In the years of scarcity after the second world war, thrift was the order of the day. But with a few leftovers and a lot of imagination, some people could rustle up almost anything. Even a huge radio telescope.
In the summer of 1951, John Bolton and Bruce Slee were working at the Dover Heights radio-physics field station, high on a cliff in Sydney’s eastern suburbs. They were exploring the origins of the enigmatic radio waves reaching Earth from space, and to make any progress they needed to pinpoint the sources of those signals. For that, they needed a very large dish. With no money to build one they set about making one themselves from cast-off gear and other people’s rubbish. For 3 months, they spent every lunch break secretly digging a huge hole in the ground. Shaped to form a dish and lined with metal ties from packing cases, the hole became the world’s second-largest radio telescope. And it worked so well, it found the centre of the galaxy.
SECRECY was vital. It was better that no one knew what John Bolton and Bruce Slee were doing in their lunch breaks. The two young scientists were working at a redundant wartime radar station perched on the cliffs just south of the entrance to Sydney harbour. Like many others who had worked on secret wartime radar research, Bolton and Slee had become accidental astronomers, putting their knowledge of radio waves to use in the embryonic science of radio astronomy.
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By 1946, the old military blockhouse on the cliff top had sprouted antennas that looked to the heavens rather than out to sea. Bolton and Slee had been given the job of monitoring the newly discovered radio emissions from the sun. But the sun was going through a quiet phase and they were getting bored. So they turned their antennas away from the sun and hunted for other celestial objects that might be producing radio waves. Their search was short-lived. When their boss, Joe Pawsey, noticed the antennas were not pointing where they were supposed to, he confined them to the lab.
The following year, Pawsey relented and allowed Bolton and Slee, now joined by the young engineer Gordon Stanley, to resume their observations and by 1952 they had earned quite a reputation for detecting new radio sources. They were still using primitive Yagi antennas – simple masts with crosspieces which they made from bits of old radar equipment. Despite this, they had picked up the big bursts of radio waves associated with solar flares and sunspots, showed that some of the new “radio stars” were objects already familiar to optical astronomers, and made the first observations of galaxies beyond our own.
In an attempt to narrow down the positions of radio sources and produce a detailed map of the radio sky, they built increasingly complex antennas. By 1951, they had an array of 12 Yagi antennas scanning the skies, making it one of the world’s most powerful radio telescopes. But Bolton wanted to investigate the structure of the objects they were finding and learn more about the processes that were generating the radio waves. For this they needed a more sensitive telescope that operated over a wide range of frequencies. “We decided to build a dish: it would have better resolution and higher sensitivity,” says Slee.
But what sort of a dish? Money was tight and the young astronomers suspected Pawsey would veto the idea. They’d have to make their dish from whatever materials they could lay their hands on, and in secret – at least until it had proved its worth. Their inspiration came from a dish built a few years earlier by Bernard Lovell in England. It was designed to detect cosmic ray showers, but had also been used with some success for radio astronomy. Lovell’s dish was 70 metres across, a size that was possible only because it was fixed to the ground. The Australians decided they would also make a fixed dish, but unlike Lovell’s, which was built on a framework of posts, they would dig theirs out of the ground.
Just over the top of the cliff, out of sight of the blockhouse, was a sandy ledge. The digging would be easy and they wouldn’t be seen. “We knew we’d have to dig a big hole and that would involve a lot of labour,” recalls Slee. Each lunch break, he and Bolton slipped off with a couple of shovels and a wheelbarrow. It took 3 months to carve out a depression 21.9 metres across. To make it parabolic they moved sand from the centre to the edges to build a low rim, and shaped the interior with a wooden template that rotated around a central pivot. “We shifted around 1500 cubic metres of sand – that’s an awful lot of barrow loads,” says Slee.
Once the hole was roughly the right shape, they packed down the sand and consolidated it with ash to make as smooth a surface as they could. Getting hold of the ash was no problem: every few days, Stanley, who was also in on the plot, scrounged a truckload of the stuff from the nearby power station. “They had to dump it somewhere so they were quite pleased that someone would take it away,” says Slee.
The dish now needed a reflective surface to focus incoming radio waves on a small dipole antenna at the top of a central mast. This time they headed to the dockside at Botany Bay and scavenged cast-off steel ties from packing cases, which they laid at 30-centimetre intervals inside the dish. They now had the world’s second-largest dish – albeit one that was rooted to the spot and able to monitor only the narrow strip of sky that passed overhead. It was time to see what it could do.
They began with a quick survey of the central region of the Milky Way at a frequency of 160 megahertz. Radio astronomers were keenly interested in this part of the heavens, because somewhere out there, shrouded in cosmic dust and gas, lay the very centre of our galaxy. The Australians were ideally placed to search for it because the region around the galactic centre passes directly over Sydney.
Although the crudity of their reflecting surface restricted them to long wavelengths, they produced a map with resolution three times as fine as they could manage with a Yagi array.
It was time to confess to Pawsey. They had expected a reprimand, but Pawsey was so delighted he suggested they might treble the resolution again if they expanded the dish and gave it a better surface, allowing it to operate at a much higher frequency. No longer limited to using leftovers, Bolton and Slee increased the dish’s diameter to 24.4 metres, building the new, higher rim on a scaffold of aluminium tubes and tensioned wires. They poured concrete over the sand-and-ash surface and replaced the steel strips with an all-over covering of chicken wire.
Newcomer Dick McGee, sometimes assisted by Slee, began to repeat the earlier survey of the central Milky Way at the new operating frequency of 400 megahertz. Now the features producing radio emissions began to come into sharper focus, and individual sources stood out more clearly. Most importantly, the astronomers could make out the small but powerful source known as Sagittarius A. Bolton had little doubt that this was the nucleus of the galaxy, the centre around which all the stars rotate. Optical telescopes had never been able to penetrate the cosmic dust to pinpoint the spot, but the hole-in-the-ground dish had done it.
In 1958, the International Astronomical Union adopted the spot as zero longitude and latitude in the galactic system of coordinates. By then, the Dover Heights station had been abandoned. The local council filled in the hole, turfed over the site and created a public playing field known as Rodney Reserve. There, a few metres from the northern goalpost of a football pitch, the hole-in-the-ground telescope lies beneath the grass.