The Keck Observatory

The Keck Observatory

From the summit of Hawaii’s dormant Mauna Kea volcano, astronomers at the W. M. Keck Observatory probe the local and distant Universe with unprecedented power and precision.

Their instruments are the twin Keck Telescopes—the world’s largest optical and infrared telescopes. Each telescope stands eight stories tall, weighs 300 tons and operates with nanometer precision. The telescopes’ primary mirrors are 10 meters in diameter and are each composed of 36 hexagonal segments that work in concert as a single piece of reflective glass.

In the middle of the Pacific Ocean, Hawai’i Island is surrounded by thousands of miles of thermally stable seas. The 13,796-foot Mauna Kea summit has no nearby mountain ranges to roil the upper atmosphere. Few city lights pollute Hawaiian night skies, and for most of the year, the atmosphere above Mauna Kea is clear, calm and dry.

Because of the large size of the 10-meter primary mirrors, the Keck telescopes offer the greatest potential sensitivity and clarity available in astronomy. Their performance, and the performance of all ground-based telescopes, is limited by the turbulence of the Earth’s atmosphere, which distorts astronomical images. Astronomers have recently overcome the affect of atmospheric blurring using an established and fundamental technique called adaptive optics (AO).

AO corrects for the image distortions by measuring and then correcting for the atmospheric turbulence using a deformable mirror that changes shape 2,000 times per second. In 1999, the Keck II telescope became the first large telescope worldwide to develop and install an AO system. The results provided a tenfold improvement in image clarity compared to what was previously possible with Keck and other large, ground-based telescopes.

Initially adaptive optics relied on the light of a star that was both bright and close to the target celestial object. But there are only enough bright stars to allow adaptive optics correction in about one percent of the sky. In response, astronomers developed laser guide star adaptive optics using a special-purpose laser to excite sodium atoms that sit in an atmospheric layer 90 kilometers above Earth. Exciting the atoms in the sodium layers creates an artificial “star” for measuring atmospheric distortions and allows adaptive optics to produce sharp images of celestial objects positioned nearly anywhere in the sky.

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 1,000 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.

Keck Observatory Article Questions

1. What types of telescopes are at the Keck Observatory?

2. Where is the Keck Telescope located?

3. How tall is each telescope? How much do they each weigh?

4. What is the diameter of the mirrors used in the Keck telescope?

5. List 2 reasons why the Keck telescope is in a good location.

6. What limits the performance of the Keck telescopes?

7. How often does the deformable mirror change shape? What is the result of that?

8. How does the special purpose laser work?

9. If the mirror was the width of earth, how high would each imperfection be?

10. What has installing adaptive optics on the Keck telescopes allowed astronomers to do?