There are moments when Andrew Huttner wonders how he ended up last month on an isolated Puerto Rican beach, down on his hands and knees in the sand for four nights, peering through $2,000 worth of new equipment.

But most of the time he can explain his impulsive expedition.

"Halley's Comet is a legend and I wanted to explore it as much as I could in my lifetime," said the Manassas Park electronics engineer who works with the Defense Department at the Defense Communications Agency in Arlington. "I've never done anything like this before. Everything went into this trip. I took a long shot and everything worked out."

Huttner, 43, returned with photographs of the comet that a professional astronomer has termed "quite good." And like lots of other star-gazers bitten by the comet bug, he also came back with a once-in-a-lifetime experience he describes as "kind of a mystical thing.

"Halley's Comet is something that's very controlled, it's always there," he said. "You know how it is, it's orderly, it shows there's something orderly in the universe."

Huttner said his interest in Halley's Comet "kind of came upon me suddenly" during a business trip to Denver last November. "I just went out in a real dark place and I looked up at the Pleiades and saw a faint blob and it was Halley's Comet."

He soon was reading up on astronomy and spent $2,000 on a telescope and camera. "I always wanted to have a telescope but I never had an excuse to buy one." He began spending nights in Sky Meadows State Park near Paris, Va., where a ranger gave him a spot to comet-watch in privacy.

One night he turned up at a meeting of Northern Virginia amateur astronomers to get some information, "but they were working on a new constitution" for their club. "I was so frustrated," he said. He finally found a mentor in Richard Schmidt, a U.S. Naval Observatory astronomer who advised him on photographing the comet.

Schmidt said he has received calls "every day" from people asking for advice on how to see and photograph Halley's Comet. Huttner stood out, he said, because he was so "meticulous in getting every detail right in a subject he didn't know much about. He drummed us for information."

Halley's arrival has drawn hundreds of new visitors to the observatory. "We turned away 100 people Monday night when we had our biggest tour on record," Schmidt said. "We can't keep up with the demand from people who want to get a last glimpse before it goes away . . . . They don't seem to care they can't see it very well."

As the time for Halley's closest approach to Earth on April 10 neared, Huttner decided he had to get a box seat. In Puerto Rico, the comet passed 25 degrees above the horizon, offering a better view than in the Washington area, where it swept by just above the horizon and was obscured by city lights and smog.

Once in Puerto Rico, he rented a car, packed the sensitive film that he had carried on ice, per instructions from Schmidt, and "drove to the southern shore. I didn't know where I was going to sleep."

In a small town called Parguera he met an American who took him to "a lonely stretch of beach" named Cana Gorda. "The same day I land in Puerto Rico, I was on this beach . . . from 10 p.m. to 4 a.m. on my hands and knees."

"They were perfect nights," he said, when Halley's passage through the constellation Lupus was crystal clear. One night he was joined by a group of Puerto Ricans who stayed until dawn. "They took me seriously," he said.

Huttner said he now looks at the past few months as a trial run. When Halley's Comet returns in 75 years, "I'll be 118 years old -- but I'll be more prepared next time."