Lassell, William

Born: 18 June 1799, Bolton, Lancashire, England
Died: 4 Oct 1880, Maidenhead

William Lassell was an amateur astronomer, who made his fortune as a brewer. He built an observatory at Starfield near Liverpool, where he constructed and mounted a 24" diameter speculum metal mirror in a reflecting telescope (1943-5). This was the first telescope with a large mirror to be to be mounted "equatorially" to allow easy tracking of the stars. It thus allowed an object in the sky, such as a star or planet, to be observed easily over long periods, move by a simple hand-cranking method. It paved the way for future development in reflector technology, leading to the large equatorial instruments seen in many national observatories around the world.

 
Lassell's polishing machine
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Lassell's 24" Telescope
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The mirror weighed nearly 500 lbs. Lassell ground and polished it himself to give a 20 foot (F10) focal length mirror of superb quality. With help from the foundryman James Nasmyth, Lassell made his own steam-driven equipment for grinding and polishing the mirror. This gave a parabolic curve accurate to better than a quarter of the wavelength of light, ideal for observing the planets.

With this telescope he discovered several planetary satellites, including Triton, moon of Neptune (10 October 1846). He found the eighth moon of Saturn, Hyperion, in 1848, one day after the American William Cranch Bond. But he was first to discover Ariel and Umbriel, satellites of Uranus (1851).

When Queen Victoria visited Liverpool in 1851 Lassell was the only local notable whom she specifically asked to meet,

Lassell built a 48" telescope (1855) and used it ini Malta, which he chose for clearer skies (1861-5), and where he made fresh discoveries in the Trapezium of the Orion Nebula.

Later he became president of the Royal Astronomical Society (1870--2).