page_title: 40-foot telescope
section_title: Decommissioning and preservation
hierarchical_section_title: 40-foot telescope / Decommissioning and preservation
caption_attribution_description: English: The remaining section of William Herschel's 40-foot telescope
context_page_description: 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 diameter primary mirror with a 40-foot-long 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 section of the tube remain.
context_section_description: The telescope's frame was dismantled at the end of 1839 by William Herschel's son, John Herschel, on his return from carrying out observations in South Africa. It was dismantled as it was feared that the frame might collapse due to rot, and John feared for the safety of his young children. A small ceremony was conducted to commemorate its dismantling.
  The tube was left lying horizontally in the garden, supported by stone blocks at either end, where it was crushed in 1867 by a falling tree. The remaining piece is a 10-foot (3.0 m) length of the mirror end, which is 3,048 by 1,465 mm (120" x 57.7"). This was still located in the garden of Observatory House in 1955, but was subsequently moved  and is now located in the Herschel Collection of the National Maritime Museum, in the Royal Observatory, Greenwich, London.
  The first mirror was last polished in 1797, and was subsequently stored away and lost. When John Herschel moved from Observatory House to Hawkhurst in 1840, a number of items (including the 40-foot telescope) were left behind. In an inventory written at the time, he recorded "In the Observatory, beneath stair, one 40-foot mirror, with case and cover." A workman later reported that only a light metal cover of a 4-foot mirror was present, rather than the mirror itself. The mirror was rediscovered on 2 February 1927: "All that could be seen on a casual inspection was a somewhat rusty iron ring, about 4 feet in diameter and 5 inches thick ... covered in front with a close-fitting lid of thin metal. The iron ring, which was not unlike the tyre of a cart-wheel, was obviously the cell of a large mirror and was quite separate from the tin cover. On removing the latter, which was provided with six handles, the mirror itself was at once seen, occupying the front portion of the cell, close under the cover."
  On 4 March 1927 the mirror was moved to the Cottage library, and was once more polished some 130 years after the mirror was last properly polished. The original mirror now resides in the Science Museum, London. The second mirror was left in place in the telescope when it was dismantled, but was removed when the tube was crushed. In 1871 it was moved into the hall of Observatory House.
  A scale model of the telescope, as well as an early photo of it that is framed in wood from the telescope, is on display at the Herschel Museum of Astronomy in Bath.
  The 40-foot (12 m) telescope was surpassed in 1845 as the largest ever built by Lord Rosse's great 72-inch (1.8 m) reflecting telescope. The image of the 40-foot telescope remains as one of the great icons of astronomy.
caption: The remaining 10 ft section of the telescope, now at the Royal Observatory, Greenwich.
exif: {"Image Make": "Canon", "Image Model": "Canon EOS 400D DIGITAL", "Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image DateTime": "2009:07:11 12:22:58", "Image ExifOffset": "163", "EXIF ExposureTime": "1/200", "EXIF FNumber": "9", "EXIF ExposureProgram": "Unidentified", "EXIF ISOSpeedRatings": "400", "EXIF ExifVersion": "0221", "EXIF DateTimeOriginal": "2009:07:11 12:22:58", "EXIF DateTimeDigitized": "2009:07:11 12:22:58", "EXIF ShutterSpeedValue": "4357/570", "EXIF ApertureValue": "1623/256", "EXIF ExposureBiasValue": "0", "EXIF MeteringMode": "Pattern", "EXIF Flash": "Flash did not fire, compulsory flash mode", "EXIF FocalLength": "21", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "2592", "EXIF ExifImageLength": "3888", "EXIF FocalPlaneXResolution": "195065/44", "EXIF FocalPlaneYResolution": "102433/23", "EXIF FocalPlaneResolutionUnit": "2", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF SceneCaptureType": "Standard"}