Keck II mirror cleaning. Technician uses a jet of pressurised carbon dioxide to clean the primary mirror of the Keck II Telescope, the largest optical telescope in the world in 1997. The mirror is 10 metres in diameter, 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.
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