Telescopes from the Ground Up

Twin telescopes provide extra viewing power

In 1977, the University of California and the California Institute of Technology teamed up to build a reflecting telescope that would be twice as large as the Hale telescope at Palomar Mountain, which has a 200-inch mirror.

The task wouldn’t be easy — or perhaps even possible. A mirror that large could cost as much as a billion dollars, and would be so heavy that it would be hard to keep it from collapsing under its own weight.

Piece by piece

So the team came up with another idea. They would build a telescope that had a single mirror made up of 36 small, thin mirror pieces, called segments. These segments would each be 6 feet (1.8 meters) in diameter, hexagon-shaped, and linked together in a grid the size of a tennis court. Computers would keep all the mirrors lined up. Together, they would weigh only about as much as the 200-inch mirror, but be four times its size.

The team received a donation from the William M. Keck Foundation, and started work in 1985.

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Multi-mirror Telescopes
Map of Mauna Kea, Hawaii where the Keck telescopes are located.
Two images of one of the Keck telescopes: and exterior view and a closeup of the primary mirror array.Enlarge picture
Keck I
Year completed: 1993
Telescope type: Reflector
Light collector: 36 aluminum-coated glass mirror segments
Mirror diameter: 400 inches
(10 m) total
Light observed: Visible
Discovery Highlights:
  • Discovered the first visual evidence of a brown dwarf, a failed star. With its light-gathering power, found planets around other stars.