English: This aerial shot of ESO’s Very Large Telescope array on top of the 2600-metre-high Cerro Paranal in the Chilean Atacama Desert beautifully shows the various stations for the mobile Auxiliary Telescopes. The largest structures are the enclosures of the four 8.2-metre Unit Telescopes of the VLT. In the middle lies the VLT Interferometer (VLTI) laboratory.

Contrary to other large astronomical telescopes, the VLT was designed from the beginning with the use of interferometry as a major goal. The VLTI combines light captured by two or three 8.2-metre VLT Unit Telescopes, dramatically increasing the spatial resolution and showing fine details of a large variety of celestial objects. However, most of the time, the large telescopes are used for other research purposes. They are therefore only available for interferometric observations during a limited number of nights every year. Thus, in order to exploit the VLTI each night and to achieve the full potential of this unique setup, some other smaller, 1.8-metre dedicated telescopes were included into the overall VLT concept. These telescopes, known as the VLTI Auxiliary Telescopes (ATs), are mounted on tracks and can be placed at precisely defined “parking” observing positions on the observatory platform (seen along the lines in the image). From these positions, their light beams are fed into the VLTI laboratory via a complex system of reflecting mirrors mounted in an underground system of tunnels.

Taken in 2005, this photo shows only two of the four ATs that are currently in operation. The enclosure on the upper right of the image will soon host the VLT Survey Telescope (VST).