Just outside the city of Cambridge stands this remarkable, significant but redundant structure. It is something of an icon for science in Cambridge : the city that gave us the electron, the atom split, the double helical nature of DNA and the rules of football!
It’s part of the Cambridge University Mullard Radio Astronomy Observatory (MRAO) site. The One-Mile Telescope has served its research purpose and astrophysics has moved on. It stands as a testament to the life and work of 1974 Noble laureates Martin Ryle and Antony Hewish for the 1963-71 synthetic aperture work which discovered and mapped the pulsar. At the other end of the array, a mile away, one of the 3 telescopes that make up the One-Mile is in occasional use by an amateur astronomy group. I like the structure for its association with the discovery of the pulsar, as an emblem of the distant radio echo of supernovae, it’s engineering excellence and it’s landscape setting.
The fist trace of the pulsar was a big surprise, it was detected on November 28, 1967, by Jocelyn Bell Burnell and Antony Hewish. The pulses were separated by 1.33 seconds, originated from the same location on the sky, and kept to sidereal time. This is a compilation of the traces Bell recorded from pulsar CP1919, the consistency of the pulses is as striking as Jocelyn Bell’s reaction:
“We did not really believe that we had picked up signals from another civilization, but obviously the idea had crossed our minds and we had no proof that it was an entirely natural radio emission. It is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly?”
They called the signal LGM-1, for “little green men”. It was not until a second source was discovered in a different part of the sky that the “LGM hypothesis” was dropped. Once astronomers had studied the facts pulsars were explained as the trace, predicted by Robert Oppenheimer and George Volkoff in 1939, of a super dense neutron star, the crushed relic of a supernova caused by the gravitational collapse of a once massive star, spinning at great speed, emitting radiation like a sputtering catherine wheel along its magnetic poles.
A single trace was one thing but how many pulsars are there? The Cambridge University Radio Astronomy Group took up the challenge to map the skies and the One-Mile Telescope, as concieved by Martin Ryle, was just the tool for the job:
“Our object was twofold. First we wanted to extend the range of our observations far back in time to the earliest days of the Universe, and this required a large increase in both sensitivity and resolution. With greater resolution we hoped that we might be able to draw radio maps of individual radio sources with sufficient detail to give some indication of the physical processes which brought them into being.”
The technique required triangulation on an East-West base line and, by using the rotation of the earth, the base could be extended to the diameter of the planet by recording signals at fixed points of the rotation. The telescope required great precision and serious processing power to control the dish orientations, decode and then synchronise the data. The base line is levelled to account for the curvature of the earth and the 2 fixed and 1 moveable antennae are aligned to 9mm.
Telescopes 2 and 3 are fixed at the centre and East end of the base, No 1 is movable by a custom Blaw Knox shunter on a half mile track. It is currently parked next to No2 at the Eastern end of its track. As ‘big science’ goes it’s pretty big: one mile long.
Observations with incremental spacings of the mobile antenna at the Western end of the array were used to observe individual radio sources with unprecedented sensitivity, angular resolution, and image quality. The work required intensive use of inverse Fourier Transforms which was only possible using the biggest computer of the day: the ‘Titan’ computer at Aldermaston and Manchester.
The One-Mile Telescope’s principle task was the compilation of the 5C (5th Cambridge) Catalogue of celestial radio sources at 408 MHz and 1407 MHz. Published between 1975 and 1995 by the Radio Astronomy Group of the University of Cambridge, the One-Mile Telescope achieved angular resolutions of 80 arcseconds and 23 arcseconds at 408 MHz and 1407 MHz respectively, mapping radio signals as faint as 2 milli-Janskys, (1 Jy = 10−26 watts per square metre per hertz) considerably fainter than any previously mapped source.
The whole thing is top quality American kit. Blaw Knox were world leaders at radio tower antennae at the time and the minimum mass framework is rock steady even today, some 50 years on. It creaks a little in gust but the galvanising has very few signs of failure. Signs of careful maintenance include oil on the bearings and congealed grease on the rack and pinion gears.
The future of this structure is far from certain; the grass is no longer cut, there are signs of recent vandalism. Some of the rack mounted relays have been ripped out. This may well be the beginning of the end for it: unless it is secured, in a short time it will be trashed for the metal and then declared a dangerous structure leading to its eventual demise. At the time of writing the control desk is intact. This antenna is a slave so apart from the dish orientation system and its power supply there’s little evidence of the epic mapping of the edges of the universe to see.
The log book has the whole history of the station’s service from 23rd April 1964 to 15th July 1973 (when the receiver feed was removed and ‘shipped to lab’) recorded as performance notes on the critical components checked during observation runs.
The last use of the One-Mile Telescope was between 1987 and 1990 as a calibration reference for the MERLIN (then MTRLI) project. Telescope No3 was not used but maintained ‘ready’, the power supply is still live and the 50cycle hum pervades the control room.
You can get some idea of the immense forces involved as a neutron star blasts out radio and xray by listening to this excellent collection of pulsar audio recodings: http://www.jb.man.ac.uk/pulsar/Education/Sounds/sounds.html
As an object in the landscape Telescope 3 of the One-Mile Telescope is a striking reminder of the vastness of the universe and the adventure of discovery. It is not scheduled or listed. Long may it stand.