A small rock in the Atlantic: Moving Mirrors at the Roque de los Muchachos

Putting the Galileo mirror onto the lorry

Carefully putting the 6-tonne Galileo mirror onto the lorry

The big telescopes at the observatory at the Roque de Los Muchachos mostly use mirrors rather than lenses. Domestic mirrors have the aluminium behind glass to protect it, but astronomical mirrors have the aluminium on top. This gives the best image, but it also means that the aluminium slowly spoils, and has to be replaced every three years or so.

This is not simple. The aluminium layer is only about 1/10,000 th of a millimetre thick, which is about 1/100th the thickness of the thinnest human hair - the sort that grows on a baby's cheeks. So taking off the old layer is fairly easy - they just have to be very, very careful not to scratch the layer underneath.

The really clever bit is getting the new layer to be absolutely even. Paint brushes won't do! The mirror is carefully cleaned and put into a very clean tank. Once all the air has been pumped out, the engineers send an electric current through a small aluminium fuse at the top of the tank, which vaporises in the vacuum. The aluminium vapour settles on everything. A few grammes falls onto the mirror, forming a new coating. Once the aluminium coating has formed, it takes 12 hours to let the air back into the tank, and the mirror is ready to put back on the telescope.

But there are only two vacuum tanks in the observatory. One is in the Grantecan building, and it's used for segments of the Grantecan mirror. The other is in the William Herschel telescope building, since the WHT has the largest single-piece mirror in Europe. So the other telescopes send their mirrors to the WHT.

It's a ticklish operation. The Italian National Telescope, (also known as the Galileo) sent their mirrors to the WHT last week. The main mirror weighs 6 tonnes, and the reflective surface is delicate, so moving it onto a lorry is not simple. Once the mirror has been loaded, the road is closed and the lorry moves at walking speed. The road will be closed again on Monday and Tuesday while the mirrors come back.