Caption: William Herschel (1738--1822) (one assumes) and his 20-foot telescope (reflector, primary diameter 18.5 inches ≅ 47 cm) at Slough, England near Windsor Castle (see also No-400).
Features:
However, the scaffolding being aligned with celestial meridian only allowed limited east-west slewing.
However, astronomical objects within the north-south range transit the meridian daily, and so can be observed for some short time by the limited east-west slewing.
Reflectors have advantages which would cause them to win out as the dominant telescope after circa 1900. Some of these advantages were appreciated by Herschel: it is easy to make large reflectors (compared to making large refractors) and reflectors do NOT suffer from chromatic aberration instrinsically.
Speculum is an alloy of copper and tin with a dash of arsenic---which gives that je ne sais quoi---invented for reflector telescope mirrors by Isaac Newton (1643--1727).
It is brittle, tarnishes easily, and reflects only about 16 % of incident light.
Newton also constructed the first working reflector telescope, Newton's reflector (primary diameter 5 cm ≅ 2 inches, 1668--1731) based on earlier ideas by Scottish mathematician and astronomer, James Gregory (1638--1675) published in 1663 (North 1994, p. 347).
The 40-foot telescope was used in the discovery (though maybe in combination with his other telescopes) in 1789 of Enceladus and Mimas, the 6th and 7th moons of Saturn (Wikipedia: 40-foot telescope: Use; (Wikipedia: Discovery: Enceladus; Wikipedia: Mimas: Discovery).
Credit/Permission: Anonymous artist,
later 18th century /
Public domain.
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