Caption: The Hooker telescope (reflector, primary diameter 2.54 m = 100 in, operational 1917--present) at Mount Wilson Observatory in Los Angeles County, California (see also the Mount Wilson Observatory page).

Features:

1. Hooker telescope was used by Edwin Hubble (1889--1953) to discover that galaxies were galaxies in 1924 (see Wikipedia: Edwin Hubble: Universe goes beyond the Milky Way galaxy) and to discover the general expansion of the universe and Hubble's law in 1929 (see Wikipedia: Edwin Hubble: Redshift increases with distance; Wikipedia: Expansion of the universe: History). When this photograph was taken in 1989, the Hooker telescope had just been mothballed. It was re-activated later for research, but since 2014, it has been used for visual astronomy for public outreach.

2. By the by, the Hooker telescope was the largest telescope (i.e., had the largest primary mirror/lens and, therefore, the most light-gathering power) from first light in 1916 until 200 inch (5.08 m) Hale telescope was completed in 1949 at Palomar Observatory, San Diego County, California.

3. Essential to Hubble's discoveries was the fact that he had access to the largest telescope of his day in Southern California which is intrinsically one of the world's best observing sites. However, air pollution (e.g., smog) and the light pollution of Los Angeles have greatly diminished the seeing (i.e., observing qualities) in the vicinity of Los Angeles.

4. Before the Hooker telescope, the largest telescope was the Leviathan of Parsonstown (reflector, primary diameter 1.83 m = 6 ft, operational 1845--c.1890). Before the Leviathan, the largest telescope was William Herschel's (1738--1822) 1.26-m reflector telescope (operational 1789--1840) which was a bit too large for Herschel to use easily.

5. All these giant telescopes were reflector telescopes: i.e., they used concave mirrors as a primaries. Herschel's 1.26-m reflector telescope and the Leviathan for the primary used speculum, 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). Speculum is brittle, tarnishes easily, and reflects only about 16 % of incident light.

6. The Hooker telescope and Hale telescope have glass primaries (for high rigidity) and have reflective coatings of aluminum (Al). Before sometime in 1935, the Hooker telescope used a silver (Ag) (see Wikipedia: Mount Wilson Observatory: 100-inch Hooker telescope).