Australian Sky & Telescope — July 2017

16 AUSTRALIAN SKY & TELESCOPE July 2017

DIAGRAM: LEAH TISCIONE / S&T; FIRST LIGHT: SKA SOUTH AFRICA (2)

Dish A

Constructive interference Destructive interference

Correlator looks for interference between signals

+ +

==

Dish B

Angle to

source

Radio

waves from

source

Distance between dishes

Longer path length to Dish A

SHOW INTERFEROMETRY WORKSWhen two radio antennae

observe a source that’s not at the zenith, the signal travels slightly

different distances to reach each of them (here, the path to Dish A is

longer than to Dish B). A supercomputer correlator compares the signals

and determines how much of a shift is necessary to make the signals

constructively interfere. That shift corresponds to the time delay, and thus

the path length difference and angle to the source. Combining this data

from many pairs of antennae improves the position’s accuracy.

WFIRST LIGHT Each

bright dot in the image

at far left represents a

distant galaxy, detected

with the first 16 dishes

of the MeerKAT array

in 2016. The close-up

(near left) zooms in on

a few of these galaxies,

revealing radio-bright

outflows powered by the

galaxies’ supermassive

black holes.

detailed ‘images’ of the radio sky. These are the techniques

behind famous facilities like the Australia Telescope

Compact Array in New South Wales, the Karl G. Jansky Very

Large Array (VLA) in New Mexico and the Atacama Large

Millimetre/submillimetre Array (ALMA) in northern Chile.

As most amateur astronomers know, a telescope’s

aperture determines both the instrument’s light-gathering

power (‘sensitivity’) and its angular resolution. But angular

resolution also depends on wavelength. While a 100-mm

optical telescope provides a resolution of approximately

one arcsecond, you would need a 50-kilometre dish to ‘see’

the same amount of detail at a radio wavelength of 21

centimetres, equivalent to a frequency of 1420 megahertz.

However, an array of smaller dishes spread out over an

area 50 kilometres across works almost as well. By combining

the signals detected by the individual antennae (the process

called radio interferometry), you can synthesise a virtual

telescope (aperture synthesis) that’s as large as the distance

between the dishes, albeit with a much lower sensitivity than

a single huge instrument would have.

MeerKAT, with its 13.5-metre dishes, uses this same

technique. More than five years ago, South African radio

astronomers and engineers completed a test facility, known

as KAT-7 (Karoo Array Telescope, with seven 12-metre

dishes). The main goal was to demonstrate the country’s

ability to design, build and operate such high-tech

instruments, in support of South Africa’s bid to eventually

host the Square Kilometre Array. MeerKAT is now one of

the four official SKA precursor telescopes. The observatory’s

first-light image, obtained with only 16 operational dishes,

was released in July 2016. It shows more than 1,300 remote

galaxies — most of them never observed before — in an area

of sky measuring approximately 2° on a side.

The preliminary results bode well for the completed

MeerKAT array, but even more so for SKA1-mid — the mid-

frequency part of the SKA’s first phase. Construction will

start in 2018, using Chinese-built antennae to expand the

existing array. SKA1-mid will provide a maximum baseline

of 150 km and a total collecting area of some 33,000 m^2 ,

equivalent to 126 tennis courts. It will have four times higher

resolution and five times higher sensitivity than the VLA.

Interferometry 101

RADIO REVOLUTION