Washington - US astronauts on Thursday were conducting the first spacewalk to upgrade and repair the ageing Hubble Space Telescope.

In a planned six-and-a-half-hour long spacewalk that began at 1252 GMT astronauts John Grunsfeld and Andrew Feustel are focussing on replacing Hubble's camera with a more advanced model. The Wide Field Planetary Camera 3 will allow astronomers to see deeper into space and to take images in the three regions of the light spectrum - ultraviolet, visible and near infrared.

The astronauts will also replace a computer that malfunctioned last year. The so-called Science Instrument Command and Data Handling Unit controls the telescope's science instruments and formats information to be sent back to scientists on Earth.

Other smaller tasks would get the Hubble ready for future work.

It is the first of five planned spacewalks in as many days to add two new instruments, repair two others and replace other hardware in frequently delicate operations to Hubble.

The crew on Wednesday used the robotic arm on the space shuttle Atlantis to capture Hubble ahead of the work.

Scientists say the upgrades, which US space agency NASA hopes will extend Hubble's life span until at least 2014, will continue to provide clues about the origin and nature of the universe.

Since its launch in 1990, Hubble has helped scientists to place the age of the universe at 13.7 billion years, learn that black holes are at the centre of most galaxies, monitor planetary formation and discover that the universe is expanding at an ever-faster pace.