Sky and Telescope - June 2017 - 29

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 just contain about 130,000
simple dipole antennas, grouped in stations of 256 each. The
relatively cheap, mass-produced antennas will look a bit like
small, slender Christmas trees and stand almost two meters
tall. A few years from now, the Australian outback will be
decorated with some 500 patches of metal forest.

SKA1-LOW

PHOTO: GOV ERT SCHILLING; INFOG R A PHIC: SK A ORG A NISATION

Spiders and Christmas Trees
The technique of using huge numbers of dipole antennas 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 meters across and containing dozens of dipole antennas.
Radio waves from a source at the zenith, right above your
head, will arrive at all the antennas 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 antennas 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 in 2016. Because of the limited capacity of optical ﬁbers, 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 propeller 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 young
kangaroo hops across the bumpy gravel road. In the harsh
sunlight, the white dishes of ASKAP are almost too bright to
look at. Technicians are using small mobile cranes to install
new phased array receivers.

Frequency range:
Location: Australia

~130,000

50 350

antennas spread
between 500 stations

MHz to MHz

Total
collecting
area:

0.4 km 2

Maximum distance
between stations:

65 km
Total raw data output:

157 terabytes
per second

4.9 zettabytes
per year

SKA1-LOW

5x
35,000 DVDs

the estimated
global internet
trafﬁc in 2016

every second

(source: Cisco)

Enough to ﬁll up

Compared to LOFAR Netherlands, the current
best similar instrument in the world:

25%

8x 135x

better
resolution

the survey
more
speed
sensitive

s k y a n d t e l e s c o p e .c o m

* JUNE 2017

http://www.skyandtelescope.com

Sky and Telescope - June 2017

Table of Contents for the Digital Edition of Sky and Telescope - June 2017