John Grunsfeld and Andrew Feustel at work 560km (350 miles) up
Astronauts upgrading the Hubble Space Telescope have made a third spacewalk in as many days to make repairs deep inside the orbiting observatory.
They ventured outside the shuttle Atlantis for more than six hours to overhaul Hubble's broken main camera.
John Grunsfeld and Andrew Feustel also installed a $88m (£58m) instrument.
The new Cosmic Origins Spectrograph (COS) is expected to provide further insight into how planets, stars and galaxies were formed.
Despite the fact it was never meant to be repaired in space, Atlantis' commander described the operation to revive Hubble's Advanced Camera for Surveys (ACS) as "smooth sailing".
The astronauts were able to restore the camera's wide field channel, with which most of its astronomical observations are made.
But the repairs failed to recover the ACS high resolution channel, which may now be down for good - despite hopes it could be retrieved another way.
HUBBLE SPACE TELESCOPE
Named after the great US astronomer Edwin Hubble
Launched in 1990 into a 600km-high circular orbit
Equipped with a 2.4m primary mirror and five instruments
Length: 15.9m; diameter: 4.2m; Mass: 11,110kg
Mr Grunsfeld, a 50-year-old astronomer, and Mr Feustel, a 43-year-old geologist, also attached a new external power source.
During the third extra-vehicular activity (EVA) of the mission, Mr Grunsfeld removed 32 tiny screws using an assortment of hand ratchets and cutters.
On Friday, the astronauts struggled to complete a critical repair to the telescope, fitting a refurbished pair of gyroscopes in the telescope after a new set refused to go in.
The troubled spacewalk - the second - was the longest yet, lasting eight hours.
"At times, I felt like I was wrestling a bear," Mike Massimino was quoted as saying by AFP news agency, as he and Mike Good struggled to install the gyroscopes, or "rate sensing units" (RSUs).
Previously, only three of the six gyroscopes worked, now Hubble has four brand new sets and two refurbished ones. Only two are needed to orient the telescope properly.
There have now been 21 spacewalks undertaken in the service of the 19-year-old observatory.
On Thursday, the telescope's Wide Field Planetary Camera 2 was replaced with the Wide Field Camera 3, giving the telescope an even deeper view into space - and back into the history of time.
A data processing unit that failed in 2008 was also replaced.
There are no plans for further trips to Hubble after this mission ends which is why the repairs are so crucial.
"We're enjoying the moment and savouring it... the goal is to make the next two days go as well as today went," senior spokesman for Nasa Preston Burch said.
If all goes well, the fifth and final spacewalk is set for Monday and the telescope will be released from the cargo bay of the Atlantis shuttle on Tuesday.