GETAWAYS : Focus on the Stars : * For 30 years, the 100-inch reflector telescope atop Mount Wilson reigned supreme. Now those heady days can be relived on weekend tours.

Driving the Ventura Freeway from Woodland Hills east you see Mount Wilson rearing up ahead in the San Gabriel Mountains’ front range, a nearly flat ridge bristling with steel transmitter towers. Half a century ago what might have caught the lowlander’s eye instead was the sun glinting on a bright observatory dome.

The observatory held the world’s largest telescope. Visitors from far and near found such high-tech hardware as fascinating as its product. The impact was doubly powerful, for both hardware and product altered the course of human knowledge. Now you can relive those heady days on a Saturday or Sunday afternoon tour by the Mount Wilson Observatory Assn.

Installed in 1917, the giant 100-inch Hooker reflector reigned supreme for 30 years. As Los Angeles slept below, Edwin Hubble and others made modern astronomy’s basic discoveries: billions of galaxies where none were known before, most of them moving away from us at astounding velocities. From these observations grew the Big Bang theory of how the universe began, which Albert Einstein--in town in 1930--endorsed.

Mount Wilson saw other feats. The Hooker observed gravity’s effect on light as predicted in Einstein’s theory of relativity. Hubble witnessed a supernova. Solar telescopes detected the first magnetic action outside Earth. Albert Michelson first measured a star’s diameter, and he set up mirrors on Wilson and on Old Baldy 24 miles away to make an accurate measurement of the speed of light, beamed there and back.

Grand in scale to suit its cosmic mission, mile-high Mount Wilson is one of the front range’s great vista points, out and down to the planet’s surface, in and up to the high country, up to the firmament. That sense builds up on the approach road along the steep Arroyo Seco to its headwall at Red Box Gap, with its awesome view east across an abyss to the snowy patriarch, Old Baldy.

From there Mount Wilson Road climbs through forest past crumbly white rock cliffs to where a white dome gleaming above the trees announces the summit. So does a forest of two dozen radio and television towers, since 1948 targeting the widest possible expanse of lowland receivers.

The observatory tour takes you through oak, pine, incense cedar and big-cone spruce--some of them truly massive old giants. You see three solar telescopes. The oldest is horizontal, the others are 60- and 150-foot towers--where the tour might look in to see the sun’s image. A small museum shows stunning celestial photography. You pass a 60-inch reflector telescope, the world’s largest when installed in 1908.

The 100-inch telescope dome is unmistakably huge. Inside, your footsteps resound on several flights in a metal stairwell up to a small gallery where you peer through glass at the imposing instrument towering above in its dim steel cavern.

During the tour you will learn of the mountain’s colorful past. Of Benjamin D. (Don Benito) Wilson, the mountain’s namesake, and his 1864 trail. Of a cool, forested upland and a succession of hikers’ camps, resorts, hotels and a tourist park that closed in 1976. Of the narrow 1891 toll road up from Altadena, in use until a paved road opened in 1935.

Word got around of Mount Wilson’s clear sky and air, and in 1889 Harvard University briefly operated a telescope. Then astronomer George Ellery Hale of Chicago’s Yerkes Observatory persuaded the Carnegie Institution of Washington to establish an observatory with himself as director. It opened in 1905 on 40 acres leased from the toll-road company and within a dozen years it had six telescopes.

The big Hooker’s startling finds at the outer limits of its vision raised questions that led to a 200-inch reflector on Mount Palomar, 130 miles away, inaugurated in 1948. For decades it was the world’s most powerful, and the two observatories operated in partnership until 1980.

But sky glow from Los Angeles began to impede deep-space work at Mount Wilson and new knowledge demanded a longer reach than even Palomar’s. Technology shifted toward today’s sophisticated optical, radio and infrared instruments, on the ground and out in space.

Mount Wilson’s role shifted too. The solar and the 60-inch telescopes are in use but the big one is not.

Now being refitted with new-generation adaptive optics, it is to be back in action later this year. Since its sharp imaging and the site’s unusually stable atmosphere fit it for such tasks as the search for planets around nearby stars, stay tuned. After a glorious past, the present operator, Mount Wilson Institute, expects a long working future.

WHERE AND WHEN

Location: Mount Wilson is 18 miles from La Canada Flintridge via Angeles Crest Highway and Mount Wilson Road.

Hours: Grounds open 10 a.m.-4 p.m. weekends. Free one- to two-hour observatory tours begin near parking lot at 1 p.m. Saturdays and Sundays, April to October.

Call: In doubtful weather, call (818) 793-3100 to confirm the tour.

Tip: Bring lunch and something to drink; nearest food service is down in town.

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