1993

Keck Telescope

Jerry Nelson (b. 1944)

The 100-inch (2.5 meter) Hooker telescope reigned as the world’s largest telescope for three decades, until the 200-inch (5 meter) Hale telescope on Mt. Palomar finally surpassed it. The Hale telescope was the world’s largest for nearly three decades more until surpassed by the Keck telescope, which began to make observations in 1993. And to do it, engineers working on the Keck telescope had to completely reconceptualize the mirror at the core of the machine. The problem with the Hale telescope is that it uses a single piece of glass to form the mirror. Even though the back of this mirror is honeycombed to save some weight, the disk is incredibly large, heavy, and problematic. Making an even bigger mirror from a single piece of glass will probably never happen.

So how do engineers make bigger, precisely curved mirrors? The Keck telescope solves this problem by being made of 36 smaller hexagonal mirrors, designed by astrophysicist Jerry Nelson, that fit together to form one immense 10-meter (400 inch) disk. In addition, the curve of each hexagon can change at a nanometer level multiple times per second in order to accomplish something called active optics. The purpose of active optics is to give a mirror the correct shape despite the effects of factors like gravity, temperature changes, and mirror movement. Mechanisms on the back of the Keck’s mirror segments change their shape.

This active system forms a perfectly shaped mirror and combines with an adaptive system to handle changes in the atmosphere over the telescope. The adaptive system looks at the twinkling of a bright star and uses it to adjust a mirror and remove the twinkling. If a bright star is not visible in the field of view, it is also possible to use a laser to create an artificial star in the upper atmosphere to serve the same purpose.

In 1996, a second Keck telescope was built, nearly identical to the first. The two can work independently. Precision engineering allows the light from the twin mirrors to combine to create an even bigger telescope. The combination gives the Keck telescope surprising power, and it has acted as a template for even larger telescope designs.

SEE ALSO Hooker Telescope (1917), The Hubble Space Telescope (1990).

Keck telescopes on the summit of Mauna Kea, Hawaii.