Test run for telescope's new coat

The aluminium coating on the mirrors will play a critical role in the performance of the so-called Large Binocular Telescope. The instrument will not be able to see without it.

Experts from the Ohio State University, US, will only get one try on the real things, so they are going to practise the fiendishly complex process on a dummy mirror.

If all goes well, the work of the researchers will help to produce one of the most powerful optical and infrared telescopes ever built on Earth.

Big weight

The aluminium coating, aside from being only a matter of a molecule thick, must also be completely uniform. Any bumps or lumps would ruin the whole project.

Vacuum technology is used to help apply an even coating - but as telescopes, and their mirrors, get bigger and bigger, the vacuum becomes harder and harder to control.

In an aircraft hangar in Columbus, Ohio, the university's engineers have constructed a test-bed comprising a 9.1-metre diameter steel cell, containing a dummy mirror with an 8.4-m diameter.

A lid for the cell has been made, with all of these components adding up to almost 70 tonnes.

The lid and cell will be placed on rail tracks, and slowly moved together until a precise connection is made.

Metal steam

Then vacuum pumps will extract most of the air inside. The remaining fraction of 1% will be absorbed by extremely cold charcoal in a special cryogenic system.

It is only in this vacuum that the aluminium can be applied.

This is done by boiling the metal in crucibles each containing one ounce.

As the aluminium "steam" rises, it will condense on the mirror surface.

A scattering gas will be used to make sure that the deposits are even.

New generation 'scope

If the test is successful, the equipment will be taken to Mount Graham in Arizona, where the actual mirrors will be coated.

The eventual Large Binocular Telescope is expected to have 25 times the light collecting area, and 10 times the image resolution of the Hubble Space Telescope.

Its two giant mirrors will mimic the performance of a telescope with a single mirror more than 11 metres across.

Mechanical manipulation of the mirrors and computer processing of the light they gather will further improve the quality of the images obtained by the telescope.

Its astronomical data, which will start flowing in 2004, should rival those of the other great telescopes now in operation, such as the VLT in Chile and the Keck twins in Hawaii.

The LBT is a joint project between Italian, US and German astronomical interests.