Two 10-metre 400-inch reflecting telescopes belonging jointly to the California Institute of Technology Caltech and the University of California. They are located at the Mauna Kea Observatories on the island of Hawaii and have been funded by the W. M. Keck Foundation. The first telescope was completed in 1992 and the second in 1996. The primary mirrors of these Ritchey-Chrétien telescopes are of a unique pioneering design, each consisting of 36 individual hexagonal segments. The precise figure of the mirrors is maintained by a combination of specially designed passive supports and active control, monitored by computer. By this means, it was possible to construct such large-aperture telescopes and assemble them at a remote mountain-top site. The use of adaptive optics makes it possible to produce images with a resolution of 0.04 arcseconds at a wavelength of 2 microns. It will be possible to use the two telescopes together as an interferometer. Because Keck I and Keck II are about 85 metres nearly 280 feet apart, they will have a resolution equivalent to a telescope with an 85-metre mirror, or about 0.005 arc seconds at a wavelength of 2 microns.