UPDATE 1-Astronauts complete tricky Hubble repairs

* Hubble gets new camera circuit boards, spectrograph

* Astronauts perform first-ever Hubble instrument repair (Recasts, adds spacewalk completion, changes byline)

By Chris Baltimore

HOUSTON, May 16 (Reuters) - Two spacewalking astronauts on Saturday tackled one of their toughest repairs to the Hubble Space Telescope -- a meticulous fix of a broken camera -- and installed a new spectrograph that can divine the properties of distant galaxies.

Astronauts John Grunsfeld and Andrew Feustel spent 6-1/2 hours outside the shuttle Atlantis for the third of five back-to-back spacewalks to upgrade the famous space observatory for another five to 10 years of work.

NASA officials had billed Saturday's spacewalk as the "hold your breath" day for Atlantis' ongoing 11-day mission, the fifth and final servicing call to Hubble before the shuttle fleet is retired next year.

But Grunsfeld and Feustel's tasks came off without a hitch, after two earlier spacewalks were beset with balky equipment that required astronauts to improvise.

Thursday's installation of a new wide-field camera was almost derailed by a frozen bolt.[ID:nN14512744]

Saturday's work required Grunsfeld to clamber into Hubble's body, dig into its electronic guts and replace sharp-edged computer circuit boards that were never meant to be repaired in space.