Issues Magazine

The Kingdom of Quietness

By Helen Sim

In their quest to hear the faintest radio whispers from the universe, radio astronomers have come to Boolardy, one of the most “radio-quiet” places on Earth.

Under the blue bowl of the sky, the flat red land, dotted with mulga scrub, stretches out in grand silence. Over a couple of kilometres, smooth white antennae sprout upwards in an apparently haphazard pattern.

This is Boolardy Station, a 350,000-hectare grazing property run by Mark and Carolyn Halleen and their children. Its permanent residents are the Halleen family, 1800 cattle, emus, kangaroos and some sizeable “bangaras” (as the locals call these lizards). Until recently it was also home to 70-odd technicians and engineers who were working to build one of the world’s great radio telescopes.

Boolardy is 350 km north-east of Geraldton, a town on the mid-west coast of Western Australia. Geraldton is 420 km from Perth, itself the world’s most isolated major city. Murchison Shire, in which Boolardy Station is located, is 50,000 km2 in area – larger than The Netherlands – and has a population of less than 160. It has no town.

For most people, “getting away from it all” means going on holidays. But the visitors to the new Murchison Radio-astronomy Observatory (MRO) at Boolardy have come here to work.

At nightfall the blank blue wall of the sky dissolves, letting through the glittering glory of the southern sky. There are no city lights to hide the faintest stars. And if you could put on “radio eyes” to see the radio waves emitted by the stars and galaxies, the sky would look equally uncluttered. In radio terms, this is the “kingdom of hush”, for with few people there are few sources of man-made radio waves to drown out what the sky is saying. “It’s one of the best places on the planet for radio astronomy,” said ASKAP Director Ant Schinckel of CSIRO, the body that has built the white antennae on the red land.

The site is so special that it was chosen as Australia’s candidate site for the Square Kilometre Array (SKA), a €1.5 billion international telescope. The SKA will comprise thousands of small antennae, with a total collecting area of 1 km2.

Australia–New Zealand and a consortium of countries in southern Africa were vying to host this telescope; in May 2012 the international SKA Organisation announced that the hosting would, in fact, be shared between the two regions.

The antennae towering over the scrub here are not yet those of the SKA, but those of a precursor telescope – the Australian SKA Pathfinder (ASKAP), an A$160 million project. “We are building ASKAP to do great science in its own right, but also to show what an excellent site this would be for the SKA,” said Schinckel. “Plus, we are prototyping some of the kinds of technologies the SKA will use.”

The need for an Australian SKA pathfinder was recognised in 2003: after careful preparation, ASKAP construction began in 2009. “ASKAP is going to be producing world-class science for years to come,” Schinckel said. “It’s a big step forward for radio astronomy.”

Negotiations around the establishment of the MRO had to take account of Native Title – the rights that the Wajarri Yamatji, the traditional owners of the MRO site, have over the land. In 2009 the Yamatji Marlpa Aboriginal Corporation , which is the legal representative group for the Wajarri Yamatji claimants, along with CSIRO and the Australian and Western Australian governments finalised an Indigenous Land Use Agreement (ILUA) that gave consent for the early radio astronomy activities on the MRO, including ASKAP. The ILUA is providing employment, educational and mentoring opportunities for the Wajarri Yamatji people, as well as financial support for the community.

At first cautious about ASKAP, the Wajarri Yamatji have now embraced it. In June 2011 a naming ceremony was held to bestow Wajarri names, such as Wilara (the Moon) and Jirdilungu (the Milky Way), on the first tranche of ASKAP antennae; the remainder received their new names at the telescope’s formal opening in October 2012. Roads and other structures have been named honouring Wajarri Elders, as well as features of the natural world. Even Ant Schinckel has been given a Wajarri name: “Minga”, which means “ant”.

ASKAP is the most recent radio telescope to be built by CSIRO, which has been involved with the field since the pioneering days of the 1940s. CSIRO radio telescopes redefined the centre of the Milky Way, discovered cosmic Faraday rotation – how magnetic fields in space rotate the polarisation angle of radio waves – and helped reveal the nature of quasars. CSIRO’s Parkes telescope was also the conduit for TV signals for the first manned Moon landing in 1969. Today CSIRO operates three observatories in eastern Australia; ASKAP is its first telescope in the west, more than 3000 km distant.

Like those other CSIRO facilities, ASKAP will be used by astronomers from around the world. The telescope’s first five years of operation will be largely taken up by ten “key science projects”. These were chosen from a large field of international contenders, and when first announced, in September 2009, they involved 363 investigators from 131 institutions. Of the ten projects’ authors, 33% were from Australia and New Zealand, 30% from North America, 28% from Europe and 9% from elsewhere. These projects will deal mainly with the formation and evolution of galaxies; magnetic fields in galaxies, including our own; understanding the interstellar medium (the space between the stars); and observing transient radio phenomena (including new kinds, it is expected).

The projects make use of ASKAP’s key characteristics – great sensitivity to faint signals, combined with a uniquely wide field of view, which will allow it to survey the whole sky rapidly – and of course the particular characteristics of radio waves, such as their ability to pass through cosmic dust, and the way they can reveal the presence of magnetic fields.

The number of objects these surveys expect to find is mind-boggling. For instance, one of these projects is EMU (Evolutionary Map of the Universe), a deep, radio-continuum survey over 75% of the sky. The largest survey of this kind done to date is the NRAO VLA Sky Survey (NVSS), which was made with the Very Large Array telescope of the US National Radio Astronomy Observatory. NVSS found two million galaxies, but EMU is projected to find 70 million.

ASKAP science is not limited to these projects, however. In fact, astronomers carried out the first experiment with ASKAP in May 2010, when only a couple of antennae were in place; this involved using an ASKAP antenna simultaneously with radio telescopes in eastern Australia and New Zealand, and processing their data together, effectively making them into one giant telescope and producing a high-resolution image of the core of a radio galaxy called Centaurus A. This is comparable to photographing the head of a pin from a distance of 20 km.

For that first experiment, the ASKAP data was recorded on disk and driven to Perth for processing. But ASKAP is now linked to the rest of the world by 380 km of optical fibre that runs to Geraldton on the coast, completing a 5000-km fibre network across Australia and New Zealand. Optical fibre links not only the site to the rest of the world, but the antennae to each other and to the specialised on-site computer, the correlator, which will combine the data from all the antennae.

ASKAP is a radio interferometer: its 36 antennae will work together as one instrument to deliver both detailed images of the sky made from radio waves and the waves’ spectra, which reveal the physical movements, and the chemical make-up of the objects under scrutiny.

ASKAP’s antennae are 12-metre diameter dishes with a pointing accuracy of 30 arcseconds. The antenna surface must be capable of operating at up to 10 GHz, which means its surface has to deviate from an ideal paraboloid by less than 1 mm (rms); the installed antennae have achieved this figure with ease.

A somewhat uncommon feature of the antenna is that it has a third axis of rotation, which runs from the centre of the receiver down through the centre of the dish. When a radio dish tracks an object across the sky, the parts of the telescope that receive the radio waves usually rotate with respect to the sky. With the way ASKAP makes images, complicated processing would be required to compensate for this motion, so it’s being cancelled out by using this extra axis of rotation. Familiarity with this feature, and an overall antenna design well matched to ASKAP’s needs, helped Chinese firm CETC 54 (the 54th Research Institute of China Electronics Technology Group Corporation) land the contract to build and install the antennae.

ASKAP’s most unusual technology, and the key to its speed as a survey instrument, is a new kind of phased-array feed – in effect, a radio camera. This sits in the antenna’s focal plane. Traditionally, feeds for radio astronomy were single metal horns, collecting radio waves from the whole surface of the dish but creating just one “point”, or pixel, of sky data. Developments in the 1990s, including leading work by CSIRO, produced feeds of up to 13 pixels, but the new CSIRO phased-array feed is a big advance on that. It’s a circuit board with a chequered pattern of flat metal squares – a ten-by-ten array of elements,

The new technology dramatically increases the field of view of a radio telescope – how much sky it can see in a single “look” – giving ASKAP a sizeable field of view, 30 square degrees – 150 times the area on sky of the full Moon. “This is 500 times more than our 64-metre Parkes telescope sees when operating at the same frequency,” said Dr Simon Johnston, CSIRO’s Head of Astrophysics.

Thanks to the phased-array feeds, each ASKAP antenna will pour forth more than 1.9 Tb of data per second, for a total of nearly 72 Tb/sec entering the correlator room of 7764 fibres. In its first six hours of operation, the full 36-antenna ASKAP will generate more information than is currently held in the whole world’s radio astronomy archives.

Moving, processing and storing all this data is one of the project’s greatest challenges, soaking up more than one-third of the project’s budget. “ASKAP differs from a conventional telescope in the final product it provides the users,” said Johnston. “Conventionally, you get raw or calibrated data out of a radio telescope, and you have to make the image yourself. With ASKAP, we can’t afford to store and handle all that data, so it’ll be processed into images immediately, and that’s what the user will get. This is a paradigm shift.”

When the full ASKAP array is churning out 72 Tb/sec, a day’s observing will generate the data equivalent of 124 million Blu-ray disks – a pile 62 km tall. Initial processing by the on-site correlator (a dedicated custom-designed and built computing system) will distil this data down to 5 Tb, enough to make an image of 30 square degrees of sky, an image that may include up to 200,000 galaxies. A thousand such images will be needed to map the entire southern sky.

In 2011 the ASKAP team began using a 100 Tflop HP machine for early ASKAP science imaging tests. This supercomputer is housed at the iVEC Pawsey Centre in Perth, Western Australia, and the petascale supercomputer was commissioned in 2013. These facilities will support ASKAP and a second radio telescope, the Murchison Widefield Array, plus other data-intensive research in nanotechnology, biotechnology and geosciences.

All 36 ASKAP antennae are now standing tall at the MRO; both the telescope and the observatory were formally opened at a ceremony in October. The precious radio-quietness of the site has been protected by the establishment by the Australian government of a Radio Quiet Zone around the observatory (with support from the Western Australian government): this is one of the few such protected spaces in the world.

“All the infrastructure is in place,” said Schinckel, “and we are now rolling out the sensitive receivers and data processors as we commission this unique new tool”. Science operations will start in 2015.

URL: http://www.csiro.au/en/Outcomes/Understanding-the-Universe/Operating-our...

First published (in French) in Le Recherche.