James Webb's mirror is revealed

By Jonathan Amos
BBC Science Correspondent

image copyrightNASA
image captionEngineers remove the last two covers used to protect the mirror segments during assembly

Revealed for the first time in all its glory - the main mirror of the James Webb Space Telescope, which will be launched in 2018.

JWST is regarded as the successor to Hubble, and will carry technologies capable of detecting the light from the first stars to shine in the Universe.

Paramount in that quest will be a large primary reflecting surface.

And with a width of 6.5m, JWST's will have roughly seven times the light-collecting area of Hubble's mirror.

It is so big in fact that it must be capable of folding. Only by turning the edges inwards will the beryllium segments fit inside the telescope's launch rocket.

The observatory is currently under construction at the US space agency's Goddard Space Flight Center in Maryland.

When in recent months engineers stuck down the segments to their support structure, each hexagon had a cover on it.

Only now, as the engineers prepare to move to the next stage of assembly, have those covers been removed to reveal the full mirror.

image copyrightNASA
image captionEach beryllium mirror segment was bonded into place

Shortly, the secondary mirror, which is at the top of the black extension poles will be collapsed into a flat configuration.

Then, the whole edifice will be flipped 180 degrees. This will permit the engineering team to attach JWST's instruments behind the main mirror.

These can be seen in a raised cage off to the left.

Leaving such a sensitive surface exposed even for a short time may appear risky. The fear would be that it might get scratched. But the European Space Agency's JWST project scientist, Pierre Ferruit, said that was unlikely.

"The main danger is to get some accumulation of dust. But it's a cleanroom so that accumulation is very slow," he told BBC News.

"They need to rotate the telescope to get access to the back, and the protective covers were only resting on the mirror segments, so they had to be removed before the rotation.

"When the mirror is upside down, the exposure to dust will be much less, and I doubt anyone will be allowed to walk underneath."

Once the integration of mirror and instruments is complete, the telescope will be sent for environmental testing. It will be shaken and blasted with sound to mimic the rough rocket ride to orbit.

Assuming that goes well, the whole train - mirror and instruments - will ship to Nasa's Johnson Space Center in Texas for some final deep-chill testing.

This will be conducted in the giant cryo-vacuum chamber built to accommodate the 1960s Apollo hardware.

Once that work is done, engineers must attach the spacecraft bus, which incorporates elements such as the flight computers and communications system. Finally, James Webb will be given an immense deployable visor - the structure that will shield its delicate observations from the Sun's light and heat.

JWST is a joint venture between the US, European and Canadian space agencies.

Each of the partners has supplied instruments for the observatory. A key additional role for Europe is to launch the telescope. An Ariane rocket will be used. It will be the launcher's most valuable ever payload.

The full life-cycle cost of the JWST project is expected to approach $10bn once all the partners' contributions are taken into account.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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