Two US astronauts safely completed their mission's third grueling space walk Saturday after reviving a crippled camera on the Hubble telescope and equipping the orbiting observatory with a new key instrument.

NASA astronauts John Grunsfeld and Drew Feustel wrapped up their six-hour, 36-minute stint in the void outside space shuttle Atlantis during which they worked on 19-year-old Hubble in a bid to extend its life by at least five years.

The outing was the third of five risky space walks to upgrade the Hubble Space Telescope, which is anchored in the cargo bay of Atlantis.

Since Atlantis engaged with Hubble on Wednesday, the seven-member crew has achieved six of their ambitious mission's highest priorities.

The two men successfully installed the new Cosmic Origins Spectrograph and repaired the Advanced Camera for Surveys, a seven-year-old imager that had an electrical problem.

"Really nice work," Mission Control told Grunsfeld and Feustel.

The new spectrograph was developed to study the grand scale structure of the universe to explain how galaxies formed and clustered.

US space agency NASA plans for it to chart the chemical evolution of the universe and in particular the stellar production of carbon and the other elements necessary for life.

The Advanced Camera for Surveys increased Hubble's field of view and observing efficiency 10-fold when it was installed in 2002.

However, an internal electrical short five years later shut down the imager's ability to observe distant galaxies, and cut short a promising future unraveling the mysteries of dark matter and dark energy.

The spacewalkers resurrected the camera in an intricate effort that involved opening up the device, replacing an internal electronics box, and attach a new power supply.

"This is the first time we have tried to repair an instrument like this in space," said David Leckrone, NASA's chief Hubble astronomer.

"It's a totally new enterprise, a repair in the sense of actually getting in to the guts of an instrument and changing out circuit boards.

"Anytime you try something brand new, when the stakes are high, that is a hold-your-breath time," he said.

The repair involved removing 32 tiny screws to access the camera's insides -- and using a specially designed cover plate in a bid to not let any of the critical parts float off into space.

As their first task in installing the Cosmic Origins Spectrograph, Grunsfeld and Feustel removed a piece of equipment installed in the telescope in 1993 to correct an optical flaw which blurred Hubble's vision.

The flaw, which nearly derailed the space telescope, was discovered in the months after the observatory's launch in 1990.

During a spacewalk Friday, astronauts Mike Massimino and Mike Good successfully installed new gyroscopes and new batteries on the revolutionary stargazer.

In the first space walk Thursday, two other astronauts equipped Hubble with a powerful new camera and computer.

Massimino and Good hope to keep the successful repairs and installations going on Sunday as they attempt a similarly intricate effort on Hubble's Space Telescope Imaging Spectrograph, a black-hole hunter.

The spectrograph was installed in the telescope during a 1997 shuttle mission, but it has been inactive since a 2004 power failure.

Although NASA has no plans for a future shuttle visit to Hubble, a new docking device installed on the first spacewalk would allow a robotic spacecraft with a propulsion module to latch onto the observatory.

When Hubble is no longer able to conduct observations, NASA plans to steer the space telescope into the Pacific Ocean rather than allow it to plunge back to Earth uncontrolled, potentially endangering a populated area.