Keck observatories. Domes of the Keck I (at right) and Keck II (far left) optical telescopes at sunrise at Mauna Kea in Hawaii, USA. Other obser- vatories are seen in the background including the NASA Infrared Facility (lower centre left) and the Canada-France-Hawaii Observatory (lower centre). Each Keck dome contains a telescope with a 10 metre diameter mirror to focus light. These two telescopes can be used to observe the same object and can have their signals combined by interferometry to obtain a more accurate image. In 1997 the Keck telescopes are the largest optical telescopes in the world.