John Grunsfeld and Drew Feustel spent six hours refurbishing the 13.2 meter (43.5 foot) long telescope, hoisted into the cargo bay of the shuttle Atlantis last week for an ambitious final overhaul during five daily spacewalks.

The astronauts are now due to raise Hubble out of the bay with the shuttle's robot arm early Tuesday, floating the telescope back into space with no plans for any future human involvement.

NASA's shuttles, facing retirement by the end of next year, launched the space telescope in 1990. Shuttle crews returned in 1993, 1997, 1999 and 2002 to upgrade the telescope with new science instruments and replace failed parts.

Atlantis will depart with Hubble equipped for at least another five years of observations.

"It's been a great achievement up here," said Grunsfeld, an astronomer-turned-astronaut who is nearing the end of his third trip to the space telescope.

"This is a real great day," shuttle communicator Dan Burbank told the Atlantis astronauts from Mission Control in a wistful reference to their final session with Hubble. "A great way to finish this out."

The Atlantis astronauts equipped Hubble with a pair of new science instruments and repaired two others sidelined by electrical problems several years ago.

The astronauts replaced all of the gyroscopes and batteries, fortifying the pointing and power systems. They replaced a hobbled data managment computer that had carried on until last September.

To ensure they did all they could for iconic telescope, Grunsfeld and Feustel started Monday's spacewalk an hour early.

The decision created just enough time to finish all of the low as well as the high priority objectives of the shuttle crew's 11-day mission.

The lowest priorities included a task that was postponed on Sunday, when fellow spacewalkers Mike Massimino and Mike Good ran into a string of obstacles as they worked on the Space Telescope Imaging Spectrograph.

Grunsfeld and Feustel quickly replaced three batteries. They joined three batteries replaced Friday.

Next, they exchanged a fine guidance sensor, an optical device that works with the gyros to provide the observatory with precision pointing.

As their final task, Grunsfeld and Feustel attached panels of stainless steel shielding to the outside of the telescope, covering blemishes created by the bombardment of solar radiation.

Massimino and Good fell behind with that task when they encountered a stubborn bolt, which had to be wrestled loose, and other obstacles when they worked to revive Hubble's Space Telescope Imaging Spectrograph.

The spectrograph, which had been inactive for five years, responded to a round of critical tests, NASA said Monday.

No task seemed of a low priority to Grunsfeld though. As he mounted the new stainless steel shielding, the deteriorating material underneath began to flake and float away -- a symbol that all things come to an end.

"I feel so bad about the debris," said Grunsfeld in a low tone. The Atlantis astronauts will aim for a return to Earth on Friday, touching down at NASA's Kennedy Space Center in Florida at 11:41 am (1541 GMT).

After they pull away from Hubble, the Atlantis crew plans another inspection of the shuttle's heatshielding in a search for signs of damage from impacts with space debris and tiny meteoroids.

The 11-day Hubble mission carries a higher risk than NASA's usual mission to the international space station because of an accumulation of manmade space debris at the telescope's higher altitude.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.