Cape Canaveral, Fla. ? Spacewalking astronauts gave the Hubble Space Telescope a more commanding view of the cosmos by installing a new high-tech instrument Saturday, then pulled off their toughest job yet: fixing a broken camera.

It was the third spacewalk in as many days for the shuttle Atlantis crew, and it was the most intricate ever performed because of the unprecedented camera repairs. Astronauts had never before tried to take apart a science instrument at the 19-year-old observatory.

Hubble’s chief mechanic, John Grunsfeld, deftly opened up the burned-out camera and plucked out all four electronic cards that needed to be replaced.

“Somehow I don’t think brain surgeons go ‘woo-hoo’ when they pull something out,” one of the astronauts observed from inside Atlantis.

To everyone’s surprise, the new cards and power supply pack went in just as smoothly, seeming to take almost no time at all.

In fact, the astronauts found themselves running ahead of schedule for a change, and their spacewalk lasted the allotted 6 1/2 hours. The first two spacewalks ended up running long because of unexpected difficulties encountered with Hubble, last visited seven years ago.

The astronauts cheered when Mission Control radioed up the news that the freshly repaired camera had passed the first round of testing.

“That’s unbelievable,” Grunsfeld said.

A second round of testing was expected to last well into the night.

The high-stakes job unfolded 350 miles above Earth. Orbiting so high put Atlantis and its astronauts at an increased risk of being hit by space junk. NASA had another shuttle on launch standby in case a rescue was needed.