Flamsteed Astronomy Society

Today’s big reflecting telescopes

— April 3, 2006

Milestones in Reflector Development 1776 — 1945

1776 William Herschel’s ‘7-ft’ Newtonian 15 cm (6-in) dia. mirror.  Using a telescope like this, Herschel discovered the planet Uranus in 1781, the first new planet found since antiquity.  Example on display in the Herschel Museum, Bath.

1789 William Herschel’s ‘40-ft’ 1.22 m (48-in)   Herschel was able to resolve several nebulae into individual stars, but the telescope mounting was very cumbersome.  Herschel preferred his smaller ’7-ft’ and  ‘20-ft’ machines for sky sweeps of stars and nebulae. Remains of the tube is on display at the Royal Observatory Greenwich.

1845 Lord Rosse’s ‘ Leviathan of Parsonstown’ 1.83 m (72-in)  The world’s largest telescope until 1917.  Rosse resolved many of the nebulae but the mounting was even more cumbersome than Herschel’s.  The telescope was effectively a transit instrument with only 5 degrees travel either side. Now fully restored at Birr Castle, Eire.

1845 Development of mountings Nasmyth’s Cassegrain 50 cm (20-in)  Nasmyth was a leading engineer, the inventor of the steam hammer.  He mounted this telescope like a cannon and used a flat mirror to turn the light-path 90-degrees out through the trunnions where he could sit comfortably.  This is still called the ‘Nasmyth focus’.  Now in the Science Museum reserve collection.

1852 Development of mountings. Lassel’s Newtonian  1.22 m (48-in)   German equatorial mounting.  Lassel moved to Malta with this machine in an effort to find suitable seeing conditions.

1857 Silver-on-glass mirrors. Foucault’s 33 cm (13-in)  Equatorial mounting by Eichens.  Earlier machines had all used metal mirrors made from ‘speculum’ metal, a mixture of copper, tin, & zinc.  Speculum mirrors were poor reflectors and had to be completely re-figured when they tarnished.  Foucault used the new technique of depositing a thin layer of silver on a glass disk.

1867 The last large metal mirror —Grubb’s Great Melbourne Reflector cassegrain 1.22 m (48-in)   Proved to be a big mistake.  The speculum mirror tarnished very quickly in the Victoria coastal air and there was no adequate expertise in Melbourne to re-polish it.  The mounting was reused but destroyed in the Mt Stromlo bush fires of 2003.

1908 Ritchey’s Cassegrain 1.5 m (60-in)  Mt Wilson CA. The first modern reflector, and one of George Ellery Hale’s magnificent productions (with Yerkes, the Hooker, and the Hale itself)

1917 The Hooker 2.5 m (100-in)  Mt Wilson CA. The largest reflector since Rosse’s ‘Leviathan’ of 1845.  Used by Hubble and Humason in their epic survey of galactic distances and red-shifts which led to the formulation of Hubble’s Law.

1945 The Hale 5.1 m (200-in)  Mt Palomar CA. A fitting tribute to George Ellery Hale and completed after his death.  It was the largest effective telescope in the world until the advent of Keck I in 1993.

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