ASKAP will be able to survey the sky 25 times faster still, and will be the world’s premier survey telescope for centimetre-wavelength radio astronomy.

ASKAP has also made a “snapshot” of a single galaxy, NGC 253, from radio waves emitted by neutral atomic hydrogen gas (HI), the fuel for making stars. This is the telescope’s first image of the HI in a galaxy.

The image captures both how much HI is present in each region and which parts of the rotating galaxy are approaching us and which are receding. “If the first image was like a black-and-white photo, this one could be compared to a colour photo,” McConnell said. “The image compares very well with one made by our established Compact Array telescope.”