Mount Wilson Observatory

Mount Wilson Observatory (MWO) on Mount Wilson (1740 meters), Las Angeles County, California, is the home of a number of historical telescopes as well as some 21st-century optical interferometers. Though light pollution affects the site, the same weather inversion layer that holds smog within the city does so by preventing it from rising to where it would affect astronomical observation. Among the observatory's historical telescopes:

• Hooker Telescope - 100-inch reflector telescope, completed in 1917, the largest aperture optical telescope until the Hale Telescope was completed in 1949.
• 60-inch telescope (1905).
• Snow Solar Telescope - 24-inch telescope with an 18-m focal length (1905).
• 60 foot solar tower (1908).
• 150 foot solar tower (1912).
• 20 foot interferometer (1920) - it was a pair of periscopes that directed light from 20 feet apart into the Hooker Telescope, within which the beams were blended to create interference.
• 50 foot interferometer (1929) - it was similar, but built around its own dedicated 40-inch telescope.

The three solar telescopes used their tower height to accommodate their long focal lengths. Modern interferometers hosted at the site:

• Infrared Spatial Interferometer (ISI, 2003).
• CHARA (Center for High Angular Resolution Astronomy, 2003).