early cambridge radio astronomy

Astron. Nachr. / AN 328, No. 5, 426 – 431 (2007) / DOI 10.1002/asna.200710761

Early Cambridge radio astronomy

F.G. Smith�

Jodrell Bank Observatory, University of Manchester, Macclesfield, Cheshire SK11 9DL, UK

Received 2007 Mar 21, accepted 2007 Mar 21Published online 2007 May 15

Key words history and philosophy of astronomy – telescopes

Radio astronomy started in Cambridge immediately after the hostilities of the World War II have ceased. Martin Ryle wasthe inspiring leader of a small group that started to develop interferometry techniques at the Cavendish Laboratory. Fromthis development came the numerous Cambridge radio source surveys culminating in the Nobel prize awarded to MartinRyle for invention of aperture synthesis. The history of this early development is the subject of the present paper.

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1 Origins

The rapid development of radar in the UK from 1935 on-wards involved several university scientists who became, af-ter World War II, leaders in radio astronomy. Martin Ryle,who was the leader and inspiration of radio astronomy atCambridge, was recruited for radar work by J.A. Ratcliffe,who was an examiner in his final exams at Oxford. Otherradar pioneers who became well-known in astronomy wereLovell, Hanbury Brown, Pawsey, and Hey.

Ryle, who was already a keen radio amateur, started inCambridge in 1939 by investigating metre wavelength an-tennas for airborne radar. These were installed in aircraftsuch as the Beaufighter for airborne interception (AI). Di-rection finding was achieved by using a pair of antennas,one on each wing, to form a simple interferometer using aswitching technique. There is a close relation between theseairborne interferometers and the first radio astronomy in-terferometers at Cambridge. Ryle moved later into counter-measures against enemy radar; one of his achievements wasto jam the in-flight guidance system of the V2 rockets.

These young men who were responsible for radarthroughout the war were high achievers, working undergreat pressure and with limited resources. An example: afew days before the D-day landings in 1944, Ryle was askedto investigate a report of a new German radar system which,if it existed, would jeopardise the whole D-day operation.Within three days he built a new scanning receiver, bor-rowed a Halifax aircraft from Lovell, assembled an aircrew(on a Sunday afternoon, when they were playing a cricketmatch) and flew on the first of several flights which showedthat, literally, the coast was clear.

James Hey, whose three discoveries, of sunspot radia-tion, meteor echoes and the discrete radio source Cygnus Awere the inspiration of early radio astronomy, was a school-master, recruited in 1940 into the Army Operational Re-

� Corresponding author: [email protected]

search Group. He continued in radio astronomy after thewar, and developed accurate position finding using interfer-ometers. One of his radio telescopes, at Defford, is still inuse in the MERLIN interferometer network.

Radar similarly provided the first radio astronomersin Australia, most notably Pawsey and Bolton, but herethe radar establishment itself continued as Radiophysicsin CSIRO, in contrast to the return of Lovell and Ryle toacademic life in the UK. In Cambridge it was again Rat-cliffe who persuaded Ryle to join his radio research groupat the Cavendish Laboratory, although Ryle then decidedto branch out from the existing programme of ionosphericwork which involved Findlay, Weekes, Bracewell, and Bud-den. He set out to investigate cosmic and solar radio noise,which had been reported on by Hey in his operational workon radar.

The Cavendish Laboratory provided an interesting back-ground. X-ray crystal analysis under Sir Lawrence Bragg(Fig. 1) led to an understanding of Fourier theory, and fa-mously to the elucidation of protein and DNA structure.Fourier methods were also applied to radio by Ratcliffe, andit was he who inspired Bracewell to write his seminal texton applications of the Fourier Transform (1965). Radio in-terferometry and, later, aperture synthesis, grew naturally insuch fertile ground.

2 Techniques

During the war Ryle had used switching techniques and in-terferometers in airborne radars at metre wavelengths, andthis can be seen as the origin of the first use of two-elementinterferometers for solar observations. The radio interfer-ometers were thought of in terms of Michelson’s stellar in-terferometer, leading to the measurement of the angular di-ameter of the radio sun by using an interferometer with avariable spacing extending eventually to 140 wavelengths(250 metres). It happened that the Sun was at maximum

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Astron. Nachr. / AN (2007) 427

Fig. 1 Sir William Bragg (centre) with M. Ryle (right) and F.G.Smith (left) at the Mullard Radio Astronomy Laboratory.

of activity in 1946 when the observations started, and Rylewas soon able to locate and measure the diameter of sunspotradiation (Ryle & Vonberg 1946). The interferometer tech-nique also allowed regular observation of the quiet sun, andit was a surprise to find that it appeared as several timeslarger than the visible disc, with a brightness temperature ofaround 106 K (Ryle & Vonberg 1948). As we now know,the radio emission at these long wavelengths originates inthe solar corona, whose temperature was later determinedby the interpretation of high excitation X-ray lines.

At this time Ryle’s research group comprised DerekVonberg and Ken Machin, with Elizabeth Smith as ResearchAssistant, all formerly in radar. Tony Hewish and I joinedthe group in 1947, but I was already involved from the startsince the Vonbergs and Smiths all lived in the Ryle housein Herschel Road, immediately adjacent to the old RifleRange in Grange Road where the interferometers were built(Fig. 2).

Fig. 2 Interferometer antennas for solar observations at wave-lengths 1.8 m and 3.7 m, at Grange Road.

Two important techniques were introduced in the so-lar observations: interferometry to isolate a discrete sourcefrom a background, and receiver switching between antennaand a variable noise source, providing a self-balancing sys-tem independent of gain fluctuations. The receiver used a

rotating capacity switch rescued from a former German air-borne radar installed on Junkers 88. Ryle’s group madegood use of surplus wartime equipment, including two Ger-man Wurzburg 7.5 m diameter antennas from fighter controlradar. These were mounted as a transit interferometer for thefirst accurate location of radio sources (Smith 1951), and thefirst angular diameter measurements of Cygnus A and Cas-siopeia A, including using a third portable antenna for am-plitude closure (Smith 1952a). They were later moved to thepresent Lord’s Bridge Observatory by helicopter (Fig. 3).

Fig. 3 A Wurzburg 7.5 m radar antenna en route by helicopterto the new radio observatory at Lord’s Bridge.

3 Cyg A and Cas A

In 1948 we made our first observations of Cygnus A usingan interferometer at 3.7 m wavelength (80 MHz), consistingof two groups of four Yagi antennas 500 metres (130 wave-lengths) apart. The first night’s record showed, to our sur-prise, not one but two prominent interference patterns threehours apart, and with different periodicities (Ryle & Smith1948). The second one was a new source, Cassiopeia A, andwe realised that the periodicity of the interference fringesand the transit time across the central fringe could give anaccurate position (Smith 1952b). Our transit telescope was,however, aligned along a local field boundary, and it was 5degrees off east-west and half a degree from level (Fig. 4).I confess that I was unaware that Bessel had long ago anal-ysed the effect of errors in a nominal east-west baseline for atransit telescope, which I proceeded to work out for myself.There is no substitute for the discipline of working fromfirst principles! As soon as we had really accurate positions,using the twin Wurzberg interferometer, we were askingfor help from Dewhirst at the Cambridge Observatory, whoconfirmed the identification of Tau A (the Crab Nebula), andfound possible optical images of both Cyg A and Cass A(Dewhirst 1951). Following advice from Redman and JanOort, we then sent our positions to the Mount Wilson and

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428 F.G. Smith: Early Cambridge radio astronomy

Fig. 4 The author with the interferometer which gave the ac-curate positions which led to the identification of Cygnus A andCassiopeia A.

Palomar Observatories, where the identifications were even-tually achieved by W. Baade & R. Minkowski (1954) usingthe 200-inch optical telescope at Mount Wilson.

4 Source catalogues

The next stage was to build an interferometer with a largercollecting area (Fig. 5), again at 3.7 m wavelength (80 MHz)(Ryle, Smith & Elsmore 1950). This consisted of two broad-side arrays 20 × 1 wavelengths, 110 wavelengths apart(Fig. 6). This showed that the sky was full of discretesources; our first attempt to catalogue them (the 1C cat-alogue), like its successor 2C, suffered disastrously fromconfusion. The new technique of phase-switching was in-troduced in these observations.

The 2C interferometer was the first deliberate attemptat a survey at Cambridge. It showed us that the sky wasfull of ‘radio stars’, with very little correspondence with thevisible sky. Was this a new population within our Galaxy?In an attempt to find out, my PhD thesis included the firststatistical count of radio sources using the first 20 discretesources discovered with this array. Unknown to us, this firstsample contained a very disparate selection of objects, butwe nevertheless fitted a 3/2 power law to it! It was only someyears later that source counts became meaningful.

A clue to the nature of the discrete sources lay in thefluctuations, first observed by Hey in Cygnus A. Were theseintrinsic, or were they imposed by the ionosphere? I under-

Fig. 5 M. Ryle and F.G. Smith (two future Astronomers Royal!)constructing the 3.7 m interferometer array at Grange Road. Thesoldering iron was heated by a blowlamp.

Fig. 6 The 2C interferometer at 3.7 m wavelength, with a record-ing showing the two major sources Cygnus A and Cassiopeia A.

took to find out using simultaneous recordings at spacedreceivers, including a collaboration with Lovell at JodrellBank and, eventually using a transatlantic baseline, when Ispent a year at Carnegie Institute’s Department of TerrestrialMagnetism. But it was Hewish who followed up the scintil-lation story and eventually extended it to the solar corona(Hewish 1958), and later to the interstellar medium.

The interferometers at Cambridge developed and grewinto the much larger survey array (Fig. 7) which producedthe 3C and 3CR catalogues (Bennett 1962), which were suf-

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Fig. 7 One of four elements of the 3C interferometer array, usedat wavelengths of 2 m and 70 cm. This interferometer producedthe 3C and 3CR catalogues.

ficiently reliable that many radio sources are still known bytheir 3C designations. The source counts became reliable,and the first application to cosmology was presented, amidsome controversy, at the 1958 URSI Paris symposium on ra-dio astronomy. The problems of confusion were understood,and a direct analysis of the interferometer recordings byScheuer (Edge, Scheuer & Shakeshaft 1958) showed a slopeconsistently greater than 3/2 for the log N/ log S relation.The radio sources were seen to be distributed uniformlyover the sky, and the integrated background was consistentwith cosmological distances. Ryle set out the significanceof these results at his Bakerian lecture in 1958, in whichhe showed them to be incompatible with the Steady Statemodel. The need for greater resolution was evident, both forthe number counts and for resolving the structure of individ-ual sources. The interferometer techniques were essential,but it was clearly impossible to build large enough primaryinterferometer elements to resolve individual sources. Thesolution was aperture synthesis.

5 Aperture synthesis

The original idea of aperture synthesis was to map the quietradio sun in two dimensions, by using a variable spacing

interferometer with two mobile elements, aligned on north-south baselines as well as east-west. This was achieved byO’Brien (1953). Then the idea developed into creating a sin-gle large aperture by using two mobile elements placed insequence on the stations of a chess-board pattern, at variousspacings on successive days; this was first demonstrated byBlythe (1957), who synthesised a pencil beam telescope at8 m wavelength and used it to make a map of the galac-tic radio emission. The basic ideas of aperture synthesiswere in fact known and appreciated by three groups at thistime: Radiophysics Sydney, the French group at Nancay,and our group in Cambridge. They had been written downby Bracewell & Roberts (1954) and by Scheuer in his thesis(1954). The practical difficulty lay in recording and Fouriertransforming what was, for the time, a very large body ofdata. This was still the era of analogue recorders, and earlydays for EDSAC, the first programmable computer at Cam-bridge. The practical application of aperture synthesis topencil beam and interferometer systems was set out by Ryleand Hewish in 1960. An account of this development isgiven by Scheuer in The Early Years of Radio Astronomy,ed. Sullivan (1964). Earth rotation synthesis came a littlelater, when digital computers had developed enough to catchup with the needs of the radio astronomers.

Fig. 8 The east-west array of the 4C interferometer.

The first Earth rotation synthesis was a spectacular suc-cess. It used sections of the synthesis interferometer built forthe 4C survey. One half of this was a long east-west array(Fig. 8) which, for the synthesis observations, was dividedinto 32 sections; the other half, which had been designed tomove on a north-south rail track, was kept fixed and dividedinto 4 sections. With the addition of a small extra antennabetween these two, all spacings up to 75 units were avail-able. The whole array was tilted to give a maximum at theNorth Pole, so that by recording during 12 hours with suc-cessive interferometer spacings and at all position angles thewhole sky over an 8◦ patch of sky round the North Celestialpole could be mapped with a resolution of 4.5 arc minutes.


430 F.G. Smith: Early Cambridge radio astronomy

The resulting map (Ryle & Neville 1962) was shown at aconference at Herstmonceux (Fig. 9). Jan Oort was very im-pressed, and on his return to the Netherlands persuaded hiscolleagues to follow the new approach in their designs forwhat became the Westerbork radio telescope.

Fig. 9 The first rotational synthesis map (Ryle & Neville 1962).

The 4C interferometer, and the later radio telescopessuch as the One-Mile synthesis telescope and the Ryle 5-km telescope, needed more space than could be found at theGrange Road site, and in 1957 we moved to the present siteat Lord’s Bridge. It happened that Sputnik I was launched inthis year, and we briefly diverted our attention into space re-search by applying our interferometric and position-findingtechniques to monitoring the orbit. Surprisingly it appeared

that we were the only source of such information, and wewere inundated by the press for many days. An interestingoutcome of our tracking technique was that we publishedin the Journal of Navigation the first paper on the use ofsatellites for navigation (Smith 1960), admittedly without afull appreciation of the potential eventually demonstrated bythe GPS system. We also ventured into space research whenNASA, through the Royal Society, offered to mount simpleexperiments on the series of six Ariel satellites. We installeda receiver in Ariel II for measuring the galactic backgroundat the low frequency of 1 MHz, using a long wire dipole thatwas deployed under centrifugal force as the satellite was de-spun after attaining orbit (Hugill & Smith 1965).

It is often remarked that these various initiatives devel-oped without much reference to conventional astronomy.None of us had any background education in astronomy,and we were immersed in the development of techniquesand astrophysical situations which were outside the experi-ence of optical colleagues; for example, coherent interfer-ometry, which had been tried and abandoned by Michelson,was not a practical possibility at infrared or optical wave-lengths for another half century. We were also very compet-itive with other radio astronomy groups, and only learnedthe full value of collaboration when interferometer base-lines extended beyond national boundaries. A detailed ac-count of the emergence of radio astronomy in Cambridge,with many references, is to be found in Astronomy Trans-formed, by D.O. Edge and M.J. Mulkay (Wiley, 1976).

I moved from Cambridge to Jodrell Bank in 1964. Bythat time we knew that we had a wonderful new window onthe universe, with the first cosmological observations sinceHubble, and we were on the way towards the VLBI worldthat we now enjoy.

Acknowledgements. I am grateful to Bruce Elsmore, who checkedthe manuscript and provided the illustrations.

6 Further reading

Astronomy Transformed. D.O. Edge & M. Mulkay, 1976, WileyClassics in Radio Astronomy. W.T. Sullivan III, 1982. ReidelThe Early Years of Radio Astronomy. Ed. W.T. Sullivan III, 1984,

Cambridge University PressJames Stanley Hey. A. Hewish, 2002. Biog. Mem. Royal Society

48, 167

References

Baade, W., Minkowski, R.: 1954. ApJ. 119, 206Bennett, A.S.: 1962, MmRAS 58, 163Blythe, J.H.: 1957, MNRAS 117, 644Bracewell, R.N., Roberts, J.A.: 1954. AuJPh 7, 615Dewhirst, D.W.: 1951, Obs 71, 212Edge, D.O., Scheuer, P.A.G., Shakeshaft, J.: 1958, MNRAS 118,

183Hewish, A.: 1958, MNRAS 118, 534Hugill, J., Smith, F.G.: 1965, MNRAS 131, 137O’Brien, P.A.: 1953, Obs 73, 106

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Ryle, M.: 1958, Roy. Soc. London Proc. Ser. A 248, 291Ryle, M., Hewish, A.: 1960, MNRAS 120, 220Ryle, M., Neville, A.C.: 1962, MNRAS 125, 39Ryle, M., Smith, F.G.: 1948, Nature 162, 462Ryle, M., Vonberg, D.: 1946, Nature 158, 339Ryle, M., Vonberg, D.: 1948, RSPSA 193, 98

Ryle, M., Smith, F.G., Elsmore, B.: 1950, MNRAS 110, 508Scheuer, P.A.G.: 1954, ThesisSmith, F.G.: 1951, Nature 168, 555Smith, F.G.: 1952a, Nature 170, 1065Smith, F.G.: 1952b, MNRAS 112, 497Smith, F.G.: 1960, J. Inst. Nav. 13, 109


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