Keck observatories. Telescope domes of the Keck I (at left) and Keck II optical telescopes at the Mauna Kea observatory in Hawaii, USA. Each telescope uses a mirror with a diameter of 10m to focus light. These are comprised of 36 hexagonal mirrors with a 1.8m 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 have their signals combined using interferometry techniques to give a more accurate image. In 1997 the Keck telescopes are the largest optical telescopes in the world.