12 reflecting telescope12/15/2023 The object I viewed was the nebula in the belt of Orion, and I found the figure of the mirror, though far from perfect, better than I had expected. The first observation with the telescope was on 19 February 1787, when Herschel pointed the then-incomplete telescope towards the Orion nebula, which he observed by crawling into the telescope and using a hand-held eyepiece: "The apparatus for the 40-foot telescope was by this time so far completed that I could put the mirror into the tube and direct it to a celestial object but having no eye-glass fixed, not being acquainted with the focal length which was to be tried, I went into the tube, and laying down near the mouth of it I held the eye-glass in my hand, and soon found the place of the focus. The telescope was located on the grounds of Observatory House, Herschel's house in Slough, between 17. Use Photograph of the 40-foot telescope's frame taken in 1839 by William Herschel's son, John Herschel. A speaking tube, up to 115 feet (35 m) in length, was used by the observer to relay measurements or observer remarks to be recorded. Īccess to the observer position was by ladders - although a chair lift was designed it was never constructed. This design has come to be called a Herschelian telescope. This saved on the severe light loss the image would suffer if he had used a speculum metal diagonal mirror. Herschel eliminated the small diagonal mirror of a standard newtonian reflector from his design and instead tilted his primary mirror so he could view the formed image when he stood in an observing cage directly in front of the telescope. The mirrors remained the largest in the world until 1845. However, this second mirror required more frequent polishing due to the fast tarnishing nature of the metal, and the original mirror was used when the second was being polished. The mirror was in use by 24 October, but continued to be polished more until 27 August 1789, when Herschel was happy with it. The first casting on 26 January 1788 cracked on cooling, but the second casting three weeks later was successful. A second mirror with twice the thickness of the original was cast a few years later, and this was used rather than the original. Over a year was spent grinding and polishing the mirror however, Herschel found it to be "much too thin to keep its figure when put into the telescope" (despite weighing half a ton). It weighed 1023 lb after being cast, but it was found to be 0.9 inches thinner at the centre than at the edge (where it was around 2 inches thick). The first was cast in a London foundry on 31 October 1785, and was made of speculum (an alloy of mostly copper and tin) with arsenic to improve the finish. Two 48-inch (120 cm) concave metal mirrors were made for the telescope, each with a focal ratio of f/10. Just prior to them entering the open mouth of the tube, the King commented "Come, my Lord Bishop, I will show you the way to Heaven!" During construction, whilst the telescope tube lay on the ground, the King as well as the Archbishop of Canterbury visited the telescope. It was paid for by King George III, who granted £4,000 for it to be made, and £200 a year for its maintenance, although the telescope remained Herschel's property. The telescope was mounted on a fully rotatable alt-azimuth mount. The telescope was constructed by Sir William Herschel, with the assistance of his sister Caroline Herschel, between 17 in Slough, with components made in Clay Hall near Windsor. 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. It may have been used to discover Enceladus and Mimas, the 6th and 7th moons of Saturn. It was the largest telescope in the world for 50 years. It used a 48-inch (120 cm) diameter primary mirror with a 40-foot-long (12 m) focal length (hence its name "Forty-Foot"). William Herschel's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope constructed between 17 at Observatory House in Slough, England.
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