Telescopes are like boats: as soon as you own one, you want a bigger one.

But if you just want to look at the stars, there is plenty to see at the Naval Observatory in Washington, D.C. Monday nights at 8:30, public tours are given for the the first 90 people who arrive. Depending on the weather, they either look through or at some of the telescopes there.

And the Smithsonian Associates have been taking people there for years. In fact the observatory holds classes four times a year for 60 lucky Associates. I was even luckier: I had a private tour with Geoff Chester, a physicist who worked for years at the National Air and Space Museum before coming to the observatory, where his great-grandfather, Colby M. Chester, was once the superintendent.

The Naval Observatory is set on a knoll on the west side of Washington and has a mansion on its grounds that has become the Vice President's residence.

The observatory is a wonderful old building with the original 15-foot ceilings, broad staircases and 19th-century air-conditioning, which mainly consists of large doorways and windows. At one end stands one of the finest astronomical libraries in the Western Hemisphere, with a fountain, a balcony and 80,000 volumes dating as far back as 1482.

The rest of the building houses astronomers and the Navy oceanography staff, evidently on the theory that the earth's blanket of air is a sea, too. The observatory, designed by Richard Morris Hunt (who also created the base of the Statue of Liberty), and the mansion, designed by Leon Dessez, were completed in 1893. For years the superintendent of the observatory lived in the house.

Unfortunately for him, in 1928 the Chief of Naval Operations happened to go to a party there and liked the place a lot. Since the chief had four stars and the superintendent was a mere captain, the latter soon found himself moving out while the former moved in.

Not until 1977 did the Vice President actually reside at the mansion, in the person of Walter Mondale. The telescopes fascinated him, too. One night when Chester was studying Jupiter on a 5-inch scope in one of the outer buildings, he heard a knock on the door. And a voice. "What you looking at? Can I have a look?"

He let the visitor in. It was Mondale.

 That's nothing. Once many years ago when the observatory was located in Foggy Bottom, the great astronomer Asaph Hall was up on the roof peering through a 9.6-inch scope. Suddenly the trapdoor opened and a certain familiar visage came into view: President Abraham Lincoln, wandering the town by night, had stopped in for a look.

Malarial Foggy Bottom, near the Potomac, was not the best spot for viewing the heavens but was still better than 17th and G streets, where the first observatory was erected in 1830.

We checked out the 12-inch scope in the present observatory's dome, a classic refractor built in 1895. This is the one the public usually gets to look through.

"This was used extensively until the '50s," Chester said, "when it was replaced by a camera that photographed the moon against the stars. You need the star background to determine the exact orbit of the moon, which, for instance, was kind of vital for the Apollo program. You wouldn't want to find yourself a mile off your calculations as you came in for a landing. After Apollo was finished the scope was dismantled." Later the staff rebuilt the scope, which Chester says is hard to beat for clarity of detail.

"It doesn't have the aperture of the 26-inch telescope, of course, but on any given night, sky conditions favor this over the larger one for a sharp picture of, say, the surface of Mars. It's the optics." The lenses for both scopes were made by Alvan Clark and Sons, and apparently Clark improved his skill in the 22 years between these jobs.

"Clark really had a knack for working glass," Chester added. "He would test a lens in his workshop, sight a star with it and throw it out of focus so he could see where the defects were. Then he would put some optical rouge on his thumb and actually feel where the error was, the tiny bump in the surface, and polish it away."

On the nights when it's too cloudy to see the stars, visitors examine the 26-inch telescope instead, an impressive sight at 30 feet long. To reach it, they must ride Washington's largest elevator: the entire floor rises and falls inside the observatory's 40-foot-wide dome. It's cold in the dome in winter, and warm in summer, because air currents that distort the view are created if the temperature is not the same inside and out.

This scope is the one with which Asaph Hall discovered the moons of Mars, Deimos and Phobos, in 1877. It is so precise that it is still used for measuring binary, or double, stars. Many measurements for the Washington Double Star Catalogue - a database that goes back over a century - were made with this instrument.

Not so long ago this data was collected the hard way, with repeated observations and notes penciled in by hand. Now, with a camera that videotapes a star area 30 times a second, and computers that interpret the data, we can fix the parameters of binary stars and determine orbit and mass in a matter of minutes. It is this function - finding the position of objects in the sky with maximum exactness, plus determining time with great precision - that is the task of the Naval Observatory.

We looked at the little meridian transit scope with a 6-inch aperture. This one is fixed on a north-south axis and measures the east-to-west movement of the stars as they cross one specific meridian. "It's probably the most accurate anywhere, said Chester, and was used for a century, until it was superseded two years ago by more sophisticated instruments."

The observatory has compiled the world's largest star catalogue, with half a billion entries contained on ten CD-ROMs. Knowing the position of stars is especially useful if, for example, you're monitoring a new celestial satellite for which you haven't been given the orbit. You need a bunch of star positions as the background against which you can work out its path.

Now we are looking at the Master Clock of the United States, accurate to a billionth of a second per day, with a roomful of computers across the hall to serve as a backup. We can get extremely accurate time by measuring the earth's rotation and breaking it down into fractions of a second. Because the earth is slowing down, every 500 days or so we declare a leap second to make up for the lag.

For those who do not need to consider the earth's rotation to measure time, cesium beams and hydrogen maser clocks, or "atomic" clocks, give us an almost perfect reading.

Chester has loved stars since he was 7 years old and saw Willy Ley's book The Conquest of Space with Chesley Bonestell's great illustrations of the planets.

"For the past ten years," he said, "I've been doing summer stargazing programs for the public out at Sky Meadows State Park near Paris, Virginia, 50 miles from Washington, D.C. That's how far out of town you have to go to see stars."

On the other hand, as instructor Richard Schmidt pointed out, for his Smithsonian Associates classes the city starscape will do well enough for instruction. There is always something to look at, a nearby planet or a constellation. There are star maps to study and slides that show what you can't see if it's cloudy.

Believe it or not, you can have too many stars up there if you're trying to point out the important ones, Schmidt said. A whole skyful of stars can be bewildering.

Bewildering but, as my grandson says, awesome.

By Michael Kernan