BALTICON 13 - April 13 to 15, Hunt Valley Inn, Hunt Valley. 301/666-7000. Take I-95 to I-695 to I-83 North. The third exit is Hunt Valley Inn. Rare science-fiction firms, early science-fiction radio shows, computer games, workshops with California animator Mike Jittlov and a giant Friday the 13th costume party are on for this weekend. Call for details. Admission for all three days is $7.

"Welcome aliens," reads the printed blue card on the door. Inside, in place of an umbrella of walking stick, a sheathed black saber leans in the corner. Rainy-day jazz floats through the suburban Maryland house, deep saxophone notes stretching down into the basement where resident Earthling Jim Wible busily prepares for Balticon.

Balticon is one of several gatherings where devotees of dystopia, lovers of H.P. Lovecraft and true believers in extra-terrestrial life congregate for three days to see films, hear speakers and swap space stories. An ominous 13 year old, Balticon is the area's oldest science-fiction convention and by far the largest. It's also the first of the year each year, followed by May's Disclave (the District), Unicon (University of Maryland) in September, Philcon (Philadelphia) in November and Artkane (free-floating).

Fun is a factor, but not the only thing that pulls the fancifully faithful to Balticon's Hunt Valley Inn headquarters every Easter weekend. A sci-fi fan is by definition a collector, ever seeking whatever may activate the cerebral zone where scientific technology blends with the favorite fantasies that are the genre's core. With the traditional Hucksters' Room filled with books, posters and prints, Balticon is the ideal place to find an obscure '50s paperback on the end of the world. It's also an ideal place for young science-fiction artists to showcase their work and, maybe, sell a few pieces. This is why Jim Wible doesn't leave his basement for months until Balticon, axcept to go to work and do his laundry.

"I hope to bring two dozen pieces," he said, hunched over his drawing table. "I used to take about a dozen, but so many people come now that I think I can sell more." Nearly 2,000 attended Balticon last year; five years ago it was 200.

One new piece has sentimental meaning for Jim. Mounted on a beveled wooden cross, a polyfoam red scorpion-like creature is awrithe with tentacles, head tilted forward in death-like repose. "It's from the first adult science-fiction story I ever read," Jim says, his blue eyes blinking with childlike pleasure behind thick lenses. "It was by Ray Bradbury and it was about an agnostic space explorer who had just missed seeing the crucifixion of Christ on another planet. He got the bright idea that perhaps every inhabitable planet was in need of redemption, and if he moved fast enough he could catch up with Christ in another world."

Another piece, a 4- x 8-inch panel, is more in Jim's usual comic-book style. As he entered a friend's kitchen one evening, Jim spied a picture on the counter. Dashing over, he found that it was merely a grocery receipt someone had used to clean a watercolor brush. "They were just marks, but I visualized a whole scene. I saw a spidery machine stretched across an empty expanse of sky, with a lone figure standing on top of it. When I got close enough to see what it really was, it vanished except for my memory." Before the memory faded, too, Jim transfered the image onto poster board with ink and brush.

As he talks, Jim is working on a piece for the convention's program.This year's Balticon guest of honor is author Poul Anderson, and the Baltimore Science Fiction Association, the festival's sponsor, has invited attending artists to illustrate an Anderson story. Jim chose "A Little Knowledge," a 1972 story, and his drawing shows the classic sci-fi trinity: an alien, an earthling and a spaceship.

Jim first discovered that he was not alone in his devotion to sci-fi when he attended Balticon 72. He found droves of fans with identities of their own, like the Illegitimate Sons of Superman, a group of middle-aged businessmen who dress up in Superman suits even though they look more like Perry White. And "Filthy Pierre," alias Irwin Strauss, who annually wanders about the conventions, melodica on his hip singing filk songs (science-fiction folk songs).

Since then, Jim has attended about 30 conventions. "I love the costume parties. I've seen all major characters brought to life, from Ursula Le Guin's ice creatures to the Cheech Wizard by Lampoon's Vaughn Bodet. Once I saw a woman wearing a very simple costume: a strategically draped live boa constrictor and nothing else. She was walking through the hotel lobby, freaking out guys attending a barbershop quartet convention who were standing around harmonizing. She won the Primeval Earth Mother award. In recent years there's been a lot of 'Star Wars' and 'Close Encounters' characters. This year I expect pod people, from the 'Invasion of the Body Snatchers.'"

Jim's favorite convention activity is the art auction. "To walk into a large hall and find it overflowing with immense surreal paintings and drawings is dizzying. You can sate your sense of visual wonder in a way you never do by merely purchasing off-the-rack magazines."

Buxom Amazons, heavily muscled warriors and assorted ungodly creatures are the chief dramatis personae in Jim's as well as in most contemporary sci-fi art. "I prefer to do other things, aliens and landscapes. But it's the familiar in the unfamiliar context that sells. Particularly cheesecake, unicorns and dragons. Thanks to writers like Tolkien, sword and sorcery has blended with rockets and rayguns for people purely interested in fantasy."

Fantasy fans now turns out in force at conventions as fully as technologically oriented sci-fi groupies. In the hotel lobby during convention time you're likely to see a blonde barbarian dressed in chain mail and brandishing a broadsword signing the register, or Maid Marion clad in forest green, bow and quiver at her side, pushing a baby carriage.

"Balticon is essentially a giant phone booth where people can shed their mild-mannered everyday identities to become anyone from the Man of Steel to Canon the Barbarian, and maybe pick up a few vintage paperbacks." CAPTION: Picture, FANTASY AND SCI-FI CONVENTIONS ARE SOARING IN POPULARITY. By James K.W. Atherton.