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How radio astronomy has changed


Working on Martin Ryle's telescope
Antony Hewish Astronomer
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The first major telescope that he [Martin Ryle] built at Lord’s Bridge didn’t do a full two-dimensional Fourier synthesis. I mean, the way to synthesise a radio telescope covering an area, the simplest way, is to have two dishes and move them appropriately. Well, in the early days there would have been too many positions of the antennas for the computer to handle, and what Martin Ryle did, when we moved to our observatory, the first antenna he built was a huge linear parabolic reflector, about, what was it, about 400 metres long and that was a real antenna, for which I designed the feed, the focus. And that was moved… that was used in conjunction with a movable element, which was about half a mile away, moved in a north-south direction and combining the data gave… gave you the effect of a huge interferometer of two… two large dishes which were much… much larger than you could possibly build, two 400-metre dishes, which… which in those days would have been fearfully… fearfully costly and impossible to build. And I had a lot to do and some quite interesting times helping Martin Ryle with these early… early telescopes and… and one thing I had to remember always is the problem of – just a pure engineering problem – the long parabolic reflector had enormous lengths of… of wire. It… it was a trough reflector which focused the radiation to… to a linear focus all… all the way down… and… all the way down the line, and… and I was responsible for the… for what you did at the feed to collect… to collect the radiation. But also the problem of… of this reflector, because you could use wires as… as a reflector to save weight and because we were only dealing with one polarisation, electric wires… lengths of wire close together would do. And there were hundreds, literally hundreds of miles of wire in that… in that first antenna of Martin Ryle’s. And we used to get winters in those days with snow on the ground and freezing fog, and the big problem that Martin Ryle was worried about, rightly, was conditions when you have freezing fog, which build up the wires. This is… this is a familiar problem if you’re used to sailing around the Horn and things like that, rigging getting heavier because ice forms on it. But if you have… Martin Ryle’s structures were engineered so that they were only just strong enough to do the job in hand, they weren’t designed to carry the load when all the wires were covered with ice. And instead of being thin, taut wires, they were… they were wires as thick as your finger made of ice. This would have been a load which actually would collapse the telescope and Martin gave me that job to solve. He said: ‘Do something about this because ice can form on that reflecting screen in a matter of hours and we can’t afford for the thing to fall down’.

Well, I spent quite a while in cold chambers here in Cambridge at another lab, seeing how much current you need to pass through the electric wires to warm them up enough to melt the ice as it formed. It turned out you’d need power generators that we couldn’t possibly install at Lord’s Bridge, to send those currents through the whole array. And, in the end, I decided that the only way of solving the problem practically would be to release the reflecting screen from the end towers so that the load was just shed, as… as it were, and the reflecting screen would just simply droop, held up by… by the structures. But it wasn’t just the weight… it wasn’t the weight of the ice that mattered, it was the tension on the end which would pull the structure together and destroy it. So I made a quick release device which you could, essentially from the ground, you could pull a ripcord and the telescope reflector wires were simply released from the end structures so they simply fell to the ground and just sagged over and… and preserved the… preserved the end towers which would have been destroyed. And that was a quick and practical way, it didn’t cost any money. The only thing it cost was man-hours and having people to be around, a crew as it were, to watch the telescope under these conditions and decide when to pull the ripcord. And, because I’d designed that equipment, it was my job to pull the ripcord. And, of course, freezing fog usually happens at night when it’s cold and I spent in the early 1960s – I think it was 1962 when we were having a very severe winter – getting myself prepared to pull the ripcord and actually spending nights at the radio telescope, essentially watching it if the weather was bad.

And this happened one night; it was early New Year 1962 when these things happened, and I had to climb the telescope at night to check what was going on. We had no lights at Lord’s Bridge, no floodlights or anything of that sort, you had to climb up the telescope with a torch and just see what it was doing. And I knew exactly how to climb up the mast and look along, because I’d been up there so many times with… under the construction period, and just watch the load building up. And it was January 1962, I was up there in a freezing fog, about 1am, and the wires were building up an ice load, the whole structure was beginning to sag. And I could see the end towers beginning to pull together because there was sag developing in everything. And I decided that after you had about a metre of sag then that was too dangerous a load to leave, things were about to break. So I simply pulled the ripcord and it all worked absolutely marvellously and the load was released. I went home to bed, not really looking forward to what would happen the following morning. Well, the following morning it didn’t look very… it didn’t look like a radio telescope, it looked like a clothes line that had fallen down and there were frozen wires littering the telescope. But the telescope was still standing and the main structure was there and we spent the rest of that day, and a couple of days after that, hacking down the wires and beginning to rethread the reflecting screen. But that was the sort of practical thing, working with Martin Ryle, that you had to do and perhaps it was as well for me that I went on sabbatical leave to America just shortly after that, so I didn’t have to do all the repair work. And I went to Yale and spent a happy year there.

Antony Hewish (1924-2021) was a pioneer of radio astronomy known for his study of intergalactic weather patterns and his development of giant telescopes. He was awarded the Nobel Prize for Physics in 1974, together with fellow radio-astronomer Sir Martin Ryle, for his decisive role in the groundbreaking discovery of pulsars. He also received the Eddington Medal of the Royal Astronomical Society in 1969.

Listeners: Dave Green

Dave Green is a radio astronomer at the Cavendish Laboratory in Cambridge. As an undergraduate at Cambridge his first university physics lecture course was given by Professor Hewish. Subsequently he completed his PhD at the Cavendish Laboratory when Professor Hewish was head of the radio astronomy group, and after postdoctoral research in Canada he returned to the Cavendish, where he is now a Senior Lecturer. He is a Teaching Fellow at Churchill College. His research interests include supernova remnants and the extended remains of supernova explosions.

Duration: 7 minutes, 25 seconds

Date story recorded: August 2008

Date story went live: 25 June 2009