About Keck

Mirror

A telescope tracks objects, sometimes for hours, across the sky as the Earth turns. This constant but subtle movement results in slight deformations of the telescope structure despite all engineered precautions. Without active, computer-controlled correction of the primary mirror, scientific observations would be impossible.

For the Keck telescopes, new techniques for manufacturing, polishing and testing their mirror segments had to be invented, including “stressed mirror” polishing. Each segment’s surface is so smooth that if it were the width of Earth, imperfections would be only three feet high.

On the telescope, each segment’s figure is kept stable by a system of extremely rigid support structures and adjustable warping harnesses. During observing, a computer-controlled system of sensors and actuators adjusts the position of each segment - relative to its neighbors - to an accuracy of four nanometers, about the size of a few molecules, or about 1/25,000 the diameter of a human hair. This twice-per-second adjustment effectively counters the tug of gravity.

Ever since their invention nearly 400 years ago, Earth-based telescopes have suffered from image blurring caused by the turbulent atmosphere above them. This is true of even the world’s best observatory sites like Mauna Kea, though to a considerably lesser extent than elsewhere. In recent years, advances in optical and computing technology have made it possible to greatly reduce this blurring through the use of “adaptive optics” (AO). At the heart of the AO system is a six-inch-wide deformable mirror that changes its shape up to 2,000 times per second to cancel out atmospheric distortion; the resulting images are therefore ten times sharper than previous images taken with the telescopes. Successfully installing AO systems on both Keck telescopes has made it possible for Keck astronomers to study objects in far greater detail than ever before.

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