ATOP MAUNA KEA, Hawaii (AP) _ The shadow of Thursday's solar eclipse will pass over a major observatory for the first time, but scientists say volcanic ash high in the atmosphere from a Philippine volcano may fog their view.

Astronomers and other scientists from around the world have gathered at the astronomical observatory at this dormant volcano's 13,796-foot summit to study the sun's outer corona with an array of high-power telescopes during the total eclipse.

But they are concerned that their four-minute view, created when the moon blocks the sun itself and allows the relatively faint outer corona to be seen, will be less than perfect because of the dusty ash from Mount Pinatubo, said Donald N.B. Hall, director of the University of Hawaii Institute for Astronomy.

''We know that more than one third of the experiments will not be affected,'' he said. ''For the other six, we expect some impact, but we don't know enough about the dust to determine what effect it will have.''

It is the first time an eclipse path of totality will pass over a major astronomical observatory, said Mark Stormon, eclipse project manager for Honolulu's Bishop Museum.

''Before, we've always had to pack up our equipment and go to the eclipse. This time the eclipse is coming to us,'' said Charles Lindsay, a scientist with the university astronomy institute.

There were no observatories here on Aug. 7, 1850, when the last total eclipse of the sun came this way. The next one visible from Mauna Kea will occur May 3, 2106.

Nine experiments using seven of the complex's nine telescopes were planned during totality, starting at 7:28 a.m. and ending at 7:32 a.m. HST.

One of the primary objectives is to learn more about why the temperature of the sun's corona, or atmosphere, is about 3 million degrees Fahrenheit, while the sun's surface has a temperature of only 10,000 degrees, Stormon said. Temperatures in the sun's interior are around 27 million degrees, he said.

Coincidentally, Pinatubo's dust was expected to have the greatest impact on three experiments trying to determine what happens to interplanetary dust as it is drawn into the sun, Hall said.

Infrared instruments will attempt to see rings believed to be created as interplanetary dust slowly spirals toward the sun and is vaporized, he said.

Other experiments involve taking photographs through the sun's lower atmosphere and watching the effect on the earth's atmosphere by the swift passage of the moon's shadow during the eclipse, Hall said.

At a solar observatory at an 11,000-foot elevation on the adjacent 13,697- foot Mauna Loa volcano, scientists plan to use the eclipse to get their first accurate baseline calibrations for an instrument that artifically blocks the image of the sun to allow observation of the corona, Stormon said.

Thousands of amateur eclipse watchers were expected to pour into Keahole Airport on the west side of Hawaii island and scramble for a good viewing position. At least four chartered jumbo jets filled with Japanese visitors were expected to arrive early Thursday.

Mike Silva, state harbor master at Kailua-Kona on the coast, said it appeared fewer than the predicted 400 to 600 yachts and pleasure boats would show up off the west coast of Hawaii Island to view the eclipse.

Silva ordered the Kailua pier closed to all but commercial traffic because of an expected onslaught of visitors who were to be bused from the airport to waiting charter boats.

Vendors were doing brisk business selling huge stocks of eclipse mementoes.

Many of the major luxury hotels dotting the coastline planned special events.

At the Mauna Kea Beach Hotel, guests were invited to paint eclipse T-shirts and were treated to an elaborate Hawaiian procession, complete with chants and hula dancing, a hotel spokeswoman said.