Very Large Telescope (VLT) at night, Chile. Despite its name, ESO's Very Large Telescope (VLT) is not a single telescope. It is actually made up of four separate 8.2-metre Unit Telescopes (the larger structures visible here) and four movable 1.8-metre Auxiliary Telescopes, two of which can be seen to the rear left of this image. Unit Telescope 4 is equipped with laser guide stars, which beam upwards and light up the atmosphere some 90 kilometres above ground. These bright laser beams reach high into the mesosphere, where they excite sodium atoms and cause them to emit photons, creating a bright artificial star that can be used for adaptive optics correction. The night sky is illuminated by a band of zodiacal light (bottom left to top left) and airglow along the horizon.