JWST will be the successor to the Hubble telescope

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.

The primary mirror will consist of 18 lightweight elements like this

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.

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.

Engineers at firm Axsys examine one of the segments

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.