Safford, Ariz. ? Firefighters widened a defensive ring around a mountaintop observatory Monday, trying to hold back two wildfires and protect a powerful telescope under construction.

The crews in southeastern Arizona used bulldozers and fire retardant around the Mount Graham International Observatory, which has two operating telescopes and the $120 million soon-to-be-completed Large Binocular Telescope.

The ground crews were helped by an air tanker plane dropping retardant.

“The building’s not going to burn, but the smoke and heat could do some real damage to the instruments inside,” said Pruett Small, a fire official.

Researchers from around the world use the observatory, which is an extension of the University of Arizona.

When fully operational in 2005, the Large Binocular Telescope will be the world’s most technologically advanced optical telescope. It’s expected to yield images nearly 10 times sharper than those from the Hubble Space Telescope.

The observatory, consisting of eight buildings, encompasses 8 1/2 acres of pine forest on Mount Graham’s 10,470-foot Emerald Peak and is surrounded by a 200-foot-wide clearing.

It also has a sprinkler system that officials said would be turned on if flames came within a quarter-mile.

Wildfires near Safford, Ariz., burn near the Mount Graham International Observatory, shown at left. The observatory has two operating telescopes and the 20 million soon-to-be-completed Large Binocular Telescope