The astronauts said fixing the telescope was like 'a game of operation'
Equipment problems bedevilled the penultimate spacewalk to repair the ageing Hubble telescope.
Two astronauts took eight hours to resurrect a spectrograph which is used to study the chemistry of astronomical objects and failed five years ago.
Work ran over by 90 minutes due to a tool running low on battery power and a particularly awkward bolt securing a handrail that needed to be removed.
A strong yanking action loosened it and tests showed the repair was a success.
Suspended outside the shuttle Atlantis more than 500km above Earth, the astronauts were delayed by a handrail blocking a cover held by 111 tiny screws that had to be undone.
Within lay the flawed wiring of the Space Telescope Imaging Spectrograph (STIS). When working, it splits light into its component colours, to reveal information about the chemical content, temperature and motion of planets, comets, stars, interstellar gas and galaxies.
After several failed attempts to unscrew the bolt on the handrail, the spacewalkers ended up tearing it off with brute force.
The astronauts, Michael Massimino and Michael Good, taped the pieces so they would not fly off into space where they could become potentially lethal projectiles whizzing about in orbit.
But there were further delays when a specialised tool designed to remove the screws of the cover plate had to be returned to the shuttle Atlantis to have its worn out batteries replaced with fully charged ones.
The astronauts upgrading the Hubble Space Telescope have now made four spacewalks in as many days.
On Saturday, they installed the new Cosmic Origins Spectrograph (COS).
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
It works in a similar way to STIS. By dissecting light, it will enable Hubble to gain new insights into the large-scale structure of the Universe, and determine how galaxies, stars and planets formed and evolved.
The astronauts also restored function to the Advanced Camera for Surveys - a workhorse instrument that took the telescope's deepest shot of the cosmos, the so-called Hubble Ultra Deep Field.
An electronics failure had closed down two of its three channels, but the Atlantis spacewalkers were able to revive one of them - the camera's wide field channel, with which most of its astronomical observations are made.
On Friday, the astronauts replaced the telescope's gyroscopes, which are used to point the observatory at targets in the sky.
On Thursday, the telescope's Wide Field and Planetary Camera 2 was replaced with the Wide Field Camera 3, giving the telescope an even deeper view into space - and therefore further back in cosmic time.
In addition, a data processing unit that failed last year was replaced.
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.
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