Mike Massimino and Mike Good emerged from the airlock of the shuttle Atlantis to begin work on the Space Telescope Imaging Spectrograph at 9:45 am EDT (1345 GMT), 30 minutes later than scheduled to allow Good to adjust the fit of the right boot on his spacesuit.

NASA said Saturday's efforts to revive a second Hubble instrument, the Advanced Camera for Surveys, had been a partial success, after two of three internal imagers were recovered.

Hubble will remain anchored in the payload bay of Atlantis until Tuesday, when the seven shuttle astronauts expect to finish their ambitious refurbishment of the telescope requiring five daily spacewalks.

The work under way aboard Atlantis is intended to extend Hubble observations for at least another five years.

"At this point, we are feeling really good," said Preston Burch, NASA's Hubble program manager. "Hubble has reached a new high in terms of capabilities. We have also made huge strides in terms of restoring the health of the observatory."

During Sunday's spacewalk, Massimino and Good will attempt to replace an internal circuit board on the spectrograph.

The spectrograph, which examines the light from stellar objects to measure temperatures, pressures and velocities as well as identify the chemisty, was installed in the telescope during a 1997 shuttle mission. It's been inactive since a 2004 power failure.

The two spacewalkers will use an assortment of custom hand tools to remove 111 small screws to gain access to the internal circuitry responsible for the failure.

Massimino and Good will replace the circuit board and install a cover plate with two latches rather than dozens of small screws.

Astronauts John Grunsfeld and Drew Feustel used a similar technique during their spacewalk on Saturday to revive Hubble's Advanced Camera for Surveys.

The Saturday spacewalk breathed some new life into the camera for surveys, a seven-year-old instrument that was crippled by a short circuit in January 2007. The breakdown left only one of three internal imagers functioning, slowing a campaign to unravel the mysteries of dark matter and dark energy.

The two little understood forces influence the structure and expansion of the universe.

Early Sunday, NASA said the work to replace four circuit boards on the survey camera and install a new power source was a partial success.

The effort revived one of the two crippled internal imagers. The still inactive high resolution imager, which was used by astronomers for about five percent of the camera's observations, is likely unrecoverable by another means, NASA said.

Since hoisting Hubble into Atlantis's cargo bay last week, spacewalking astronauts have achieved six of the mission's highest priorities.

In addition to the partial revival of the Advanced Camera for Surveys, they have installed two new science instruments and a crucial science computer as well as replaced gyroscopes and batteries to sustain the the observatory's pointing and power systems.

The overhaul will prepare Hubble to search for the oldest and most distant galaxies, map the large scale structure of the universe and study the planet forming processes around other stars.

Hubble is a cooperative project between NASA and the European Space Agency.

earlier related report
US astronauts equip Hubble on third spacewalk
Spacewalking astronauts revived a crippled camera and installed a second new scientific instrument aboard the Hubble Space Telescope on Saturday, extending an ambitious effort by the crew of the shuttle Atlantis to overhaul the 19-year-old observatory into the weekend.

The nearly seven-hour spacewalk by John Grunsfeld and Drew Feustel will be followed on Sunday by the fourth in a series of five daily excursions intended to equip the high flying telescope for at least another five years of observations.

Since hoisting Hubble into Atlantis's cargo bay last week, the shuttle's seven astronauts have achieved six of the mission's highest priorities.

In addition to the Saturday's revival of the Advanced Camera for Survey and installation of the new Cosmic Origin Spectrograph, the list of achievements include the addition of the Wide Field Camera-3, a new science computer, gyroscopes and power storage batteries.

Hubble, a cooperative project between NASA and the European Space Agency, is headed for a new scientific summit, a perch from which astronomers can search for the oldest star systems, map the large scale structure of the universe and study planet forming processes around other stars.

"We are batting 1000," said Dave Leckrone, NASA's chief Hubble astronomer, told a news briefing. "We have just an extraordinary capability to image the universe, much more powerfully that what we had before by far."

Saturday's spacewalk breathed new life into camera for surveys, a seven-year-old instrument that was crippled by a short circuit in January 2007.

The breakdown slowed a campaign to unravel the mysteries of dark matter and dark energy, two little understood forces that influence the structure and expansion of the universe.

During the repair, Grunsfeld, a 50-year-old astronomer, and Feustel, a 43-year-old geologist, opened the camera to extract and replace four circuit boards and attach a new external power source.

Grunsfeld, who is visiting the space telescope for a third time, removed 32 tiny screws using an assortment of hand ratchets and cutters to revive the imager. A custom cover plate kept the tiny fasteners from floating into the telescope, where it could have caused harm.

Saturday's outing began with the installation of the new Cosmic Origins Spectrograph, an instrument that analyzes the light from stars, planets and other astronomical objects rather than taking pictures.

The new instrument will rely on the light from distant quasars to illuminate the vast web-like structures that extend across the universe encirling regions of interstellar gas and dust.

The new spectrograph promises to aid astronomers as they track the stellar genesis of carbon and the other chemical elements needed for life.

The spacewalkers made room for the spectrograph by removing an optical module installed in Hubble in 1993.

Astronauts on the long ago mission equipped the observatory with the Corrective Optics Space Telescope Axial Replacement, an instrument that overcame an optical flaw in the telescope's light gathering mirror.

The flaw surface in the weeks following Hubble's 1990 launching.

Saturday's spacewalk featured a marked absense of the stuck bolts and equipment misallighments that turned spacewalks on Friday and Thursday vexing and pushed the outings well beyond their planned six and a half hour limits.

"You guys are tracking wonderfully on the time line," Mission Control told the two men at one point Saturday.

Fellow spacewalkers Mike Massimino and Mike Good hope to keep that streak alive on Sunday as they attempt a similarly intricate repair of Hubble's Space Telescope Imaging Spectrograph, a black hole hunter.

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

The repair plan is similar to the one that revived the Advanced Camera for Surveys. However, Massimino must extract 111 small screws without losing one.

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