LOS ANGELES (AP) _ Mary Firth will photograph Halley's comet from a California mountainside. Mark Coco plans to watch it from the Galapagos Islands. Ruthi Moore may plant her lawn chair in a Hawaiian sugar cane field.

The three amateur astronomers belong to International Halley Watch, a loosely organized group mounting history's largest study of a comet, says Stephen Edberg, an astronomer at the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena.

About 900 professional and nearly 600 amateur astronomers from more than 50 nations, including Eastern bloc countries, will participate in the Halley Watch, which is coordinated by the Jet Propulsion Laboratory and West Germany's University of Erlangen-Nurnberg.

With so many eyes turned to the skies, scientists hope to keep the comet under 24-hour watch, Edberg said.

''The reason is that we don't know very much about comets,'' he explained. ''We have very nice models of what we think a comet is and how it works. We've got a very nice edifice, but it really needs a foundation'' of solid facts.

Edberg said information amassed by Halley Watch participants will complement data from five space probes - two Soviet, one European and two Japanese - that will fly by the comet next March, as well as from various U.S. spacecraft that will watch the comet from afar.

Halley Watch ''is an excellent opportunity for an amateur to contribute data which has real meaning,'' said Mrs. Firth, a 57-year-old grandmother from suburban Canoga Park. She plans to haul her telescope and cameras onto the foothill slopes of Mount Pinos, 75 miles northwest of Los Angeles, two weekends a month starting in August.

Coco, 25, a Redondo Beach computer firm employee, said he's excited about watching and photographing the comet while leading a trip to Peru, Ecuador and the Galapagos next April because ''comets are a lot of fun. They're strange visitors and poorly understood. They're different from the average, run-of- the-mill objects we normally observe.''

Mrs. Moore, 39, wife of a Navy submarine commander, said she will record details of fall and spring meteor showers associated with Halley's comet while sitting in a lawn chair ''either in my backyard (in Honolulu), Barbers Point Naval Air Station or on the island of Lanai in the middle of a cane field.''

Her husband thinks her hobby ''is nice,'' Mrs. Moore said. ''When I come home at 3 a.m. in the morning, he asks all the appropriate questions and then goes back to sleep.''

John Brandt, astronomy and solar physics chief at NASA's Goddard Space Flight Center in Greenbelt, Md., said amateur astronomers will make ''a vital contribution'' to the study.

''These people are doing this largely out of the goodness of their hearts,'' he said.

Halley Watch's professional astronomers have been observing the comet since it was first detected on its current trip around the sun in October 1982, Edberg said. The most careful amateurs will be able to see it through their telescopes starting in August and continuing well into 1986, he added. It will be visible to the naked eye next January, then again in March and April, although springtime views from the Northern Hemisphere are expected to be poor.

The Halley Watch astronomers are organized into several groups, each assigned to study a different aspect of the comet. The amateurs will mail their photos and information to ''recorders'' such as Mrs. Moore, who will judge the quality of the material before turning it over to scientists

The scientists will compile the photos and information into an archive, which will published in a multivolume set for future study.

''It will be the most thoroughly observed comet ever studied, and the archives will be the most complete collection of data on a comet ever compiled,'' Edberg said.

''It gives astronomers 10, 20, 30, 50 or 75 years from now the chance to study the data of Halley's appearance,'' he added.

Edberg is the author of NASA's ''International Halley Watch Amateur Observer's Manual.'' He described it as ''the recipe book'' that contains an application form to join Halley Watch and tells participants how to observe the comet and record what they see scientifically. Halley Watch members also get regular updates through a mailed news bulletin.

Halley Watch ''will show that amateurs have a high degree of proficiency to do good work,'' said John Sabia, a Clark Summit, Pa., amateur astronomer who will photograph the comet from an observatory in Keystone, Pa.

But for Charlie Oostdyk, a Costa Mesa computer programmer, participating in Halley Watch simply provides another opportunity view the heavens and ''see all the wild and wonderful things out there.''

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EDITOR'S NOTE - Amateur astronomers who wish to join International Halley Watch should mail the application form from the manual. Some bookstores carry the manual in a single volume. It also may be purchased in two volumes, priced at $4.50 each, by writing Superintendent of Documents. U.S. Government Printing Office, Dept. 33, Washington, D.C. 20402. Request stock numbers 033-000-00888-1 and 033-000-00889-9.