Two's company
The team built two similar telescopes on top of the dormant volcano Mauna Kea in Hawaii. The first was completed in 1990, the second in 1996. The 36 mirror segments that make up each individual mirror are connected to 168 electronic sensors and 108 motorized adjusting devices. The sensors on each segment constantly compare its height with the heights of the segments around it.
If the heights don’t match — even by a difference a thousand times thinner than a human hair — the sensors send that information to a computer. The computer calculates what has to be done to put all the mirrors back in alignment and directs the adjusting devices to make the changes. The entire process happens twice every second.
Like the Multiple Mirror Telescope, the Keck telescopes use a compact, computer-controlled mounting, called an altitude-azimuth mounting, that can move up and down or left and right, like the gun turret in a naval boat. This means that although the Keck telescopes have larger mirrors than the Hale telescope, they can be kept in much smaller buildings because they need less space in which to operate.
and Keck II.