Keck II telescope mirror. The primary mirror (at right) and its supporting frame of the Keck II Telescope, the largest optical telescope in the world in 1997. Overall hexagonal in shape and 10 metres in diameter, the mirror is composed of 36 smaller hexagonal mirrors each 1.8m across. Each small mirror is computer controlled by a network of pressure sensors and motorised actuators to maintain the correct position of the entire mirror. The Keck II Telescope is twinned with Keck I and both can combine their signals to obtain a more accurate image. The Keck Telescopes are situated at 4200 metres altitude on the summit of the dormant volcano Mauna Kea in Hawaii, USA.