Herschel's seven-foot telescope, 1795-1816.
© Science Museum / Science & Society Picture Library
The famous English astronomer Sir William Herschel (1738-1822) made this telescope for his sister Caroline Herschel (1750-1848). She was William's lifelong observing asistant and an astronomer in her own right, having discovered eight comets. Her telescope has a seven foot wooden tube and stand that is operated using pulley ropes and gears. The heart of the telescope is a 6 1/8-inch (156mm) diameter mirror made of speculum metal; a bronze alloy with arsenic added to it to give a more reflective surface. The instrument is made of painted deal - a type of pine - and is thought to be similar to the telescope that Herschel used to discover the planet Uranus in 1781.