Geoffrey Chester is starstruck, has been since he was seven and saw his first heavenly body. Anything cosmic or mindboggling, like the wonders of the universe, say, falls within his orbit of specialty.

Chester, vice president of the National Capital Astronomers, will be stargazing at Lake Fairfax this Saturday night. His eye on the sky is a homemade, eight-inch reflecting telescope made of scrap parts and stuff he bought at Hechingers. Jim Crowley, also of the NCA, will bring along a six-inch telescope for peering at the phases of Venus, the rings of Saturn, the moons of Jupiter and, of course, the craters of the moon.

"If it's clear," says Chester, "We'll be looking for some far-out galaxies and clusters. There'll be globular clusters, giants that orbit the hub of the galaxy -- great spherical clouds with 500,000 to a million stars. They'll appear as very compact masses of pinpoints of light. It took from 25,000 to 50,000 years for the light to get to us. And you can see this in my telescope."

You can also see such astral reflections in Rock Creek Park, where NCA president Robert McCracken and other members regularly set up their telescopes for all comers in a field near Glover and Military roads. Despite city lights, it's possible to see the planets and, later in the summer, a star called Albireo in Cygnus (the Swan). "It looks like one star to the naked eye, but through a telescope it's seen to be two lovely stars, one a golden color and the other a beautiful blue," says Chester. On cloudy nights, there's a constellation prize -- a sky show in the park's Nature Center planetarium, where you learn about Orion, Gemini, Cassiopeia, the Dippers and so on.

"I consider the night sky the world's biggest national park. And it's free for anyone and always open," Chester says. "With an ordinary pair of binoculars on a dark, clear night, sweep across the Milky Way and you will find clusters of stars. If you look toward the south, there are two of the most magnificent clusters of all . . . like jewel boxes. You can see two nebula with your naked eye, like fuzzy spots. With binoculars, you'll see their stars, and with a telescope, you'll see swirls and eddys.

"It's the most magnificent structure in nature. And knowing its secrets makes the night a little more friendly." ROCK CREEK PARK -- Explore the night sky at 9 on July 16 and August 6. Meet at Grove 13 on Glover Road. Star shows will continue through September. Free. Call 426-6829. LAKE FAIRFAX -- Waterside starwatch from 8:30 to 9:30 on June 18th and again on July 16. If it's cloudy, a slide show will be presented. Meet at the camp store in the campgrounds. Call 759-3211. ALGONKIAN REGIONAL PARK -- A naturalist will give a talk on the constellations at 9 p.m. on August 13. Call 450-4655. MEADOWSIDE NATURE CENTER -- This Rockville regional park, which offers planetarium shows, all summer long, plans an outdoor "Sky Scan" on July 5 from 8:30 to 9:30 p.m. Call 924-4142 for reservations, which are required.