The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 1,740-metre peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles. The observatory contains two historically important telescopes: the 100-inch , which was the largest aperture telescope in the world from its completion in 1917 to 1949, and the which was the largest telescope in the world when it was completed in 1908.