What is the largest telescope?
The largest visible-light telescope currently in operation is at Gran Canarias Observatory, and features a 10.4-meter (34-foot) primary mirror.
The Hobby-Eberly Telescope at McDonald Observatory near Fort Davis, Texas, has the world's largest telescope mirror. Because of the way HET is designed, however, astronomers use only 9.2 meters of the 11-meter (36-foot) mirror at any one time, making HET the world’s fourth-largest telescope. Scheduled upgrades to the telescope, however, will improve its performance to that of a 10-meter telescope.
Two larger ground-based mirrors are in the planning stages: the Giant Magellan Telescope and the Thirty-Meter Telescope. The first would consist of eight individual mirrors working together, while the latter would consist of a large segmented mirror. Each would have an effective aperature of roughly 30 meters (100 feet), giving them as much surface area as a small office building.
Hubble Space Telescope looks at the nether regions of the universe with a 2.4-meter mirror. The James Webb Space Telescope, which NASA plans to launch as early as 2013, will have an eight-meter (25.6-foot) primary mirror.
The largest refracting telescope in the world is at Yerkes Observatory in Williams Bay, Wisconsin. Instead of a mirror, it gathers light with a 40-inch glass lens.
Astronomers also gather radio waves from space using dish-shaped antennas, the largest of which is the Arecibo Observatory in Puerto Rico. Featured in the movie “Contact,” Arecibo's dish is 1,000 feet in diameter.