CSIRO Digital Systems Engineer John O'Sullivan and a prototype phased array feed being developed for ASKAP.
Credit: Chris Walsh, Patrick Jones Photographic Studio
This image shows a 'chequerboard' array with receivers. By combining the signals from a number of receivers (those encompassed by the yellow circle) individual beams may be formed. The two beams formed using the sets of receivers will be separated on the sky.
Credit: Russell Gough
In this image, six overlapping sets of receivers are indicated. The six beams formed using these receivers will be adjacent on the sky and there will be no gaps between them. This has been achieved by using the signals from some of the receivers to help form a number of different beams.
Credit: Russell Gough
SYDNEY: An international team of researchers are introducing phased-array feed receivers to radio telescopes for the first time, enabling large areas of sky to be surveyed with unprecedented sensitivity and speed.
Unlike the single-pixel feeds used in current radio telescopes, the instrumentation - commonly known as PAFs - have many separate, simultaneous beams to detect cosmic radio waves, which will allow astronomers to catch sight of transient objects such as supernovae and X-ray binary star systems more efficiently than ever before.
“The difference between single-pixel astronomy and a 100-pixel panorama is a matter of observing time – an astronomer can observe with a 100-pixel panorama in one day what would take him three to four months with a single-pixel receiver,” said CSIRO principal research engineer Russell Gough, leader of analog systems for ASKAP, the Australian Square Kilometre Array Pathfinder project.
Viewing extended regions of space
It takes single- or dual-pixel receivers in current radio telescope dish antennas a long time to map extended regions of space, said Gough, comparing the technology to “a camera with just one pixel”.
Plus, objects like comets, molecular clouds and nearby galaxies are large in relation to the telescopes's resolutions, making them very difficult to observe.
In 2008, astronomers used the Green Bank radio telescope - the world's largest fully steerable land-based movable structure, based in West Virginia, USA - to carry out an extremely detailed study of hydrogen in Smith's Cloud, a large gas cloud in the constellation Aquila that, for some unexplained reason, moves faster than the rotation of the Milky Way.
Nearly 40,000 individual pointings of the giant telescope were required to observe this high velocity cloud of hydrogen gas - 30 times the width of the full Moon - with enough sensitivity.
No gaps astronomical images
“Some of the signals of most interest to astronomers are very, very weak, and astronomers may spend a lot of telescope time to detect and map them,” said Gough.
Australian researchers built a 13-beam receiver for the 64-metre Parkes radio telescope in rural New South Wales. This array of receivers allow astronomers to work 13 times faster than those working with single-pixel receivers.
There are problems, however, said Gough: "There is a limit to how large we can make the receiver and we can’t put the receivers as close together as we would like, so there are gaps between the beams (or radio-camera pixels)," he said.
And this is where the PAFs come in – “This technology allows us to make astronomical images with no gaps between the pixels,” he said.
