William Herschel's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope constructed between 1785 and 1789 at Observatory House in Slough, England. It used a 48-inch (120 cm) diameter primary mirror with a 40-foot-long (12 m) focal length. It was the largest telescope in the world for 50 years. It may have been used to discover Enceladus and Mimas, the 6th and 7th moons of Saturn. It was dismantled in 1840 by Herschel's son John Herschel due to safety concerns; today the original mirror and a 10-foot (3.0 m) section of the tube remain.
40-foot telescope
Photograph of the 40-foot telescope's frame taken in 1839 by William Herschel's son, John Herschel.
The telescope's first mirror on display in the Science Museum, London
The remaining 10 ft section of the telescope, now at the Royal Observatory, Greenwich
Frederick William Herschel was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover, William Herschel followed his father into the military band of Hanover, before emigrating to Great Britain in 1757 at the age of nineteen.
Portrait by Lemuel Francis Abbott, 1785
Original manuscript of Symphony No. 15 in E-flat major (1762)
Replica in the William Herschel Museum, Bath, of a telescope similar to that with which Herschel discovered Uranus
Herschel's mirror polisher, on display in the Science Museum, London
