Near the summit of Mauna Kea, a long‐dormant volcano on the island of Hawaii, the first of the two Keck Telescopes has begun doing astronomy, and construction of its identical twin, a stone's throw to the north, is well under way. The individual Keck telescopes, with effective primarymirror diameters of 10 meters, are by far the largest optical telescopes ever built. And together they will serve as an interferometer with a baseline of 95 meters.