MIRA Issue 111

MIRA Issue 111
2021.2


A History of the Jodrell Bank Observatory
Part 1: Cosmic rays to Sputnik,1945 - 1957
By Mark Edwards


The story of Jodrell Bank as a radio observatory started when the electric tram came to Oxford Road in Manchester.  As every tram passed the University of Manchester's old physics building on 12th September 1945 it produced radio interference that ruined Bernard Lovell's experiment set up outside.
That experiment was to use an ex-army gun laying radar, working on a wavelength of 4.2 metres with a peak power of 50 kW, to try to detect the ionised trails left by cosmic ray showers as they passed through the Earth's atmosphere. 
Looking for a quieter site, Lovell contacted Richard Rainford, the deputy bursar of the university, who suggested a place owned by the horticultural department at High Legh near Knutsford.  However, when he visited it on a misty day in the autumn of 1945 he found that there were high voltage power lines sizzling with sparks as they crossed the land.
Returning to Rainford he suggested another site run by Frederick Sansome, a lecturer in horticulture in the botany department and also by chance a radio amateur.  This site of 11 acres made up of three fields had been acquired in 1935 for experimental research into horticulture and was located at Jodrell Bank near Lower Withington in Cheshire. 
So it was that in mid December 1945 Lovell had the army move the radar in three trailers (one containing the transmitter, one the receiver and one the diesel generator) to Jodrell Bank.  The receiver was put next to the wooden botany huts, but the transmitter and generator were dragged along a muddy track leading to a manure heap.  He then discovered that the generator would not start, but after the local farmer, Ted Moston, removed ice from its fuel line he switched the radar on and instantly saw, not interference but a series of transient echoes.  The date was 14th December 1945.

Meteor echoes
However, there seemed to be many more echoes than he had expected, several per hour rather than one or two per day.  After talking to James Hey, who had used the radar to locate V2 rockets during the war, he discovered that Hey too had noticed the same spurious echoes and had written a secret report that linked these to the entry of meteors into the Earth's upper atmosphere.  The report also referred to the work of JP Shafer and WM Goodall in the USA who in 1932 had come to the same conclusion. 
This is when the date of his observation becomes significant as it was the peak of the Geminid meteor shower.
In early 1946 Lovell along with John Clegg and CJ Banwell (a New Zealander) moved the radar equipment into the adjacent field, now marked by the power house.  They also acquired for accommodation a large army vehicle known as a Park Royal (as it was made by the Park Royal Vehicle company of Abbey Road, London) which became stuck in the mud when they tried to drive it across the field.  When in 1949 the control building for the 218 ft telescope was constructed near that position it became known as Park Royal.
As at this time Lovell was not an astronomer he enlisted the help of Manning Prentice who was the director of the BAA's meteor section to help out.  Prentice suggested that they observe the Perseid meteor shower that summer, both visually and with the radar to look for coincident events.
These all-night observations started on 29th July 1946 and each night they found that the number of echoes seen increased when the radiant was at its highest and that overall the counts reached their maximum on 14th August.  Also of note was that only the echoes of longest duration seemed to be associated with visible meteors.
Before he left, Prentice suggested observing the Giacobinids over the night of 9th October as the Earth would pass close to the head of its source comet and a short, but brilliant display of meteors was expected.  Meanwhile, Lovell was still wondering whether some of the echoes might be from cosmic ray showers when Patrick Blackett (who won the Nobel prize in 1948 for his discoveries in nuclear physics and cosmic rays), the head of physics research at the university produced a letter from Thomas Eckersley (that he had kept since 1942!), which pointed out that the strength of the expected reflections had been overestimated. 
Needing a more sensitive receiving setup to see these weaker signals, Lovell turned to Clegg for help.  Clegg had a wealth of experience of aerial design during his war work, so he mounted an array of five 6-element Yagi aerials on a borrowed Army searchlight.  This searchlight was mounted on a wheeled trailer, but it was soon taken off and set in concrete.  Years later it was still in use as a rotating stand for aerials under test.  As he had completed the aerial in time for the Giacobinid meteor shower it was used for the observations.
Looking at the cathode ray tube just after dusk they were disappointed to see only two or three echoes per hour, when just after midnight suddenly echoes started to appear and the sky was ablaze with streaks of light.  By 3 am the meteors were so numerous that they were unable to count either the meteors in the sky or on the tube.  Turning the aerial to the radiant the echoes all but disappeared but when pointed at right angles to the meteor trails the count shot up to over a thousand per hour.  By 6 am it was all over and the counts went back to normal levels.
These observations were conclusive proof of the meteoric origin of the mysterious echoes and established Jodrell's reputation as a centre for astronomical research.

The Transit Telescope
Still looking for more sensitivity for the cosmic ray experiment they continued to build even larger aerials, starting with a broadside array of dipoles (which they instantly abandoned as too dangerous), but then progressing to build a horizontal paraboloid whose size was determined by the distance between the Park Royal and the hedge.  This gave one of 218 ft in diameter made out of wires mounted on scaffolding poles.  The focus aerial being held on top of a 126 ft long steel pole mounted on a concrete base and secured by guy wires. 
When they started to use this dish with the radar they could see nothing unusual except for the occasional meteor echo.  However, they noticed that each time the Milky Way went overhead the noise level increased dramatically and so started a new chapter in the history of Jodrell Bank.
In 1948 Victor Hughes joined Jodrell as a research student and he was given the task of studying this cosmic noise from the zenithal strip of sky that the telescope could see.  There was one problem.  They needed to determine the polar diagram of the telescope to make useful measurements.  This is where Lovell's war time connections came into play as he had soon acquired an aircraft containing a transmitter that made more than a hundred flights at 3000 ft over the dish.  Its track plotted on the table of a camera obscura giving the wanted polar diagram.
The main problem with the telescope was that it always looked at the same strip of sky, but when Hughes left and Robert Hanbury Brown took his place in May 1949 things were about to take a dramatic turn.  Not long after a research student, Cyril Hazard, also joined the group.
The first thing the pair did was to reduce the size of the beam to 2 degrees to increase its resolving power.  That meant changing the receiving wavelength to 1.89 metres which was the shortest that would work with the wire dish design.  The second thing was to make the beam steerable away from its constant 53 degrees north declination (the latitude of Jodrell). 
The only way to do this was by adjusting the 18 guy wires that held the aerial mast in place, each shift of 2 degrees taking about two hours.  Eventually they managed to get paper charts of the radio noise received in strips between declinations of 38 and 68 degrees north.  What they noticed from this map was that although most of the emission was from the Milky Way, scattered all over it were bright points of radio emission that were not associated with the visible stars.  These became known as radio stars, the two brightest of which were called Cassiopeia A (a supernova remnant) and Cygnus A (a galaxy with an active supermassive black hole at its centre).  
Suspecting that the Andromeda galaxy might also emit radio waves like the Milky Way they spent 90 nights observing it in the autumn of 1950 and in the process produced the first radio map of another galaxy.
Design for a new telescope
It was the limitations of a fixed telescope that drove Lovell to think of building a fully steerable version.  After having been told that it was not possible by several engineering firms, on 21st July 1949 D. Roberts from Coubro and Scrutton (the company that had supplied the aerial mast for the 218 ft telescope) recommended a consulting engineer by the name of Henry Charles Husband. 
So on the afternoon of 8th September, standing by the transit telescope, Lovell explained to Husband that he wanted to have a telescope of that size, mounted so that it could be directed to any part of the sky.  Husband replied that he did not think the task was impossible, but "about the same problem as throwing a swing bridge across the Thames at Westminster".
By January 1950 Husband had produced sketches of the proposed design for a 250 ft dish and at a meeting at Jodrell on 14th June between Lovell, Husband and Blackett, proposed that it would cost about £100,000.  Consequently they applied to the DSIR (Department of Scientific and Industrial Research) for a sum of £120,000 and in response the DSIR agreed a sum of £3,300 to enable Husband to produce a detailed design for the construction of the telescope.

Lunar echoes
Around the same time as these plans were being developed another line of research was taking place, this was that of the variation of lunar echoes.  A group in Australia had found that radio waves reflected off the Moon experienced two types of fading, one was rapid with a period of seconds, the other was much slower of about 30 minutes.
It was thought that the short period was due to the Moon's libration and the long something to do with the Earth's ionosphere, so to explore these effects many attempts were made at Jodrell to obtain radar reflections from the Moon and on 18th July 1949, using a long pulse 4.2 metre transmitter and a narrow band receiver on the searchlight aerial, they succeeded.
Later, when the festival of Britain took place in the summer of 1951, one of the exhibits in the Dome of Discovery was to be the live display of echoes from the Moon from signals transmitted by a 30 ft dish on the shot tower next to the Royal Festival Hall in London.  Jodrell helped in setting up the exhibition and it was meant to start transmitting on 4th May, except that they had been unable to get the necessary radar equipment and the best they could do was to display radio noise received from the Sun, starting on 4th June.  
After the exhibition the dish was moved to Jodrell where it was mounted on top of the power house and eventually, in March 1954, used for its original purpose by William Murray and JK Hargreaves.  They had been studying the lunar echoes since the summer of 1953 at a wavelength of 2.5 metres, but using the dish they discovered that the long period fading was caused by the rotation of the plane of polarisation of the radio waves as they passed through the ionosphere.

Optical telescopes
In September 1951 the university council accepted the offer of an 18 inch reflecting telescope from GT Smith-Clarke of Coventry, believing that it would be useful to Professor Zdenek Kopal, the head of the astronomy department at the university.  It cost £500 to dismantle and transport it to a new £1000 observatory at Jodrell, but it was rarely used and was finally donated to the University of Salford in 1970.
Smith-Clarke also donated a spectrohelioscope which was erected in late 1953 in a darkroom extension to the cosmic noise hut.  This was used in combination with a solar radiation monitoring programme at 3.7 metres. 
Intensity interferometer
To try to find the true nature of the radio stars Hanbury Brown decided to build an interferometer consisting of two aerials, but he feared that if they were really stars the aerials would have to be separated by hundreds of kilometres to give sufficient resolution to measure their diameter.  He thought that would be impossible, but he had a marvellous incite, he thought that the fluctuations in intensity of the noise from two separate aerials must be correlated and as these fluctuations were at a relatively low frequency they would be easy to send over a radio link or landline to a correlator.
To build such an interferometer he called on one of his research students, Roger Jennison, who with a young research student from India, Mrinl Das Gupta built two independent receivers at 125 MHz to work with their own aerials measuring 120 ft x 41 ft, one of which was in a fixed position at Jodrell, the other was transported by truck from farm to farm around the Cheshire countryside.
After starting in the summer of 1952, when they obtained their first results in August they were surprised to find that the two brightest sources were much larger than stars.  Cygnus A was double, the two components separated by 88 seconds of arc and Cassiopeia A was roughly circular with a diameter of 3.5 minutes of arc.
To try to identify the fainter radio stars Hanbury Brown was joined by a new member of staff, Henry Palmer (my supervisor!) and two students, Richard Thompson and David Morris to build a small portable 25 ft aerial linked by radio to the 218 ft telescope.  By 1955 they had reached a baseline of 13 km, but only two of the five sources not in the Milky Way were resolved, showing they were less than 25 seconds of arc across.
By 1956 they had established their remote station 20 km away, next to the highest pub in England, the Cat and Fiddle in Derbyshire, which has a direct view across the Cheshire plain to Jodrell, but the three remaining sources still remained unresolved, making them smaller than 12 seconds of arc.  Eventually these compact sources became know as quasars (quasi-stellar radio sources), later to become recognised as the active nuclei of galaxies.
Hanbury Brown then had another crazy idea.  If the intensity interferometer worked with radio, then why not with light?  To try this out he borrowed two anti-aircraft searchlights containing 5 ft diameter mirrors, removed their arc lamps and replaced them with photomultipliers.  He then pointed the searchlights at Sirius and observed it on every possible night from November 1955 to March 1956.  However, he discovered that Cheshire was no place to do optical astronomy, as during all this time he only managed to get 18 hours of acceptable observing.
To his delight and contrary to what most quantum physicists had said, the intensity fluctuations of the light were indeed correlated (now called the Hanbury Brown-Twiss effect) and agreed reasonably well with what he had expected.  The correlation was at a maximum at the shortest baseline of 8 ft and decreased as the searchlights were separated reaching a very low value when they were 30 ft apart.  This showed that the angular size of Sirius was 0.0071 seconds.
To follow up on these researches Hanbury Brown eventually emigrated to Australia on 13th January 1962 where the weather was better and he could get the required funding to build an interferometer with 7 metre diameter mirrors.

Building the 250 ft telescope
To help with the design of the 250 ft telescope Husband built a 30 ft diameter dish as a small-scale prototype.  This was mounted on a concrete base next to the cosmic noise hut and originally had a reflecting surface made of chicken wire, that was later replaced by one of expanded metal.  The theory here was that at the proposed operating wavelengths of 1 to 10 metres leakage through the mesh would be negligible.  However, with the discovery in 1951 by HI Ewen and EM Purcell at Harvard University of the 21 cm radiation from neutral hydrogen, the design for the 250 ft telescope was changed to give it a solid metal surface, made from welded sheets of 14-gauge mild steel, to be able to work at that wavelength.
This change of design had great consequences for the cost of the telescope, which by the time construction started had risen to £335,000, half to be paid for by the DSIR, the other half by the Nuffield Foundation (the observatory was called the Nuffield Radio Astronomy Laboratories from 1966 to 1999 in its honour) whose vice-chairman happened to be Sir John Stopford, the vice-chancellor of the university.
The preparation of the ground for the 250 ft telescope was supposed to start on 1st July 1952, however the first of endless problems occurred right from the start as Lovell decided that he wanted to move the telescope into a field not owned by the university.  However, the woman who owned the field had just died and he ended up in a family dispute among her five sons.  Three weeks later the dispute was resolved in the high court and eventually on 3rd September construction started in earnest.  The original site is now the visitors' car park.
It was not until 12th June 1957 that the telescope was sufficiently complete to move under its own power in azimuth.  Then on 20th June it made its first trip out of the zenith when the elevation motors were started up.  At this time the twelve outer rings of steel plates had not yet been attached to the surface of the dish, but by the end of July the whole paraboloid was complete.
These successful tests meant that on 2nd August the first radio observations were made of the Milky Way and on subsequent nights the polar diagram of the telescope was measured using Cassiopeia A, which happily confirmed that the telescope was working exactly as was expected.
However, these successes came at a cost, £750,000 to be exact and could have resulted in Lovell being imprisoned for debt and his career ruined had it not been for a fortuitous event a few months later.

Sputnik
That event was of course the launch of Sputnik I on 4th October.  Although the beep-beep signals transmitted from the satellite were easily received on a standard radio, what exercised the Government was that there was no radar in the country capable of detecting the carrier rocket and that was the rocket of a Russian intercontinental ballistic missile.
The newly completed telescope could do this, but for the problem that it would take days to move the heavy Moon radar equipment from one of the laboratory buildings on to the telescope. In the end the transmitter was left where it was and instead connected by transmission lines across the field and up to the focus of the telescope. 
By 6 pm on 9th October the telescope was moving automatically from the control room and a few minutes after midnight a test on the Moon was successful, with strong echoes being received.  However, before they could try it on the rocket an obscure fault that they could not find put the equipment out of action.
The day was saved, though, by JS Greenhow.  He had joined Jodrell as a research student and operated a small meteor radar to measure the height of their ionised trails.  This radar of 150 kW operated on a lower frequency of 36 MHz rather than the 120 MHz of the Moon radar and give a transmitted beam almost four times wider making it much easier to search for the rocket.
With this transmitter mounted in the elevated laboratory, suspended under the dish, they searched for the rocket on the night of 11th October.  In amongst all the meteor echoes was one which was unmistakeable that of the rocket and on the next evening they saw it again travelling at 5 miles per second. a hundred miles high over the Lake District.
This success was acknowledged by the Prime Minister Harold Macmillan who on 29th October said in the house of Commons: "Hon. Members will have seen that within the last few days our great radio telescope at Jodrell Bank has successfully tracked the Sputnik's carrier rocket."
This was repeated a few weeks later when the rocket of Sputnik II (carrying the dog Laika) was detected on 2nd November, this time with the repaired lunar radar, travelling over the Arctic Circle at a distance of 1000 miles.  They also observed its final orbits as the carrier rocket burned up in the Earth's atmosphere in the early hours of 1st December.
With these very public successes there was tremendous pressure on the Government to release the funds to wipe out Jodrell's debt, once this had been done it was able to move on to its next phase of astronomical research.


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