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Very Large Telescope Interferometer (VLTI)

The European Southern Observatory's (ESO's) Very Large Telescope (VLT) has interferometric tunnels connecting the four 8.2-m telescopes with each other via their so-called Coude foci. In addition, 4 smaller telescopes (1.8-m siderostats) are included in the interferometer, to add more baselines. More than 20(!) mirrors are required to combine the signals from each two telescopes for measurements of interference patterns (fringes). For more details on interferometry, try this page.

VLTI siderostat base stations in the foreground on the top of Paranal. The mountain top was blasted away to have such a flat surface, under which tunnels are located for transporting the light beams from each telescope to the delay units and from there on to the detectors.

Delay lines are devices that delay the signal coming from one telescope with respect to that of another so as to make both signals arrive at the detector simultaneously. This way both telescopes in one interferometer baseline form part of a single synthesised telescope surface. Part of a VLTI delay line is shown in the photo below.

Part of a VLTI delay line, with rails on which a carriage can be moved with a mirror ontop. To see a (smaller) carriage, have a look at a photo from SUSI.

The siderostats can be used at any of the base stations visible in the first photo above. Here a sketch of their locations on the mountain top and an impression of the layout of the light tunnels.

Layout of VLTI base stations (click on image for higher-resolution version).

After a long path (and more than 20 mirror reflections) the light from two telescopes is finally combined in an optical bench like the one shown below and then registered by a scientific instrument.

View of a VLTI detector assembly.

Northern Chile being an earthquake area, the VLT is built onto a concrete platform sitting on a solid rock mountain top. The VLTI experiment is sheltered within the concrete foundation of the observatory. The entire observatory is designed to withstand a magnitude 7 (on the Richter scale) earthquake.