World's largest telescope

The twin Keck Telescopes atop Mauna Kea, Hawaii. 

A plan by California and Canadian universities to build the world's largest telescope at the summit of Hawaii's Mauna Kea volcano won approval from the state Board of Land and Natural Resources on Friday.
But the telescope may not hold the world's largest title for long. A group of European countries plans to build the European Extremely Large Telescope, which will have a 138-foot (42-meter)-long mirror.
The University of California system, the California Institute of Technology and the Association of Canadian Universities for Research in Astronomy are spearheading the telescope.
The University of Hawaii is involved because it leases the summit land from the state of Hawaii.
Mauna Kea's peak already hosts about a dozen telescopes.