40-foot telescope

William Herschel built a 12-metre reflecting telescope in Slough between 1785 and 1789. The iron tube stood on a rotating mount in the garden of Observatory House. King George III provided £4,000 for its construction and £200 a year for upkeep. Caroline Herschel assisted her brother during the work. The telescope’s 1.2-metre primary mirror was cast in London and weighed over 450 kilograms. A second, thicker mirror replaced the first after it proved too thin. The telescope was the largest in the world for half a century. It may have revealed two moons of Saturn, Enceladus and Mimas. Herschel described Sirius shining like the rising sun through the eyepiece. Caroline received a £50 annual pension as William’s assistant, becoming England’s first paid female astronomer. Visitors on their way to Windsor Castle often stopped to see the giant instrument. Ordnance Survey maps marked its position for travellers. Today a 3-metre section of the iron tube remains in the garden. The second mirror still lies inside this fragment. The original mirror vanished after being stored beneath the stairs. A 1.2-metre iron ring from the first mirror was rediscovered in 1927 and now belongs to the Herschel Collection in Greenwich. The site once echoed with ladders and a planned chair lift that was never built. A speaking tube carried the observer’s voice to recorders up to 35 metres away.