SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
The 80cm reflecting astronomical telescope of Leon Foucault (1819-1868), the French physicist, was built in 1862 in Paris and was moved to the Marseilles Observatory in 1864. Here it remained in service until 1962, giving it a working life of 100 years. This instrument has been called the first modern reflecting telescope due to the advances Foucault had made in the manufacture and testing of telescope mirrors. Previously, mirrors had been made from speculum, a copper and tin alloy but they were optically limited. Foucault found that glass was easier to work and had a better surface finish which, by depositing an ultra-thin layer of silver on the front surface, Foucault was able to produce an optical-quality, first-surface parabolic mirror, far superior to speculum mirrors. Equally important was his âknife-edgeâ test to measure the shape of telescope mirrors.
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