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Space telescope mirrors ready for polishing

James Webb's eye is lookin' good

After a rather difficult week, some good news for NASA: its engineers have just finished making the mirror that will be the eye of its next space observatory, the James Webb Space Telescope (JWST). All that remains is for it to be ground and polished.

The telescope will be used, as Hubble was and still is, to examine the farthest reaches of the universe. But its increased sensitivity, due to its larger mirror, means it will be able to see the first galaxies that formed after the big bang.

The mirror of the JWST will be so big it could not be launched in one piece. Instead, NASA has built 18 separate hexagonal mirrors, made of beryllium, which will be assembled once the telescope is in orbit.

Each segment is 1.3 metres across and weighs 20kg. With a span of 6.6 metres, the fully assembled mirror will be more than two and a half times the size of Hubble's but will only weigh* half as much.

Just how big is big?

"The James Webb Space Telescope will collect light approximately nine times faster than the Hubble Space Telescope when one takes into account the details of the relative mirror sizes, shapes, and features in each design," said Eric Smith, JWST program scientist at NASA Headquarters, Washington.

NASA says the scope has the potential to revolutionise studies of how stars and planetary systems form and evolve.

The completed mirror pieces have already been transferred to California where they will be ground and polished. Next, the polished pieces will be taken to Ball Aerospace where they will be assembled, NASA says. After that, they'll go to back to the Goddard Space Centre for final assembly before the observatory's 2011 launch. ®

*Assuming one g acting on each mirror. Pedants, begone.

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