The Great Melbourne Telescope (GMT)

The Great Melbourne Telescope sketched in the Adelaide Post, 19 May 1868.

From many years from 1869 onwards, the City of Melbourne could boast of having the largest steerable telescope in the world. Known as the Great Melbourne Telescope, it was truly large with a total weight of 10 tonnes and a tube that was nine-metres long. The Irish telescope makers Thomas and Howard Grubb of Dublin, had built the huge telescope, under the supervision of a committee of eminent British astronomers. In Melbourne, it was placed in a building with a roll-off roof that was located some distance away from the other buildings of Melbourne Observatory, so that the mass of metal should not disturb readings by magnetic instruments.

The maker’s plate on the GMT. Photo Nick Lomb, cleaned up by Gemini.

The telescope had interchangeable metal mirrors, a 1.2 metre diameter primary mirror and an 0.25-metre diameter secondary mirror. There were two of each of these mirrors, as the metal mirrors tarnished easily and were hard to repolish. So, while one set of mirrors were in use in the telescope, the other set of mirrors were being repolished.

It should be noted that modern telescope mirrors are made of glass (or some low expansion equivalent) and then coated with a thin layer of aluminium. A similar technology with glass mirrors and silver coatings was already available when the GMT was built, but the committee supervising the building of the telescope did not want to risk the then new and little-tried technology on such a large mirror.

The surviving GMT mirror, collection of the Museum of Victoria. Photo Nick Lomb.

In Melbourne, the telescope achieved success with two short exposure photographs, one of the Moon and one in 1883 of the Great Nebula in Orion, which was the first photograph of a nebula in the southern sky. Unfortunately, the main observing aims of the telescope were flawed. These were to reobserve and sketch the nebulae that Sir John Herschel had observed from South Africa in the 1830s. However, sketches made by different observers with different telescopes cannot be directly compared and it would not have been possible to ascertain if the nebulae had shown changes in the intervening few decades.

The GMT House at Melbourne Observatory. Photo Nick Lomb

The GMT was rarely used after the 1880s, and in the 1940s sold for scrap to Mount Stromlo Observatory. There, the Observatory used parts of it for a 50-inch (1.3-metre) telescope, which itself went through a number of rebuilds over time. This telescope was destroyed in the Stromlo fire of 18 January 2003. Later, the burnt-out parts of the telescope were brought back to Melbourne. Since then, a team of dedicated volunteers from the Astronomical Society of Victoria, supported by Museum of Victoria staff, have been restoring the telescope. Engineering students have also been assisting with the restoration since 2019.

A future post will discuss Project Phoenix, the restoration of the telescope.

Acknowledgement

Simon Brink, Project Manager, Great Melbourne Telescope Restoration, Strategic Capital Programs, Museums Victoria, for corrections.

Further Reading

Richard Gillespie, The Great Melbourne Telescope, Museum Victoria 2011.