Imagine having to melt 24 tonnes of glass to make an 8.4-metre-wide telescope mirror. Now imagine if that were the easy part of the project.

The mirror-making for the ambitious Large Synoptic Survey Telescope (LSST) project is now underway, but the greatest challenges still lie ahead.

By 2015, the new $400 million observatory should begin recording the entire night sky every three days from its perch in the Chilean Andes. It will provide a vast treasure trove of data invaluable for tracking near-Earth asteroids, watching supernovae explosions and mulling the mysteries of dark matter and dark energy.

Before then, though, LSST project managers must perfect how to gather the faint starlight, capture it for posterity and share the images with the world.

Spin Cycle

At the telescope's heart is its primary mirror. Its unusual design will incorporate a tertiary mirror ground into the middle, as if someone had taken a giant-sized ice cream scoop from the glass surface. To accommodate this extra depth, the mirror was built especially thick – almost a third of a metre at its centre, and about a metre at its edge.

Late in March, workers at the University of Arizona's Steward Observatory Mirror Laboratory fired up the furnaces around the mirror mould, heating the glass until it melted to the consistency of heavy molasses.

Then, they set the mould spinning, giving the mirror surface a gentle curve and eliminating the need for nearly 14 tonnes of extra glass that would otherwise have been ground away.

With the melting and rough shaping now complete, the mirror will rest in a computer-controlled cool-down for the next three months to avoid generating any stresses within the glass.

Trickier than Hubble

So far, the mirror appears to be in good shape, says LSST Project Manager Don Sweeney. "We'll only know for sure after it's down to room temperature," he told New Scientist. "But everything looks very, very good; it's very unlikely that there will be a problem."

For mirror lab director Roger Angel, though, the hard work is just beginning. Once the mirror cools, it will have to be ground and polished to gather light perfectly.

"There's quite a lot of testing there to make sure we get the curves right," he told New Scientist. "The mirrors that we make now are way more difficult in testing than the Hubble mirror."

Wide-eyed

With its vast primary mirror, 3.4-metre secondary mirror and sizable tertiary mirror, the LSST will be able to see a patch of sky about 40 times the size of the Full Moon (other large telescopes can only see a fraction of the Moon at one time).

Capturing and storing such large images is not easy, though. Developing sensors for the camera, which has a 60-centimetre-wide focal plane covered in 10-micron pixels, is one of the biggest challenges left to be solved before the telescope is up and running.

Another is developing a way to process and publish the 30 terabytes of data captured each night. "Scientifically, it is the world's biggest database," Sweeney says. "You have to process all of that data as fast as it comes up, because if you get behind, you'll never catch up."

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