Telescopes from the Ground Up

A southern-sky telescope shows the wisdom of glass mirrors over metal

In 1862, authorities in Victoria, Australia decided to build a large telescope to study the Southern nebulae — cloudy patches in the sky whose nature was still unknown. A committee of telescope-makers and astronomers, including Lord Rosse, was formed to decide on the design of this telescope, whose view of the southern sky would help complete the picture of the heavens. Many observatories were focused on the northern sky, but the southern sky had received less scrutiny.

The committee chose a reflecting telescope with a 48-inch Cassegrain mirror. Though glass mirrors were becoming popular at the time, the committee preferred a metal mirror. The members assumed that the silver layer of a glass mirror would tarnish faster than a metal mirror, that the silver might disintegrate or be damaged by the weather, and that such a large, heavy, glass mirror would be difficult to manipulate.

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Early Reflectors
Map of Melbourne, Australia, where the Melbourne reflector was located.
Illustration of the Melbourne reflector.Enlarge picture
The Melbourne Reflector
Year completed: 1868
Telescope type: Reflector
Light collector: Metal mirror
Mirror diameter: 48 inches
(1.2 m)
Light observed: Visible