Spacewalkers replace gear to steady Hubble's gaze

*Spacewalkers to replace six positioning gyroscopes

*Second spacewalk of five under way

*Astronauts tackle NASA's top job for Hubble fix-up

By Irene Klotz

HOUSTON, May 15 (Reuters) - A second pair of spacewalkers from the shuttle Atlantis tackled the most important task on NASA's fix-up list for the Hubble Space Telescope on Friday, installing new positioning gear to steady its gaze.

The six gyroscopes are not the most exciting part of the telescope, which another pair of spacewalkers on Thursdayoutfitted with a new panchromatic camera that can see light in infrared, visible and ultraviolet wavelengths.

Key to the telescope's success is its ability to point and hold steady on a target, which project managers say is like illuminating a dime with a laser beam from 200 miles (320 km) away.

That trick is accomplished with spinning gyroscopes, all six of which are expected to be replaced by astronauts Michael Massimino and Michael Good, during the second of five spacewalks planned during shuttle Atlantis' ongoing Hubble telescope servicing mission.

"Those gyros are absolutely critical," said Hubble project manager Preston Burch.

Gyroscopes have been replaced during NASA's previous house calls to Hubble, including a 1999 mission that restored the observatory to service after four of its six gyros died.

The telescope is designed to operate with three gyros, but engineers devised a plan to use two, and if necessary, even one.

The change-out is not particularly difficult work, but getting access is tricky.

Massimino will have to crouch to get himself positioned inside the observatory, where there is a small platform to anchor his boots. He will wedge himself against the inner structure, then try to avoid moving.

"My mantra is 'Be a statue,'" he told Reuters in a preflight interview.

Massimino worked on the telescope during NASA's 2002 telescope servicing mission. Good is making his first spaceflight.

The astronauts floated into Atlantis' cargo hold shortly before 9 a.m. EDT (1300 GMT) to begin what was expected to be a 6-1/2 hour spacewalk.

"It's a beautiful day outside," radioed Massimino to his crewmates on the shuttle.

The spacewalkers also plan to replace three of Hubble's 19-year-old batteries, which have degraded to the point that they can only be partly charged. The remaining three batteries will be replaced during the mission's last spacewalk on Monday.

Atlantis blasted off on Monday for an 11-day mission, NASA's fifth and final visit to Hubble before the shuttle fleet is retired next year. (Editing by Jane Sutton and Vicki Allen)