Astronauts John Grunsfeld and Andrew Feustel left the shuttle"s air lock at about 8:30 a.m. EDT (1230 GMT), almost an hour ahead of schedule. Their tasks concluded at 3:22 p.m. EDT (1932 GMT). During their walk in space, Grunsfeld and Feustel installed a battery group replacement, removed and replaced a Fine Guidance Sensors and three thermal blankets protecting Hubble"s electronics.

The shuttle astronauts will set Hubble free Tuesday and return to the Kennedy Space Center in Florida on Friday.

During four previous spacewalks, astronauts installed a new camera and light-splitting spectrograph, replaced Hubble"s positioning system, repaired two instruments and attached a docking ring so a robotic spacecraft can be sent to remove Hubble from orbit at the end of its operational lifetime.

U.S. shuttle Atlantis lifted off on May 11 on a mission to upgrade the 19-year-old Hubble Space Telescope for the last time.

Hubble, launched by NASA in 1990, was named after U.S. astronomer Edwin Hubble whose work revolutionized our understanding of the size and structure of the universe. After its first two months of tests in 1990, the initial images from Hubble were a blurry disappointment. A slight flaw in the telescope"s main mirror -- barely the width of a human hair -- fouled the observatory"s vision.

In 1993, NASA sent a shuttle up to Hubble, where astronauts added corrective lenses -- essentially glasses -- to sharpen its vision. The result was crystal clear: 16 years of stunning cosmic photos followed.

Since that first orbital fix, astronauts returned to Hubble four more times; in 1997, 1999, 2002 and 2009.

Hubble has given the world amazing insight into the origins of our universe. Among its greatest discoveries are determining the age of the universe (13.7 billion years); finding that virtually all major galaxies have black holes at their center; discovering that the process of planetary formation is relatively common; detecting first ever organic molecule in the atmosphere of a planet orbiting another star; and providing evidence that the speed at which the universe is expanding is accelerating--caused by an unknown force that makes up more than 75 percent of the universe.