U.S. astronauts repair Hubble imaging device

In this photo provided by NASA, astronaut Andrew Feustel, STS-125 mission specialist, retrieves a power tool as he navigates near the Hubble Space Telescope on the end of the remote manipulator system arm, controlled from inside Atlantis' crew cabin, Saturday, May 16, 2009. Astronauts Feustel and John Grunsfeld were continuing servicing work on the giant observatory, locked down in the cargo bay of the shuttle. (AP Photo/NASA)

In this photo provided by NASA, astronaut Andrew Feustel, left, STS-125 mission specialist, navigates near the Hubble Space Telescope on the end of the remote manipulator system arm, controlled from inside Atlantis' crew cabin as astronaut John Grunsfeld, right, signals to his crew mate from just a few feet away, Saturday, May 16, 2009. Astronauts Feustel and Grunsfeld were continuing servicing work on the giant observatory, locked down in the cargo bay of the shuttle. (AP Photo/NASA)

WASHINGTON, May 17 (Xinhua) -- A pair of U.S. astronauts on Sunday conducted the fourth spacewalk of space shuttle Atlantis' Hubble-upgrade mission to repair a failed imaging device on the Hubble Space Telescope used to discover black holes.

The device, known as the Space Telescope Imaging Spectrograph (STIS), splits light into its component wavelengths. It was shut down in August, 2004 after electronics problems cut off its power. Since then, STIS has been in "safe mood."

The spacewalk was conducted by Michael Massimino and Michael Good. It lasted eight hours and two minutes.

Sunday's spacewalk was fourth of five back-to-back spacewalks planned for Atlantis' 11-day mission, NASA's fifth and final visit to the observatory before the shuttles are retired next year.

On Saturday, astronauts installed Cosmic Origins Spectrograph for Hubble. It the most sensitive such instrument to ever fly in space, which delivers precise astrophysical data on the temperature, density and speed of distant cosmic bodies.

On Friday, astronauts replaced six gyroscopes that will allow Hubble to steady its gaze on distant galaxies.

On Thursday, astronauts installed a new camera in Hubble. They also replaced a science data-handling unit that broke last fall and hooking up a docking ring so a robotic craft can guide Hubble into the Pacific years from now.

U.S. shuttle Atlantis lifted off on May 11 with seven-member crew onboard from Kennedy Space Center in Florida, 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.