Keeping The VLT's Eyes Clear: Recoating A Giant Mirror

Every night, all year round, the ESO Very Large Telescope, or VLT, opens its four giant eyes to scrutinise the beautiful southern skies. Each eye is a huge mirror, 8.2 metres in diameter, that gathers the light of the night sky, and reflects it into optical systems that form ultra-sharp images of the Universe. But keeping the VLT´s eyes clear requires each mirror to be cleaned and recoated occasionally, a delicate and complex procedure.

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The world's most advanced visible-light astronomical observatory

The skies over the ESO sites in Chile are so dark that on a clear moonless night it is possible to see your shadow cast by the light of the Milky Way alone.

The Very Large Telescope array (VLT) is the flagship facility for European ground-based astronomy at the beginning of the third Millennium. It is the worlds most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter and four movable 1.8m diameter Auxiliary Telescopes.

The telescopes can work together, in groups of two or three, to form a giant interferometer, the ESO Very Large Telescope Interferometer, allowing astronomers to see details up to 25 times finer than with the individual telescopes.

The light beams are combined in the VLTI using a complex system of mirrors in underground tunnels where the light paths must be kept equal to distances less than 1/1000 mm over a hundred metres.

With this kind of precision the VLTI can reconstruct images with an angular resolution of milliarcseconds, equivalent to distinguishing the two headlights of a car at the distance of the Moon.

The 8.2m diameter Unit Telescopes can also be used individually. With one such telescope, images of celestial objects as faint as magnitude 30 can be obtained in a one-hour exposure. This corresponds to seeing objects that are four billion (four thousand million) times fainter than what can be seen with the unaided eye.

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