The Mark III telescope, the same size as the Mark II, was constructed to be transportable but it was never moved from Wardle, near Nantwich, where it was used as part of MERLIN. It was built in 1966 and decommissioned in 1996. The Mark IV, V and VA telescope proposals were put forward in the 1960s through to the 1980s to build even larger radio telescopes. The Mark IV proposal was for a diameter standalone telescope, built as a national project. The Mark V proposal was for a moveable telescope. The concept of this proposal was for a telescope on a railway line adjoining Jodrell Bank, but concerns about future levels of interference meant that a site in Wales would have been preferable. Design proposals by Husband and Co and Freeman Fox, who had designed the Parkes Observatory telescope were put forward. The Mark VA was similar to the Mark V but with a smaller dish of and a design using prestressed concrete, similar to the Mark II (the previous two designs more closely resembled the Lovell telescope). None of the proposed telescopes was constructed, although design studies were carried out and scale models were made, partly because of the changing political climate, partly to the financial constraints of astronomical research in the UK and it became necessary to upgrade the Lovell Telescope to the Mark IA, which overran in terms of cost. A 50 ft (15 m) alt-azimuth dish was constructed in 1964 for astronomical research and to track the Zond 1, Zond 2, Ranger 6 and Ranger 7 space probes and Apollo 11. After an accident that irreparably damaged the 50 ft telescope's surface, it was demolished in 1982 and replaced with a more accurate telescope, the "42 ft". The 42 ft (12.8 m) dish is mainly used to observe pulsars, and continually monitors the Crab Pulsar. When the 42 ft was installed, a smaller dish, the "7 m" (actually 6.4 m, or 21 ft, in diameter) was installed and is used for undergraduate teaching. The 42 ft and 7 m telescopes were originally used at the Woomera Rocket Testing Range in Australia. The 7 m was originally constructed in 1970 by Marconi Company. A Polar Axis telescope was built in 1962. It had a circular 50 ft (15.2 m) dish on a polar mount, and was mostly used for moon radar experiments. It has been decommissioned. An reflecting optical telescope was donated to the observatory in 1951 but was not used much, and was donated to the Salford Astronomical Society around 1971.