The VLT “Venus” and the Belt of Venus

Before another clear, starry night falls at ESO's Paranal Observatory, home of the Very Large Telescope (VLT), the sky produces a palette of intense colours, putting on a beautiful show for observers. These colours can only be seen with such depth from sites such as Paranal, where the atmosphere is extraordinarily pure. Looking to the west, over the Pacific Ocean, the sunset sky turns bright orange and red. However, this photograph shows the view to the east instead, looking away from the Sun after it has just set. The grey-bluish shadow above the horizon is the shadow of our own planet. Above this is a pinkish glow known as the "Belt of Venus", a phenomenon produced by the reddened light of the setting Sun being backscattered by the Earth's atmosphere.

In the centre of the image is the fourth 8.2-metre Unit Telescope (UT4), part of the VLT. The Mapuche name given to UT4 is Yepun, which means Venus. As well as working as individual telescopes, groups of two or three UTs can combine their light using a technique called interferometry, which allows astronomers to see details up to 25 times finer than with the individual telescopes. The VLT also has four 1.8-metre Auxiliary Telescopes (ATs), housed in ultra-compact mobile enclosures, which are fully dedicated to interferometric observations. Two of the ATs are visible in the background, with a third mostly hidden.

The yellow frame-like structure in front of Yepun is the "M1 Lifting Platform", used when the giant 8.2-metre primary mirror (M1) of the telescope is periodically recoated. The delicate mirror and its support structure, which together weigh 45 tonnes, are removed from the telescope enclosure and slowly driven about two kilometres to a maintenance building at the Paranal base camp. This process is, unsurprisingly, performed with the utmost care.

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