By Jonathan Amos
Science reporter, BBC News, Le Bourget
Europe's new billion-euro Herschel space observatory, launched in May, has achieved a critical milestone.
The telescope has opened the hatch that has been protecting its sensitive instruments from contamination.
The procedure allowed light collected by Herschel's giant 3.5m mirror to flood its supercold instrument chamber, or cryostat, for the first time.
The observatory's quest is to study how stars and galaxies form, and how they evolve through cosmic time.
The command sent on Sunday to fire two pyrotechnic bolts holding down the hatch was arguably the key moment in the European Space Agency (Esa) mission since the 14 May launch from Earth.
"We need the lid open or we can't see the sky, so it's a really important event," said Professor Matt Griffin, the principal investigator on SPIRE, one of three instruments inside the cryostat.
There is a YouTube video that shows what would have happened - in slow motion.
News of the hatch opening came on the eve of the Paris air show, a big event in the space calendar when Esa and the space industry come together to celebrate their achievements.
A Herschel display is a prominent feature in the Esa pavilion which the public can visit from Friday 19 June, after the trade days here at Le Bourget that run from Monday to Thursday.
Herschel's instruments sit inside a tank kept at supercold temperatures
During final ground testing (L), the cryostat was sealed and evacuated
Only in the vacuum of space (R) can the cryostat be opened up safely
The lid release allows light from the mirror to reach the instruments
It was a critical procedure - failure would have killed the entire mission
Scientists stress it will be a while yet before they are ready to release a "first light" image from the telescope. Herschel is little more than half way through its check-out phase and it is still several weeks away from beginning full operations.
The astronomical community - and the public - will have to be patient as they wait for Esa's flagship space telescope (which is bigger than Hubble in mirror diameter) to show off its capability.
Herschel is sensitive to light at long wavelengths - in the far-infrared and sub-millimetre range.
This will allow it to see past the dust that scatters visible wavelengths, and to gaze at really cold places and objects in the Universe - from the birthing clouds of new stars to the icy comets that live far out in our Solar System.
Herschel was launched on an Ariane 5 rocket in May
The observatory is tuned to see the Universe in the far-infrared
Its 3.5m diameter mirror is the largest ever flown in space
Herschel can probe clouds of gas and dust to see stars being born
It will investigate how galaxies have evolved through time
The mission will end when all the superfluid helium boils off
For the observatory to see these phenomena requires that it, too, be very cold. Superfluid helium is used to take its instruments very close to "absolute zero" (-273C). This is done inside a huge evacuated tank.
For nearly two years, the instruments have been locked away in the top of this cryostat to maintain their frigid state and protect them from contamination. Only now - a month into the mission - was it considered safe to open the lid.
"Anything that's launched into space always has some water vapour and various other contaminants - volatile gases - absorbed in its materials; and in space the water and these volatiles slowly boil away into the vacuum of space," explained Professor Griffin, who is affiliated to Cardiff University, UK.
"It is conventional and necessary to wait for this to happen, to make sure these contaminants don't find their way inside the cryostat where they could condense in the instruments."
Herschel is heading for an observation position some 1.5 million km from Earth. It is more than 90% of the way there.
Indeed, its distance from home is now so great it took almost five seconds for the pyro command to reach Herschel. A slight temperature rise and a shaking detected on the spacecraft indicated to controllers that the lid opening was successful.