The biggest telescope in the world is the Large Binocular Telescope located in southeastern Arizona. Like its name suggests, it uses two 8.4 meter mirrors mounted side by side to produce one image with the light gathering capacity of an 11.8 meter aperture. The massive, multimillion dollar project began in 1996 and took its first look at the sky in March 2008.
The LBT beats out another brand new project, the Grand Telescopio Canarias, a 10.4 meter telescope located on La Palma in the Canary Islands. This telescope took 7 years to complete and has had its “eye” open on the sky for less than one year. From its 2400 meter mountain perch, it is poised at one of the best locations in the world for astronomical viewing. It is scheduled to officially open sometime in May 2008.
As a point of reference, the W.M. Keck Observatory on Mauna Kea in Hawaii has two, identical 10 meter telescopes that can work individually or together as one large telescope. Keck I was operational in 1993 and Keck II became operational in 1996.