During the second spacewalk of mission STS-103, astronauts successfully replaced the Hubble Space Telescope's main computer and installed an improved fine guidance sensor used to track stars for accurate pointing of the telescope. Images from NASA TV.

Brain surgery Perched on the end of Discovery's robot arm, astronaut Michael Foale works inside one of Hubble's equipment bays to replace its main computer.
Out with the old Foale carefully removes Hubble's aging computer, before replacing it with an improved 486 system.
Moving the piano European Space Agency astronaut Claude Nicollier removes the telescope's old fine guidance sensor, which is often described as being the size of a baby grand piano.
Hanging around Towards the end of the spacewalk Nicollier stands at the end of the shuttle's robot arm.