Light of the silvery Moon

Nautical twilight, above the Very Large Telescope platform. Near the horizon the bright moon is shining above the Antu telescope, the first one near the centre. At his left, above the horizon are visible some of the Auxiliary Telescopes (ATs) of 1.8 m aperture. At the right side of Antu, is the telescope Kueyen, with a mirror of 8.2m diameter. Both, are opening and preparing for a night of observations. This telescopes are generally used separately, but can be used together to achieve a very high angular resolution. Looking from outside, they are reflecting a silver colour coming from the moon reflection on its metallic surface. In the ground, at the left side of the image, we can see part of the interferometer (VLTI) complex, where the movable Auxiliary Telescopes can be placed. The Very Large Telescope (VLT) is a telescope operated by the ESO - European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT is the world's most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter, which are generally used separately but can be used together to achieve very high angular resolution. The four separate optical telescopes are known as Antu, Kueyen, Melipal and Yepun, which are all words for astronomical objects in the Mapuche language, with optical elements that can combine them into an astronomical interferometer (VLTI), which is used to resolve small objects. The interferometer is complemented by four movable Auxiliary Telescopes (ATs) of 1.8 m aperture. 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. The telescopes can work together, 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.