Astronauts John Grunsfeld and Drew Feustel were working on decking out Hubble in a bid to extend Hubble's observations for at least five years.

The new science instrument to install, the Cosmic Origins Spectrograph, was developed to study the grand scale structure of the universe to explain how galaxies formed and clustered.

It will also chart how the chemical elements, especially those necessary for life, emerged and changed in abundance over time.

The Advanced Camera for Surveys, next up for repair, 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 hoped to resurrect the camera in an intricate effort to open it up and replace 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 involves 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.

Saturday's walk got under way at 1335 GMT in the latest in a series of five daily spacewalks by the Atlantis astronauts that was expected to last 6 1/2 (six-and-a-half) hours.

As their first task, Grunsfeld and Feustel removed a piece of equipment installed in the telescope in 1993 to correct an optical flaw. The flaw, which nearly derailed the space telescope, was discovered in the months after the observatory's launching three years earlier.

They replaced the no longer needed optical box with the new Cosmic Origins Spectrograph, an instrument designed to study the large scale structure of the universe, the largely unseen forces that govern the distribution and grouping of star systems.

The new spectrograph is also equipped to chart the chemical evolution of the universe and in particular the stellar production of carbon and the other elements necessary for life.

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

During the first of the mission's five daily spacewalks on Thursday, two other astronauts also equipped Hubble with a powerful new camera and science computer, the mission's second and third highest priorities.

The seven Atlantis astronauts rendezvoused with Hubble on Wednesday and hoisted it aboard the shuttle for a final overhaul.

Although NASA has no plans for a future shuttle visit to Hubble, a new docking device also 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 allowing it to plunge back to Earth uncontrolled, potentially endangering a populated area.

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