The observatory dome at the end of the Charles County farm field emits a satisfying electric hum as a motor retracts a section of its roof.

For now the exercise is, strictly speaking, of no practical importance: There is no telescope in the small observatory in Nanjemoy, about 20 miles west of the county seat of La Plata.

But members of the Southern Maryland Astronomical Society expect that will change before too long. They are tantalizingly close to accomplishing a dream of several years' standing: establishing a working astronomical observatory for use by students, hobbyists and others.

Club members estimate they are about $1,000 short of completing the dome-topped building at the county's Nanjemoy Creek Environmental Education Center, and finishing work on a shed-sized control building a few feet away.

Once the construction is finished, the club can place its 16-inch reflector telescope in the dome. The white cylindrical device now sits unused in a corner of the education center's main building, looking much like an unused water heater.

"With this we expect to see all kinds of wonderful things," said Pamela Humbert, of Indian Head, the secretary for the astronomical society, which has about 25 members from throughout Southern Maryland. "Jupiter and Saturn should be spectacular."

Telescope users might be able to see massive storms on Jupiter's surface, the so-called canals that line the surface of Mars, and enjoy distinct views of Saturn's rings, Humbert said. Those viewing deeper space will see disks, clouds, spirals and other formations made by star clusters and galaxies.

The observatory will be available to young students who each year use the education center, which is run by the county's public schools.

It also will be a convenience for astronomy club members who now pack their telescopes into car trunks and travel to Nanjemoy for regular viewing get-togethers.

The relatively remote site lets astronomers avoid man-made lights that illuminate night skies in the more populated La Plata and Waldorf, overwhelming faint star light.

Without a permanent observatory, the hobbyists need to set up their telescopes each time they gather at Nanjemoy for stargazing--a time-consuming process, because some of the instruments have sophisticated tracking motors that keep space objects in view despite Earth's rotation.

The repeated setting up and breaking down "is not too good for the equipment, and it doesn't do our backs much good either," said Michael Conte, president of the Southern Maryland Astronomical Society.

The dome once was part of a Naval Research Laboratory in northwestern Charles County, off Bensville Road. The lab shut down a number of years ago and a former astronomy club president heard the dome might be available, Conte said.

The Navy donated the dome--built in the 1960s by an Illinois company--with the understanding it would be used for education.

Club members worked for about three years to find an observatory site, obtain donations of labor and equipment, and get permits to move the 16 1/2-foot-wide dome over county roads to the site.

The move was completed in May of last year. Club members then took up hammers and saws--and spent about $10,000--to construct the small circular frame building that supports the dome. They also built the shed-like control room that will shelter computers used to control the telescope.

The control building will be heated; the dome building is to remain unheated because optical distortion is minimized when lenses share the temperature of the surrounding air.

These days the project's incomplete status shows, with wall studs exposed, no ceiling in the control building, and new wiring running to and from unconcealed outlet boxes.

Beauty is in the eye of the beholder.

"We just got power . . . in here," Humbert said recently, smiling as she flicked on lights in the dome building.

She gazed up at the dome itself, a gleaming industrial artifact made of aluminum-clad steel and clean converging arcs, and expressed her admiration.

"It's about 35 years old and it still works," she said.

A UniverseOf Knowledge

Members of the Southern Maryland Astronomical Society say the observatory they are building at the Nanjemoy Creek Environmental Education Center can be used for a number of educational projects for school children. Possible activities include:

* Scale mapping of constellations

* Learning the mythology of the constellations

* Observing craters on the moon

* Finding the distances from Earth to other planets

* Observing Venus's phases

* Viewing Jupiter's Great Red Spot

* Observing sunspots

* Discerning colors of stars

* Surveying binary, or twinned, stars

* Comparing galaxies

* Taking photographs of celestial objects

CAPTION: Pamela Humbert, of the Southern Maryland Astronomical Society, shows off the group's partially completed observatory in Nanjemoy.