California scientists, celebrating the largest private grant for a scientific project, today announced plans to build the world's biggest optical telescope, with a mirror 10 meters in diameter and sitting atop an extinct Hawaiian volcano.

The $70 million grant from a foundation established by the late W.M. Keck, an oil wildcatter who founded Superior Oil Co., would allow astronomers to look 12 billion years into the past and probe the origins of the universe, California Institute of Technology President Marvin L. Goldberger said.

The mirror is to be placed in a new dome on a 13,600-foot ridge of Mauna Kea volcano, and its 394-inch diameter would be a giant leap from Caltech's 200-inch Hale Telescope at Mount Palomar, Calif., or the reportedly flawed Soviet six-meter Velenchukskaya Observatory telescope in the Caucasus Mountains.

The 10-meter telescope will be so sensitive, Keck Foundation Chairman Howard B. Keck said, that it could detect the light of a single candle on the moon. It would have four times the light-gathering and magnification power of the Mount Palomar instrument.

To keep the enormous Keck Telescope from sagging under its own weight, scientists said they plan to use new technology developed at the University of California's Lawrence Berkeley Laboratory that allows construction of the mirror in 36 separate hexagonal sections.

A scientific breakthrough known as "stress-polishing" is expected to give each section the proper surface, and each is to be controlled by a computer that can make as many as 100 adjustments a second of as little as one-thousandth the width of a human hair.

Goldberger, whose snagging of the record grant made him the envy of college presidents nationwide, today joined a Caltech conference-room gathering that included Keck, University of California President David P. Gardner and some of the state's most prominent astronomers to celebrate what Goldberger called a "historic occasion."

Goldberger said he could not find evidence of a previous such grant even as large as $50 million. The grant is contingent on specific contractual agreements between Caltech, the University of California and the Keck Foundation, but the principal officers said they anticipate no serious problems.

Under a proposed agreement, Caltech would provide the $85 million estimated construction cost from the Keck grant and other sources, while the University of California would contribute funds for operation of the facility, expected to be completed by 1992. The two schools would share use of the facility with scientists from the University of Hawaii, which owns the building site.

The National Optical Astronomy Observatories in Tucson called for construction of an even larger device, perhaps as wide as 15 meters, but that project has yet to be funded.

Caltech officials called the volcanic ridge on the island of Hawaii "the best observatory site in the world."

Several other telescopes there enjoy a usually stable, dust-free and cloud-free sky. More than two miles above sea level, the site allows excellent readings of infrared light, usually diluted by water vapor in the lower atmosphere.

Jerry Nelson, the University of California astronomer who has led the team that is planning construction of the new telescope, said the remote site would also reduce risk of light pollution, such as the glare from high-voltage lighting in new housing developments that has hampered work at Mount Palomar.

Although astronomers have emphasized expected bonuses from space-based telescopes, such as the Hubble Space Telescope to be launched in 1986, Nelson noted that the proposed telescope could see much farther at far less cost than any telescope put in orbit.

The new telescope would complement the space telescope, he said, by making follow-up measurements of new objects detected from space.

Nelson said the instrument would aid in the search for planets around other suns. With electronic recording instruments that have replaced the human eye in "seeing" through modern telescopes, the new telescope should be able to detect light from objects as far as 12 billion light years away, Goldberger said.

It will thus be looking that far back into the history of the 16-billion-year-old universe, studying infant galaxies and primordial clouds of gas and dust where stars were born.

Its infrared detectors could probe dark interiors of interstellar clouds and even the mysterious center of Earth's galaxy.

William Myron (W.M.) Keck established his foundation in 1954 after working as a drilling crewman and risk-taking foreman and creating a multinational oil empire. He had made use of several scientific innovations, such as the reflection seismograph, in making his fortune and directed the foundation to support educational research in earth sciences, engineering and medicine.

He died in 1964 at age 84, but the assets of his foundation and trust continued to grow and total more than $500 million. His son, foundation chairman, said its largest previous award was $7 million to the Colorado School of Mines.