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40 foot telescope
William Herschel's 40-foot telescope, also known as the "Great Forty Feet" (in English: 40-foot telescope, or also "Great Forty-Foot"), was a reflector telescope built between 1785 and 1789 at the House of Observatory in Slough, England. He used a primary mirror of 120 cm (47 inches) with a focal length of 12 m (40 feet; hence his name "Forty-Feet").
It was the largest telescope in the world for 50 years. It could have been used to discover Enceladus and Mimas, the sixth and seventh moons of Saturn. It was dismantled in 1840, retaining only the original mirror and a section of the 3 m long tube.