The ESO VLT in Chile was described in Section 7.4.1. In addition to the main 8.2m utility telescopes, the last of which is almost complete, the site also includes the Italian VLT 2.6 m Survey Telescope (VLST) and the British 4 m Visible and Infrared Telescope for Astronomy (VISTA), which were both installed in 2007/2008. One of the most exciting developments at the VLT, however, is the construction of the VLT Interferometer (VLTI). The VLTI will, for the first time, provide a long-baseline interferometric capability to the visible/infrared, routinely used in microwave observations, and thus greatly increase the angular resolution of ground-based observations at these wavelengths. Among other things, the VLTI provides an important precursor to possible future terrestrial planet-finding missions such as NASA's TPF and ESA's Darwin (Section 8.7). The VLTI system will be able to combine light from any of the four main unit telescopes (UTs), together with light gathered by any of three 1.8 m auxiliary telescopes (ATs), which may be moved on a rail system to be placed at any of 30 stations, from which light may be directed to the
Beam Combination Laboratory (Figure 8.1). The maximum baseline achievable with the UTs is 130 m, while the ATs may be placed as far apart as 200 m, giving angular resolutions theoretically as high as 0.001" at visible wavelengths. While the UTs will only occasionally be used for interferometry, the ATs are dedicated to the VLTI alone. Before the beams from two telescopes can be combined, the light from one must first be passed through optical delay lines, as shown in Figure 8.2, in order to compensate for the different distances between the observing stations and the Beam Combination Laboratory. The accuracy of pathlength compensation must be to within 0.05 ^m over a distance of 120 m and the VLTI delay line system uses a retro-reflector fixed on a carriage running on two stainless steel rails. The carriage is driven by a 60 m linear motor and a piezo-transducer element, and a laser metrology system monitors the instantaneous distances between the mirrors. The Beam Combination Laboratory houses a number of instruments for performing interferometry across the wide wavelength range, and the proposed GENIE experiment (den Hartog et al., 2006; Wallner et al., 2006) will test the principle of "nulling" interferometry, proposed for use by both the TPF and Darwin missions to cancel the light of stars allowing direct imaging of orbiting extrasolar planets. Nulling interferometry will be discussed further in Section 8.7.3.
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