Spacewalkers James Newman and Michael Massimino successfully installed a new $75 million camera inside the Hubble Space Telescope today, accomplishing the primary scientific goal of the fourth Hubble servicing mission.

The 870-pound Advanced Camera for Surveys, inserted into the telescope by Newman with guidance from Massimino, took the place of the now obsolete Faint Object Camera in an axial instrument bay behind Hubble's 94.5-inch primary mirror. Video from a television camera mounted on Massimino helmet gave flight controllers a bird's eye view as he plugged in the electrical cables needed to bring the new camera to life.

Engineers at the Goddard Space Flight Center in Greenbelt, Md., then carried out an "aliveness test" to confirm the camera was healthy and properly installed.

"Very good guys, you did an excellent job," astronaut Richard Linnehan radioed from Columbia's flight deck. "You guys have paved the way for a lot of Ph.D.s in the years to come."

After permanently stowing the Faint Object Camera in a cargo bay carrier, Newman and Massimino will trade places on the end of the shuttle's robot arm and press ahead with the final major objective of their spacewalk, installation of an electronics control module needed by an experimental instrument refrigerator that will be installed during a spacewalk Friday.

The refrigerator, known as a cryocooler, is designed to revive a now dormant infrared camera-spectrometer that ran out of nitrogen ice coolant in 1999.

Today's spacewalk began at 4 a.m., about a half-hour behind schedule.

"Good to be back on the arm, Nance, it looks like a beautiful night for a spacewalk," Newman told arm operator Nancy Currie as he anchored himself to the end of the Canadian space crane.

It took Newman and Massimino nearly an hour to get set up and to get the telescope's aft shroud doors open to gain access to the Faint Object Camera. The FOC was removed just after 5:37 a.m. and temporarily stowed on a mounting fixture in Columbia's cargo bay. The ACS then was pulled from its cargo carrier around 6:22 a.m. and installed in Hubble shortly after 7 a.m.

The spacewalkers later installed an electronics package to control an experimental refrigerator that will be attached to Hubble Friday.

Rrepressurizatin of the shuttle Columbia's airlock began at 11:30 a.m. to officially end the fourth of five planned spacewalks to service the Hubble Space Telescope.

Today's excursion ran seven hours and 30 minutes, pushing the crew's total spacewalk time to 28 hours and 35 minutes through four outings since Monday.