Looking down the VLTI tunnel

At the Very Large Telescope (VLT), the telescopes can work together, in groups of two or three, to form a giant ‘interferometer’, the ESO Very Large Telescope Interferometer (VLTI). This allows astronomers to see details up to 25 times finer than with the individual telescopes. The light beams are combined in the VLTI laboratory using a complex system of mirrors in underground tunnels (shown here) where the light paths must be kept equal to distances less than 1/1000 mm over a hundred metres.