Australian Sky & Telescope — July 2017

http://www.skyandtelescope.com.au 19

PHOTO: GOVERT SCHILLING; INFOGRAPHIC: SKA ORGANISATION

In May 2012, after a lot of political tugs of war, it was

finally decided to build the Square Kilometre Array on both

continents. “That wasn’t necessarily the cheapest option,”

admits SKA director Diamond, “but it was a nice way to

exploit the best qualities of both sites.”

The original goal was to expand ASKAP to a 96-dish array.

With its huge field of view, it would have become the survey

part of the Square Kilometre Array — a full-fledged third

observatory next to SKA-mid and SKA-low. But two years

ago that plan was scrubbed — at least for the foreseeable

future — because of budgetary issues. Instead, the first phase

of the Australian part of the SKA will contain about 130,

simple dipole antennae, grouped in stations of 256 each. The

relatively cheap, mass-produced antennae will look a bit like

small, slender Christmas trees and stand almost two metres

tall. A few years from now, the WA desert will be decorated

with some 500 patches of metal forest.

Spiders and Christmas trees

The technique of using huge numbers of dipole antennae to

create maps of the low-frequency radio sky has been pioneered

over the past decade by the Low Frequency Array (LOFAR) in

the Netherlands, which was inaugurated in the summer of

2010. The concept is pretty straightforward. LOFAR consists
      of some 50 individual ‘stations,’ each about 100 metres across
      and containing dozens of dipole antennae. Radio waves from a
      source at the zenith, right above your head, will arrive at all the
      antennae in a particular station at exactly the same time. But
      for every other position in the sky, there will be tiny differences
      in the wavefront’s arrival time. By combining the signals
      from individual antennae according to the specific pattern of
      time-of-arrival differences that corresponds to a particular sky
      position, a LOFAR-like array can be virtually ‘pointed’ in any
      direction — a process called beam forming.
      In fact, smart computer processing allows LOFAR’s dipoles
      to ‘look’ in eight different directions at once. To further
      enhance resolution, astronomers then combine observations
      from the individual stations interferometrically.
      The required computer processing power for this technique
      is huge: SKA1-low will produce an incredible 157 terabytes
      of raw data per second, or five times the estimated global
      internet traffic for all of 2016. Because of the limited capacity
      of optical fibres, part of the data reduction will be done by
      smaller processors at the actual antenna stations.
      The Murchison Radio-astronomy Observatory feels even
      more remote than MeerKAT in South Africa: Murchison
      Shire spreads about 110 inhabitants across an area larger
      than the Netherlands. A small aircraft takes me from
      Perth to Boolardy Station, a former sheep farm east of
      the settlement. It’s a spectacular 90-minute flight over an
      orange-red semi-desert covered with low, shrubby vegetation.
      From Boolardy, it’s another 30 minutes by 4WD truck to the
      actual observatory. Eagles soar high in the sky; a kangaroo
      hops across the bumpy gravel road. In the harsh sunlight,

Location: Australia

Frequency range:

~130,

antennae spread

between 500 stations

Total

collecting

area:

0.4 km^2 Maximum distance

between stations:

65 km

Total raw data output:

157 terabytes

per second

4.9 zettabytes

SKA1-LOW per year

Enough to fill up

35,000 DVDs

every second

the estimated

global internet

traffic in 2016

(source: Cisco)

Compared to LOFAR Netherlands, the current

best similar instrument in the world:

better

resolution

more

sensitive

the survey

speed

MHz to MHz

50 350

8x 135x

5x

25%

SKA1-LOW