Derby and District Astronomical Society


DDAS Visit to the Mullard Radio Astronomy Observatory in Cambridge - Saturday 12th September 2015

Article by Mike Lancaster


On Saturday, 12th September 2015 a group of 24 DDAS members visited the Mullard Radio Astronomy Observatory (MRAO) in Cambridgeshire. The trip by mini-coach from Derby was organised by DDAS secretary Brian Dodson. This is the third trip made by the DDAS to this facility, the previous ones having been in 1984 and one in the early 1990s. Our first stop for this latest visit was at the White Horse pub in the village of Barton where we had an excellent lunch, after which we were met by our guide for the day, Peter Doherty. Peter is an instrument engineer at MRAO, and has worked on many projects including instrumentation for other radio astronomy facilities around the world. Our first stop on the tour was the MRAO lecture room at the former Lord's Bridge railway station. Peter gave us a fascinating talk on the history of the site and MRAO. In the immediate post war years this area of Cambridgeshire was used to dump mustard gas and munitions, which remain a potential hazard to any excavations even to this day! The Cavendish Laboratory under Professor Martin Ryle had been pioneering radio astronomy since 1945 and with a generous grant of 100,000 from Mullard Ltd. the MRAO was founded in 1957. Various radio telescopes were then set up in the area in the ensuing years. The Lord's Bridge railway station comes into the picture in 1968 when the Varsity Line linking Oxford to Cambridge closed, and a section of line and station buildings were taken over for use by MRAO. This section of line would then become the baseline for the 5 km Array of telescopes. After the talk we had a chance to view some of the old pieces of equipment on display including pieces of a WWII German radar array and parts from EDSAC, Marconi Myriad and Titan computers from a bygone age.



On the platform at Lord's Bridge.  Image Copyright: Mike Lancaster



Early computer parts on display at Lord's Bridge.  Image Copyright: Tony Barker


Not far from the Lord's Bridge station Peter showed us the now obsolete control room of the 5 km Array which opened in 1972. This array (also known as the Ryle Telescope) consisted of eight 13 m dishes mounted on the former railway line, which afforded a moveable baseline. The control room included a large console that would not have looked out of place in the original Star Trek series and racks of 'correlators' used to combine the signals from the separate dishes into a single 'image'. Today the eight dishes have been moved into a more compact and fixed arrangement and form the Large Array component of the Arcminute Microkelvin Imager. We took a short bus ride in our coach down the former railway line and got a good look at this impressive array, which was actually in use while we were there. So how is the array controlled today? It can be done remotely from a laptop! On our way to these dishes our coach had to thread its way under a somewhat narrow bridge and also past the antennas of the Cambridge Low-Frequency Synthesis Telescope (CLFST)!



Plaque outside the 5 km Array control building.  Image Copyright: Mike Lancaster



The now obsolete control console of the 5 km Array. Our guide Peter Doherty explains the contents of the control room.  Image Copyright: Tony Barker



Equipment racks in the 5 km Array control room.  Image Copyright: Bill Miles



Radio dishes of the 5 km Array (now the AMI Large Array).  Image Copyright: Mike Lancaster (top) and Tony Barker



The antennas of the Cambridge Low-Frequency Synthesis Telescope (CLFST). Originally built in 1980 CLFST comprised an array of 60 antennas spread across a 4.6 km baseline. CLFST ceased operations in 2000 but some of the antennas have been preserved here at Lord's Bridge.  Image Copyright: Mike Lancaster


Next stop on our tour was a 32 m diameter dish that forms part of the e-MERLIN network of radio telescopes that are spread across England. MERLIN stands for Multi-Element Radio Linked Interferometer Network and comprises seven radio telescopes including the 76.2 m Lovell Telescope at Jodrell Bank, from where the array is controlled. The telescopes were originally networked by microwave links but by 2009 these had been replaced by fibre optic cables - hence the name e-MERLIN.



The Cambridge 32 m dish contributing to e-MERLIN.  Image Copyright: Mike Lancaster

We moved on to another building on the site where we were able to refresh ourselves with a drink and some biscuits before visiting AMI's other half - the AMI Small Array. This array of ten 3.7 m dishes is housed inside a single storey tennis court sized enclosure whose walls and floor are metal to shield the facility from terrestrial interference and the whole structure is open to the sky. I was quite moved by this structure - the scientific equivalent of entering a great shrine or temple. The metal walls and floor of the structure were brilliantly lit by sunshine and one could easily go 'snow blind' without wearing sunglasses! The constant whirr of cooling pumps also filled this astronomical arena, reminding us that these instruments were operating at the time - mobiles off please! Working in concert with the Large Array of the Ryle Telescope the combined AMI array observes highly distant galaxy clusters in the very early universe by the 'shadows' they cast on the radio emission of the Cosmic Microwave Background.

Inside the AMI Small Array.  Image Copyright: Tony Barker (top) and Mike Lancaster



Plaque inside the building by the AMI Small Array.  Image Copyright: Bill Miles


Next we made a trip to the COAST - in this case the Cambridge Optical Aperture Synthesis Telescope. Conceived by the late Professor John Baldwin of the astrophysics group at the Cavendish Laboratory in Cambridge, COAST is a multi-element optical interferometer with baselines of up to 100 metres, designed to observe stars with angular resolutions as high as one thousandth of one arcsecond. This is a much higher resolution than even the Hubble Space Telescope can deliver - and all from the corner of an English field! The idea was to apply the same physics used in combining the signals from several radio telescopes to optical and infrared wavelengths. COAST was completed in the 1990s and some examples of the research conducted include surface imaging of the star Betelgeuse and an infrared image of the binary star Capella. Today COAST is no longer operational but the bunker-housed beam combining laboratory is still used as a testbed for equipment destined for other observatories around the world such as the Magdalena Ridge Observatory Interferometer. In a field adjacent to this site we also saw a test array for the low frequency component of the multi-national Square Kilometre Array (SKA) project.

Cambridge Optical Aperture Synthesis Telescope (COAST). The white boxes housed two of the several reflecting telescopes making up the array and the light from each was sent down beam lines into the bunker at left where the images were combined into one.  Image Copyright: Mike Lancaster


A test array for the low frequency component of the multi-national Square Kilometre Array (SKA).  Image Copyright: Mike Lancaster


Our next encounter was with a number of radio dishes making up components of the One Mile and Half Mile Telescopes. The One Mile Telescope comprises two fixed and one moveable dish, each 18 m in diameter. The moveable dish could be positioned along a half mile section of purpose built rail track, being slowly moved into place by a small locomotive. One of the fixed dishes of the One Mile Telescope sits at the eastern end of this track and the other sits in a field a half mile further east. Other smaller dishes on this same track make up the Half Mile Telescope, two of which were fixed and two moveable. The track itself is straight to an accuracy of 0.9 cm and is raised by 5 cm at one end to compensate for the Earth's curvature. Both telescope arrays were completed in the 1960s but are no longer operational. The One Mile Telescope was the first to use the Earth's rotation as a means of aperture synthesis, and the first to produce radio maps of the sky with a better resolution than the human eye's optical view of the sky. The One Mile Telescope was also employed in producing the 5th Cambridge Catalogue (5C catalogue) of radio sources. Operating at the 21 cm line the Half Mile Telescope mapped the distribution of hydrogen in galaxies such as M31 and M33.

One of the fixed dishes of the One Mile Telescope sits at the eastern end of the half mile track.  Image Copyright: Mike Lancaster


From the eastern end of the half mile of track we can see the base of one of the fixed dishes of the One Mile Telescope in the foreground, with the moveable One Mile Telescope dish and two of the smaller Half Mile Telescope dishes in the distance.  Image Copyright: Mike Lancaster


The base of the moveable One Mile Telescope dish (foreground) complete with locomotive.  Image Copyright: Mike Lancaster


Then came what for many of us was the 'holiest' place on the entire site. Through an un-prepossessing gap in the hedge on the south side of the Half Mile Telescope track we entered a field overgrown with hawthorn bushes, weeds and grass. Stretching into the distance on either side and partly hidden by the vegetation was a ramshackle array of wooden poles and wires which was once the scene of one of the greatest discoveries in astronomy. These were the remains of the Interplanetary Scintillation Array with which Jocelyn Bell discovered the first pulsar in 1967, while working as a graduate student for Anthony Hewish. The array originally consisted of over four thousand dipole antennas and covered up to nine acres, but is now retired and in a very sorry state. We also saw the wooden control shed in which Jocelyn Bell would have worked, again in a sad state today. It is such a pity that even only a small corner of this site could not be preserved. In my opinion it should have World Heritage Site status, such was its importance to science - although that could probably go for the entire observatory site!

The Interplanetary Scintillation Array - which enabled the discovery of the first pulsar in 1967.  Image Copyright: Mike Lancaster


Following our encounter with the Interplanetary Scintillation Array we moved on to take a look inside the control building of the Half Mile Telescope. This was filled with racks of electronics from a bygone era. A punched tape machine sat next to a slide rule and boxes of old exam papers had been dumped on the floor. I found a diary from 1970 with entries such as 'midnight - M31'.

The Half Mile Telecope control room.  Image Copyright: Mike Lancaster


In the next field on the north side of the Half Mile Telescope we viewed yet another piece of radio astronomical history - the 4C Array. This was actually the first telescope to be installed at the Lord's Bridge Site and was built in 1958. It is a 450 m long, 20 m wide cylindrical paraboloid and was originally covered in reflector wire although that has since been removed. During the 1960s it contributed nearly 5000 radio sources to the 4th Cambridge (4C) catalogue.

The 4C Array.  Image Copyright: Mike Lancaster


Our penultimate stop on the tour was the dish of the Cosmic Anisotropy Telescope (CAT). This was a three element interferometer which during the 1990s made the first measurements of small-scale structure in the cosmic microwave background. It has since been decommissioned. Following that we had a look at another testbed for some new instrumentation and then it was back to the coach for our trip home.

The Cosmic Anisotropy Telescope (CAT).  Image Copyright: Cmglee (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons.
Cmglee Cambridge MRAO CAT


Back on the coach - like many of the telescopes at Mullard this had also seen better days!  Image Copyright: Tony Barker (top) and Mike Lancaster



Our thanks go to the Mullard Radio Astronomy Observatory and especially to Peter Doherty for the excellent tour he gave us, and for giving up a Saturday afternoon to do so! Also many thanks to DDAS secretary Brian Dodson for organising another superb day out.


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