The European Extremely Large Telescope (E-ELT) project is the largest funded optical telescope in the World. With a diameter of 42 m and being fully adaptive from the start, the E-ELT will be more than one hundred times more sensitive than the present-day largest optical telescopes. Decision to build is expected by late 2010, with a start of operation in 2018. The European Southern Observatory has announced that the E-ELT will be built high on Cerro Armazones, a 3060-metre peak near the heart of Chile's Atacama desert.
The Thirty Meter Telescope (TMT) has also been funded and the Giant Magellan Telescope (24.5 meter diameter)
The Giant Magellan Telescope (GMT) is a ground-based telescope planned for completion in 2018. It will consist of seven 8.4 m (27.6 ft) diameter primary segments, with the resolving power of a 24.5 m (80.4 ft) primary mirror and collecting area equivalent to 21.4 m (70.2 ft) one. The telescope is expected to have over four times the light-gathering ability of existing instruments.
The current largest optical reflecting telescopes is the large Binocular telescope with 11.8 meters of effective apperture. The E-ELT will have 13 times the light gathering area. The E-ELT would have the area of three basketball courts and almost the area of a north american hockey rink (1385 square meters is 87% of the 1586 square meters.)
Wikipedia has a list of telescopes under construction or proposed and planned.
Science and technology drivers for future giant telescopes (10 page pdf)
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