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Last Updated: Thursday, 8 February 2007, 02:15 GMT
Telescope mirror nears completion
JWST artist's impression   Image: Nasa
JWST will be the successor to the Hubble telescope
A giant mirror that will fly on the James Webb Space Telescope (JWST) - Nasa's next space observatory - is a step closer to completion.

Engineers have finished making the 18 hexagonal elements that will come together to form the telescope's 6.6m primary mirror.

The size of a mirror determines how much light a telescope can collect, and therefore how much detail it can see.

JWST is seen as the heir to the hugely successful Hubble Space Telescope.

JWST will study every phase in the history of our Universe.

Its large primary mirror will help it "see" further into the cosmos than other telescopes, to detect some of the first stars to emerge in the Universe.

JWST could even shed light on the origins of life in other planetary systems.

The orbiting observatory is due to launch in 2013 on an Ariane 5 rocket from Kourou in French Guiana.

JWST mirror blank   Image: Axsys
The primary mirror will consist of 18 lightweight elements like this

JWST's primary mirror will be 6.6m (22ft) in diameter, compared with Hubble's 2.4m (7.9ft) mirror.

Hubble's primary mirror was made in one piece. JWST's primary mirror consists of 18 different elements, allowing it to be folded up into the rocket that will carry it into space.

This design also gives the telescope more flexibility in space, because each segment can be moved by ground controllers to tweak up its performance.

Infographic, BBC

Each of the hexagonal mirror segments measures 1.3m (4.3 feet) in diameter and weighs just 20kg (46lbs).They are made of beryllium - one of the lightest of metals known to science.

Although the completed primary mirror's diameter will be 2.75 times larger than that of Hubble, it will weigh roughly half as much.

Beryllium has been used in other space telescopes and has worked well at the super-frigid temperatures of space in which JWST will operate.

Hexagonal mirror blank  Image: Axsys
Engineers at firm Axsys examine one of the segments
The telescope is designed to operate at cryogenic temperatures, on the order of 40 Kelvin (-233C, -388F).

This prevents JWST from emitting its own infrared radiation, swamping faint astronomical signals.

Now the mirror elements are complete, they have been taken to a firm in California to be ground and polished.

After grinding and polishing, the completed mirror segments will be assembled at a separate facility.

They are then transported to Nasa's Goddard Space Flight Center in Maryland to be integrated into the telescope.

JWST will study the first light to emerge after the Big Bang, the first stars to form in the Universe, investigate how galaxies are assembled and look for clues to the origin of life in other planetary systems.

Infographic, BBC
JWST is named after James E Webb, Nasa Administrator during the Apollo lunar exploration era; he served from 1961 to 1968
It will be placed 1.5m km from Earth, at Lagrange Point 2, an area of gravitational balance that keeps it in a Sun-Earth line
The telescope will be shaded from sunlight by a shield, enabling it to stay cold, increasing its sensitivity to infrared radiation
Three principal instruments will gather images of the Universe in the infrared region of the spectrum
These will yield new information about how stars and galaxies first formed a few hundred million years after the Big Bang

First stars in telescope's sights
12 Jan 07 |  Science/Nature
Large camera delights astronomers
10 Nov 03 |  Science/Nature

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