40-foot telescope

For the telescope at Green Bank Observatory, see 40-foot radio telescope.
40-foot telescope
Alternative namesGreat Forty-Foot telescope Edit this at Wikidata
Location(s)Slough, Borough of Slough, Berkshire, South East England, England
Coordinates51°30′30″N 0°35′43″W / 51.5082°N 0.5954°W / 51.5082; -0.5954 Edit this at Wikidata
Built1785–1789 (1785–1789) Edit this at Wikidata
First light19 February 1787 Edit this on Wikidata
Decommissioned1840 Edit this on Wikidata
Telescope stylealtazimuth mount
reflecting telescope Edit this on Wikidata
Diameter1.2 m (3 ft 11 in) Edit this at Wikidata
Focal length12 m (39 ft 4 in) Edit this at Wikidata
40-foot telescope is located in the United Kingdom
40-foot telescope
Location of 40-foot telescope
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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 (hence its name "Forty-Foot"). 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.