Astronauts have ventured out on the third and final spacewalk of their mission to unwrap the International Space Station's phenomenal new lookout - the best window on the world that orbiting crews have ever had.

Robert Behnken and Nicholas Patrick had a multitude of chores to complete outside, but the most anticipated was the unveiling of those seven windows, a fitting grand finale to the shuttle mission's spacewalks.

"Let's take the covers off," shuttle commander George Zamka urged as the spacewalk got under way.

It was expected to be a fairly straightforward job. Behnken and Patrick needed to remove insulation from the windows and unlock the shutters. Then their colleagues inside were going to crank open the shutters, and if everything went well, enjoy the biggest and best views of Earth afforded orbiting astronauts.

Mission Control wanted the shutters opened while the spacewalkers were still outside so the two men could intercede if something jammed. Barring a problem, Behnken and Patrick were under orders to stay a safe 10 feet or more from the windows after the shutters were raised. The last thing Nasa wanted was to have one of them inadvertently kick a window or bang it with a toolbag.

The £17 million observation deck is part of the new space station room, Tranquillity. Space shuttle Endeavour delivered the European compartments last week. The Italian-built dome - 5ft tall and nearly 10ft in diameter - is designed to offer sweeping 360-degree views of the home planet and outer space, as well as the space station itself. It is not just for the crew's viewing pleasure - a robotic work station will be installed, providing direct views for astronauts when they operate the station's big mechanical arm.

Six trapezoid-shaped windows encircle the dome. In the middle is a round window 31ins across - the largest window flown in space.

During normal operations, the space station crew will be able to keep the round window unshuttered most of the time, along with a couple of others. But the windows facing along the direction the outpost is orbiting will need to be closed, except during robotic operations, to protect against a micrometeorite strike.

The final spacewalk represented the last of the Endeavour crew's space station construction work. The shuttle will depart on Friday.

Back at the launch site, meanwhile, Nasa has delayed the next space shuttle flight. Discovery had been scheduled to blast off in mid-March, but a string of unusually cold weather stalled preparations. Lift-off is now targeted for April 5. Only four more shuttle flights remain.