Shooting lasers, guiding the eye

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 back left of this image.

Unit Telescope 4 is equipped with laser guide stars, which beam upwards and lights 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, which can be used for adaptive optics correction.

Settled on a mountain peak in the remote Atacama Desert, the VLT experiences little or no light pollution. However, the exquisite sky isn’t completely dark. This picture shows the beautiful, diffuse glow of sunlight being scattered by interplanetary dust —  zodiacal light — while the atmosphere emanates a faint, steady shine — airglow — that can only be seen on the darkest of nights.