TO A BORN entrepreneur, that theater sitting there vacant all summer at the new Sidwell Friends School art center was a crime.

So a couple of teachers have launched Washington's newest repertory movie palace: the Sidwell Cinema.

With 415 seats and a big screen that makes 16 mm look like 35, the theater at 3825 Wisconsin Ave. has everything but popcorn.

Tonight at 8 "The Magic Christian," that Terry Sothern curiosity with Peter Sellers, will be featured. Wednesday and Thursday it's a Max Fleischer animation festival concentrating on his more interesting surrealistic work. And over the weekend there will be a Woody Allen double bill.

"The Sidwell Film Society, which is run by the students, has been showing films every other Saturday night here since the place opened last fall," said Ellis Turner, who with George Lang (no relation to Fritz) runs the cinema. "Everyone liked it so much that we thought we'd try this in the summer."

The full-time season lasts through Aug. 7, when it will revert to the biweekly schedule. Since the theater advertises, it pays the regular commercial rate on rentals, which is usually the big problem for people who want to start a small-scale repertory theater. So far, things are going well for Sidwell, situated as it is in a theatrical dead area far from the crowded streets of Georgetown and Friendship Heights.

"We have a good time picking films," said Turner. "We're both film nuts."

This month will see a '50s festival, with Jayne Mansfield in "The Girl Can't Help It" and appropriate shorts, plus a Paranoia Night featuring the classic anti-marijuana picture "Assassin of Youth" and Jack Webb's 1957 "Red Nightmare." There will be a Dada festival, too, Man Ray, Melies, Clair and Duchamp,and a nostalgic look at movies of small-town America--with a Preston Sturges comedy for starters.

Already the Sidwell has drawn good houses with Patriotism Night, a vehicle for Ronald Reagan, George Murphy and Kate Smith in "This Is the Army" plus some Nixon speeches, not to mention "Sunset Boulevard," "All About Eve," Hitchcock and Bunuel.

"We want to make it a sort of community thing," remarked Lang, "and we're encouraging people to suggest pictures. We have two shows a night sometimes, but we find the hours are kind of hard for audiences, either too early or too late, so we've been experimenting with doing one show at 8."

The one thing that amazes him is the number of people who ask about popcorn (which for a large number of commercial theaters is literally the margin of profit).

The entrepreneurs hope people will take their movies neat. This radical notion, pioneered here by the American Film Institute Theater, means you can enjoy a picture without feeling you're in a horsebarn at feeding time, and when you get up, your feet don't slip out from under you in a sea of butter.