Washington  - US astronauts on Friday began a delicate spacewalk to replace three pairs of gyroscopes aboard the Hubble Space Telescope.

The spacewalk began at 1253 GMT, after about a half-hour delay.

In the second of five planned spacewalks, astronauts Mike Massimino and Michael Good will remove and replace the gyroscopes that keep the telescope aligned and pointed toward areas being examined by astronomers.

NASA officials described it as the most delicate of all the planned spacewalks, with Massimino required to crawl inside the telescope to do work while being careful not to disturb its instruments.

They will also change one of the giant batteries that power the telescope.

On Thursday, spacewalkers John Grunsfeld and Andrew Feustel replaced 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. They also replaced a computer that malfunctioned last year.(dpa)