Not enough faint stars to make up the Milky Way's dark corona. On Mauna Kea, Hawaii, altitude of 14,000 feet. Started operation in 1993. Keck's major strength is its light gathering power , which comes from its enormous main mirror. It can take very deep images, but its greatest importance is for measuring spectra of faint objects, where every photon counts.
Quasars are the most luminous objects in the universe.
They can thus be seen at enormous distances, in light they emitted when the universe was much younger. Hydrogen gas clouds at different redshifts produce absorption lines in spectra of background quasars. Keck can examine these absorption lines with unprecedented accuracy and detail. Summary is at end of lecture 27.