Fenlandphil's Blog A blog from the low country

Visit to the Mullard Radio Telescope

The Cambridge Astronomical Association had organised the tour of the Mullard Radio Telescope. Jeff and I were there before 9.30, waiting for the gate to open at half past nine.

When I had an ironwork business, I had driven past the telescope on occasion when travelling between Cambridge and Sandy. The huge structures visible from the road with what appeared to be radar dishes on top, sitting beside the A603, fascinated me. I can’t remember if I knew it was a radio telescope at the time, but Mullard was definitely a familiar name. Mullard had made and possibly still does radio valves, the forerunners of transistors. It was not just the large structures that interested me but that some were mounted on a trolley, albeit a substantial one, with flanged wheels running on very widely spaced iron rails.

Once the gate had been opened and the visitors allowed in, it was closed and locked, and we gathered outside the former Lord’s Bridge Station building to be greeted by our host, Peter. Peter gave us a brief history of the site as we stood outside the former station building on what had been, before its closure, the Cambridge-to-Oxford railway line, the one now being rebuilt at enormous public expense.

Before and during the Second World War, the site had been used to store munitions and explosives, notably many containing mustard gas. This extensive 30-hectare site, occupied by the radio telescope, is owned by Cambridge University.

Following the brief introduction, at the former station building standing outside on what would have been its platform, we were shown into the station building itself. In there, we were given more details about the site’s history and that of the telescope by Peter. The telescope isn’t one single instrument but a variety of configurations of apparatus and structures, each measuring different wavelengths of signals emitted by the various planets, stars, galaxies and other bodies that make up the cosmos.

The telescope was opened in 1957, it is part of the Cavendish Laboratory and was initially funded by Mullard Limited. It was originally under the direction of Professor Sir Martin Ryle, who, together with Professor Antony Hewish, went on to receive the 1974 Nobel Prize for Physics for their work there on radio astronomy.

Radio telescopes can observe parts of the cosmos not visible in the optical spectrum and can look back at the very early universe, as it was billions of years ago. A lot of the work at the Radio Telescope is on the Cosmic Microwave Background (CMB), which is observed using some of the Ryle Telescope and other instruments on site.

We moved from the station building to look around the site itself. The first stop was the Ryle telescope, which consists of large dishes with some mounted-on rails, also known as the Large Array. Using these clusters of dishes, they can achieve the same results as one large dish, the diameter of the distance between the smaller dishes in the array. So, the one-mile-long array produces results equivalent to a single dish one mile in diameter.

The site is so large that we drove from one part of the site to another, although even so, there was a fair bit of walking involved. Back in the cars, as we moved on to our next stop, we passed the Merlin 32m dish, part of a Jodrell Bank project involving a number of these dishes in different parts of the country.

After passing various wartime bunkers or their remains, the next stop revealed some very interesting receivers, which seemed to be structures in the shape of large diameter bowls constructed of wooden frames covered in wire mesh. These turned out, I found out later, were Part of the prototype HERA, the Hydrogen Epoch Ionisation Array developed on site at Lord’s Bridge. HERA looks back at the very early universe just after the Big Bang. The shape of the antenna above the centre of one of these dishes is the result of extensive research.

The next stop was to look at the memorial plaque to the telescope and, below it, one celebrating the work of Grote Reber. Reber built the first radio telescope in his backyard in Wheaton, Illinois, USA, in 1937. This followed the discovery of radio signals from the Milky Way by Karl Guthe Jansky in 1933.

The Small Array was next on the list. Consisting of ten smaller dishes used together with two of the Ryle telescope’s receivers it forms the Arcminute Microkelvin Imager (AMI). This project primarily measures temperature fluctuations in the Cosmic Microwave Background (CMB).

We had a short biscuit break before looking around some other telescopes, none of which looked like what you would imagine a telescope to look like. Probably the closest to an optical telescope was the Cambridge Optical Aperture Synthesis Telescope (COAST), which combines the signals from several telescopes. The telescopes track the same object and combine the images to produce one that overcomes atmospheric effects, yielding images equivalent in resolution to those from a 70m-diameter telescope.

Although much of the site is active, there was also a historical connection. The mile-long telescope is no longer in use but the structures, the dishes and rails are still in place as is the data collection centre, kept locked but opened for our visit. This building is a time capsule housing what was, in its day, cutting-edge technology, now largely obsolete, including eight-inch floppy discs, punched-tape machines and some very old electrical instruments.

To one side of the mile-long track is the 4C telescope, a series of masts with oddly shaped arms sticking outwards at right angles to the line of masts, which appear to be connected with cables at the top and struts lower down. This telescope was constructed in 1957 and was one of the first to be built there. 

Further along, we crossed the tracks and were shown down a narrow pathway to view a much simpler, low-tech design. The Interplanetary Scintillation Array, built in 1967, covered four acres. It was enlarged in 1978 to nine acres and refurbished in 1989. It was designed by Antony Hewish, who was awarded a Nobel prize for the discovery of pulsars, although the credit should arguably have gone to Jocelyn Bell Burnell. Jocelyn Bell Burnell not only analysed the results from the data gathered by the array, but she also helped build it. Originally, it had consisted of over a thousand wooden poles connected with cabling to measure another part of the wavelength of signals received from space. Time has taken its toll on this telescope and what remains of it is in poor condition.

This completed our tour. After returning to the former Lord’s Bridge Station building, the gate was unlocked and we headed home. It was a fascinating visit. I was amazed by the depth and quantity of knowledge that our guide, Peter, not only the knowledge he had acquired and retained but also how much he shared with us.