Building record 7756/14 - Jodrell Bank Observatory: Mark II Telescope

<1> Historic England, 2011, The National Heritage List for England, 1443087 (Web Site). SCH6528.

The Mark II Telescope was instigated in 1960 by discussions between Sir Bernard Lovell and Sir Charles Husband of structural engineers Husband and Co of Sheffield, who had previously designed the Mark I Telescope (since renamed the Lovell Radio Telescope), built in 1952-57. When approaching the design of this second telescope various issues were addressed. The idea of an oval (elliptical) reflector arose as a response to the design problems of constructing a very large reflector. Husband wanted to keep down the vertical size to avoid constructional and aerodynamic problems. He also preferred to construct the whole turntable of pre-stressed concrete rather than structural steel for reasons of rigidity and economy. It was intended as the prototype of a larger telescope, later referred to as the Mark IV, which was never constructed. Meanwhile, in 1960 the General Post Office asked Husband to build a radio telescope at Goonhilly, Cornwall, for transatlantic communication via the first active communication satellite, known as Telstar, with a summer 1962 deadline. Husband declared this impossible unless he was allowed to use the basic design for the Mark II Telescope, scaling the design so the bowl was circular with the diameter of the minor axis of the Mark II. It was subsequently built to the 1962 timescale and is recognised as the birthplace of satellite communication (Antenna No.1 at Goonhilly Satellite Earth Station listed Grade II*). Funding for the construction of the Mark II Telescope was requested on 19 December 1960, but the Research Council of the Department of Scientific and Industrial Research (RCDSI) did not pass this until December 1961.

The telescope was built on the site of the earlier 218ft Transit Telescope and was operational by summer 1964. It had a steel sheet paraboloidal reflecting bowl with an elliptical aperture of major axis 125ft (38.1m) and minor axis of 83ft 4in (25.4m). The focal length was 40ft (12.2m), the mounting was alt-azimuth with 420 degrees of travel in azimuth and 0 to 90 degrees in elevation. The drive was a Ward-Leonard system with digital computer control giving great accuracy of positioning. The Mark II was the first telescope in the world to be steered by digital computer. The computer used was the Ferranti Argus 104, which was one of the very first computers designed for real time control, with a storage capacity of 12 Kbytes.

A combination of greater surface accuracy of the bowl and the design of more precise control devices meant that the Mark II was able to observe higher frequency wave lengths in the 21cm band and shorter than the Mark I Telescope and thus complement it. As well as operating as a solo instrument, the telescope was intended to work as an interferometer with the Mark I Telescope (two or more telescopes that combine their signals to produce a resolution equivalent to that of a telescope of diameter equal to the largest separation between its individual telescopes). In 1971 the Mark II’s computer was upgraded to an Argus 400 computer, also capable of carrying out data acquisition for the telescope’s observing programs, with the original 104 transferred to the upgraded Mark I. During the 1970s the telescope worked in conjunction with the Mark III telescope (built to the same dimensions in 1966 at Wardle, Nantwich, Cheshire and intended to be portable but never moved). This work led to the development of the Multi Telescope Radio Linked Interferometer (MTRLI) and then Multi-Element Radio Linked Interferometer Network (MERLIN) arrays of which it is an integral part.

In 1974 a proposal was made to upgrade the Mark II to Mark IIA, by giving it a 100ft circular aperture which could be used on wavelengths down to 6mm. However, funding could not be secured.

In 1987 the bowl was upgraded with a circular surface of new aluminium panels mounted on top of the original steel surface. The new surface had an accuracy of 1/3mm and was set using a holographic technique. It was built in a collaborative project between Jodrell Bank and Racal Antennas and resulted in the telescope being able to observe at the 22GHz MERLIN frequency. In the late 1990s a new, more compact carousel for the receivers was installed in the aerial focus. The building was refurbished in 2014.

The Mark II Telescope continues in daily use with the majority of current observational time spent either on e-MERLIN (the upgraded MERLIN programme) or Very Long Baseline Interferometry (VBLI), joining radio telescopes around the world.

<2> Chris Blandford Associates, 2016, Jodrell Bank Observatory, Conservation Management Plan (Client Report). SCH8560.