Keck observatories. Domes of the Keck I (at left) and Keck II optical telescopes seen by moonlight at the Mauna Kea observatory in Hawaii, USA. Both domes are open. Star trails are seen in the sky. Each telescope has a 10 metre diameter mirror to focus light. The mirrors are made up of 36 hexagonal units with a 1.8 metre diameter which are mounted on computer-controlled motors that make tiny adjustments to keep astronomical objects in precise focus. The 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.