A folksy, funky wiseacre who is widely credited with the popularity of backyard astronomy, John Dobson created the do-it-yourself design that made it possible for anyone to build powerful amateur telescopes. Now at age 89, he still spends many nights on San Francisco street corners, enticing suspicious passers-by to "come see the moon!" in his beat-up old 14-inch scope, made with a large cardboard tube and some concave mirrors on a pivoting mount.

"A Sidewalk Astronomer" is both an informal biography (his time spent as a monk in a Hindu monastery comes up as if in passing) and a tantalizing primer on the science that Dobson has spent his life bringing to the people. Director Jeffrey Jacobs combines footage of Dobson at outdoor stargazing conventions, in telescope-making workshops and in casual classroom lectures, as his layman's explanations of the universe (he makes sound arguments against the Big Bang theory and calls God "the exterior decorator") are accompanied by breathtaking images (mostly from the Hubble Space Telescope, not Dobsonian scopes) that zoom in, and in, and in on sun spots, planets, galaxies and nebulae, revealing their wondrous structures.

Although the filmmaking is basic and Dobson expresses some off-kilter and occasionally confusing notions, Jacobs indulges his subject's goofy spirit, and this documentary is better for it. If "A Sidewalk Astronomer" can't turn you on to astronomy, I feel sorry for you.