Where is the largest optical telescope in the world?

The Gran Telescopio Canarias (GTC) is the largest filled-aperture optical telescope. It has a mirror with a diameter of 10.4 meters (34.1 feet), and is located at the Roque de los Muchachos Observatory on La Palma in the Canary Islands of Spain.

A close second are the twin W. M. Keck telescopes on top of the dormant volcano Mauna Kea in Hawaii. At an elevation of 13,800 feet, the two Keck telescopes each have a diameter of 33 feet.