The telescope's light-gathering mirror, much lighter and cheaper than a conventional mirror of the same size would be, is relatively flexible.

The diameter of the Hubble's light-gathering mirror is 94.5 inches, while the diameter of the Keck's segmented mirror will be 33 feet.

But a flaw, apparently in the Hubble's main light-gathering mirror, has prevented the 43-foot telescope from functioning properly.

The finished instrument, which will cost about $187 million, will consist of four optically linked telescopes, each one containing a light-gathering mirror 8.2 meters (27 feet) in diameter.

The $1.5 billion space telescope, its vision slightly impaired by a flaw in its main light-gathering mirror, has lost the use of two of its six principal gyroscopes.

On Thursday, two astronauts are to make a space walk to try out tools to repair the $1.5 billion Hubble Space Telescope, whose main light-gathering mirror is flawed.

Perkin-Elmer has repeatedly denied any wrongdoing and said the Government was fully informed of all data needed for judging whether the telescope's main light-gathering mirror had any imperfections.

As director of Cerro Tololo, Dr. Williams supervises a huge instrument with a light-gathering mirror four meters in diameter.

Its main optical component is a light-gathering mirror 6 meters, or 236 inches, in diameter.

The new observatory, while only half as big as the 24,000-pound Hubble, will have a primary, light-gathering mirror 20 feet in diameter compared with the existing telescope's 8-foot reflector.