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:

1. As you can see in the image, Herschel had NO dome. Everything was so much simpler without a dome.

2. Herschel just stood at the top end of the tube and viewed the real image created by the primary mirror with an eyepiece he held in his hand???---simple, but it worked (Wikipedia: Optical telescope: Principles).

3. As you can see, the telescope was supported by quasi-permanent scaffolding, pulleys, and rope.

4. The telescope has an equatorial mount and can be slewed a fair ways along the (celestial) meridian (i.e., along the north-south direction).

   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.

5. The 20-foot telescope (reflector, primary diameter 18.5 inches ≅ 47 cm) is, of course, a reflector: i.e., its primary is a mirror, NOT a lens as for reflectors.

6. Most professional telescopes up to circa 1900 were refractors. Reflectors have advantages which would cause them to win out as the dominant telescope after circa 1900 (Wikipedia: Reflecting telescope: History).

   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.

7. Early reflectors were all made of speculum for the mirror.

   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, operational 1668--c.1731) based on earlier ideas by Scottish mathematician and astronomer, James Gregory (1638--1675) published in 1663 (North 1994, p. 347).

8. The 20-foot telescope (reflector, primary diameter 18.5 inches ≅ 47 cm) was probably Herschel's most useful telescope. The 40-foot telescope (reflector, primary diameter 1.22 m ≅ 48 inches, operational 1787--1815)---the record largest telescope of its time---was simply difficult to use because of its size given Herschel's primitive telescope mounting

   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).

9. The 40-foot telescope (reflector, primary diameter 1.22 m ≅ 48 inches, operational 1787--1815) was surpassed by the Leviathan of Parsonstown (reflector, primary diameter 1.83 m = 6 ft, operational 1845--c.1890) in 1845.

Credit/Permission: Anonymous artist, later 18th century / Public domain.
Download site: Wolfgang Steinicke's List of NGC/IC observers.
Image link: Itself.
Local file: local link: telescope_william_herschel.
File: Telescope file: telescope_william_herschel.html.