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View Professor Chris Christiansen (1913-2007)'s photo gallery
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Professor Chris Christiansen was interviewed in 1997 for the Interviews with Australian scientists series. By viewing the interviews in this series, or reading the transcripts and extracts, your students can begin to appreciate Australia's contribution to the growth of scientific knowledge.
The following summary of Christiansen's career sets the context for the extract chosen for these teachers notes. The extract covers his first job in CSIRO and the development of Earth rotational synthesis as a way to view radiation from the sun. Use the focus questions that accompany the extract to promote discussion among your students.
Chris Christiansen was born in 1913 in Melbourne. From the University of Melbourne he received a BSc in 1934, an MSc in 1935 and a DSc in 1953. In 1980 he was awarded a DScEng from the University of Sydney.
After graduating with his MSc he was a physicist at the Commonwealth X-ray and Radium Laboratory in Sydney for two years. In 1937 he became a research engineer at Amalgamated Wireless (Australasia) Ltd (AWA) where his first project was to produce a direct-reading field intensity meter. He worked at AWA until 1948 and during his years there he was involved in many of the technical aspects of radio transmission and reception, including being the resident expert in directional aerial design.
In 1948 he moved to the CSIRO Division of Radiophysics where he worked until 1960. He began his work in radioastronomy by investigating the quiet sun. To study solar radiation, he developed a new type of aerial array known as a grating interferometer, it consisted of 32 parabolic dishes each 6 feet in diameter and spaced at equal intervals in a line. In 1953, by adding a second array of aerials at right angles to the original array, he was able to scan the sun in two dimensions. This novel technique was called Earth rotation synthesis and it has become an extremely powerful tool of modern radioastronomy. He developed the innovative cross-type radio telescope, known as the Chris Cross, which was completed at CSIRO's Fleurs field station near Sydney in 1957. It produced high-resolution two-dimensional pictures of the sun each day, which were used in Australia and sent overseas.
Christiansen became Professor and Head of the Department of Electrical Engineering at the University of Sydney in 1960, a position he held until 1978. In 1963 the CSIRO handed over the Fleurs field station and its radio telescopes to his department. He further developed the telescopes into sophisticated instruments of great resolving power and used them to contribute greatly to international astronomy.
He retired in 1979 and moved to Canberra where he was a Visiting Fellow at the Mt Stromlo Observatory of the Australian National University until 1983.
In addition to his distinguished career in scientific research and development of large-scale projects in radioastronomy, he was also committed to forging strong international links and friendships. He served as Vice-President of the International Astronomical Union from 1964 to 1970. He was President of the International Union of Radio Science from 1978 to 1981, and subsequently honoured as Honorary Life President in 1984. Together with J A Högbom he is the author of Radio Telescopes, a widely cited book covering the basics of radio astronomy.
Elected to the Fellowship of the Australian Academy of Science in 1959, he served as Foreign Secretary of the Academy from 1981 to 1985.
What was your first job in CSIRO?
Joe Pawsey's group was at that stage studying the sun, although I think he stayed on it too long. That was one of the few things I could criticise him for. Thanks largely to Joe, it had been discovered by this time that most radio waves from the sun came as radiation from a pretty uniform hot body. Secondly, there was a thing called the slowly varying component, which changed slowly from day to day. It was known that when there were a lot of sunspots, it was highest, and when there weren't, it was lowest.
Still not identified to any location but just generally coming from the sun?
Yes, but known to be associated with sunspots. Thirdly, an occasional blow-up of the sun would always be associated with a big sunspot and with sudden enormous increases in energy that would vary like mad. Of those three main factors I was told to investigate this slowly varying component. The first thing I did was to record the radiation during an eclipse of the sun. I tried using three stations, but well separated – one in Tasmania, one in Sydney and so on – so that the eclipse shadow of any part of the sun would occur at different times at each station and we should be able to locate the emitting regions. That worked out very well and it also gave me an idea of the size of these regions, roughly three minutes of arc.
Was that very much larger than the dimension of a sunspot?
Yes. Then I thought, 'We can't wait for these damned eclipses. We ought to be able to look at the sun every day,' and I started to wonder how we could get such a high resolving power from a radio aerial that we could get down to three minutes of arc. You would need an enormous aerial if you made it in one piece but I thought, 'Well, if you have a series of aerials, not together but spaced out, you get a number of very narrow responses from that. If I can so space those that there is only one response on the sun at a time, we should get a series of scans across the sun as long as we like, provided we point the aerials in the right direction.' I worked out that we could collect the energy we would need with a lot of six-foot diameter dishes as aerials.
And they had to be steerable. Were they steered automatically?
No, we took it in turns to change them and the running kept us thin and healthy!
Where was the array, Chris?
I discovered that we could get the use of the side of the Potts Hill Reservoir, which contained the drinking water of Sydney. Only one of our people ever fell in.
You've got a picture there showing a close-up of one of the dishes. I think you have told me that they were made from 6 x 3 aluminium sheets welded together and then spun. Who are the three people in this picture?
I'm the one demonstrating part of the telescope to Professor van der Pol, the famous Dutch engineer and scientist. And there, with his black hat on, is Sir Edward Appleton, looking as though he were going to a morning party. This was at the 1952 General Assembly of the International Union of Radio Science (URSI).
That is just one linear array, but you refined it later on, didn't you?
That just gave, in effect, a knife-like beam across the sun, and so I built another one on another side of the Potts Hill Reservoir – this time we used mechanical help to turn the aerials. With this one we did something that hadn't been done before. By looking at the sun during about a 12-hour period, we were scanning it in every direction during the day as it went round the sky. By doing a bit of mathematical jugglery called Fourier synthesis we could get a real picture of the sun, with its hot spots and so on. This method of using the Earth's rotation to produce a picture is called an Earth rotational synthesis and now all the really big aerials in the world use it. But that was the first time.
Approximately how many individual sources on the sun were you looking at?
Usually not more than 10 or so. They were always associated with either the sunspots or where sunspots had been, the previous time the sun turned round.
Select activities that are most appropriate for your lesson plan or add your own. You can also encourage students to identify key issues in the preceding extract and devise their own questions or topics for discussion.
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