oldest of the three ESO observatories and comprises a number of optical telescopes of diameter between 0.6 m and 3.6 m, which are listed in Table 7.1. The 12 m Atacama Pathfinder Experiment (APEX) is based at Llano de Chajnantor (23°00'S, 67°46'W) at an altitude of 5,104 m in the Atacama desert and is the largest submillimeter telescope in the southern hemisphere. Finally, the Paranal Observatory (24°40'S, 70°25'W) at an altitude of 2,645m is home to ESO's Very Large Telescope (VLT) facility and was chosen for its excellent atmospheric conditions and remoteness, ensuring that its operation is not disturbed by the effects of human settlement such as dust and light from roads and mines.

The VLT is the world's largest and most advanced astronomical observatory (Figure 7.11). It comprises four 8.2m reflecting unit telescopes, which may operate individually or in combined mode providing the total light-collecting power of a single 16 m telescope. In addition, the telescopes can be used in an interferometric mode together with three movable 1.8 m auxiliary telescopes to form the VLT Interferometer (VLTI). The four large telescopes, initially designated UT1-UT4, have been renamed in the indigenous Mapuche language as Antu (the Sun), Kueyen (the Moon), Melipal (the Southern Cross), and Yepun (Venus). A schematic of the observatory showing the telescopes and instruments that will eventually be available is shown in Figure 7.12. To save weight, the primary mirrors of the unit telescopes are rather thin and thus flexible. Hence, their shape is dynamically controlled using active optics to apply correcting forces to the primary mirror and move the secondary mirror in order to cancel out the errors. The active optics system was originally developed for the ES0 3.5 m New Technology Telescope (NTT) at La Silla. The VLT telescopes use the Ritchey-Chretien optical design and each telescope may operate in the Cassegrain, Nasmyth, or Coude focus. The image quality of the VLT telescopes is very impressive with a record angular resolution of 0.18". More usually, the seeing is in the 0.5" to 1.0" range, but the VLT may use its adaptive optics to achieve near diffraction-limited observations of angular resolution 0.05". Should no guide star be available then UT4 (Yepun) incorporates a laser system to generate a laser guide star. UT1 (Antu) achieved first light on May 25, 1998, and the last completed telescope, UT4 (Yepun), achieved first light on September 3, 2000. The VLT provides excellent data on the giant planets due to its high angular resolution, and is equipped with a number of instruments for imaging and spectroscopy from the UV to approximately 25 ^m.

Figure 7.11. The European Southern Observatory Very Large Telescope (VLT) at La Paranal, Chile. Courtesy of ESO.

7.4.2 The Mauna Kea observatories

The Mauna Kea volcano on Big Island, Hawaii is an excellent place for astronomical observations due to its high altitude of 4,200 m and its dry, stable atmospheric conditions. Hence, the site is currently home to the world's largest and most powerful telescopes (Figure 7.13). The largest of these are the twin Keck telescopes (Figure 7.14), which have primary mirrors that are 10 m in diameter and are composed of 36 individual actively controlled hexagonal elements that operate together as a single, high-precision mirror. The Keck Observatory is jointly operated by NASA and the California Institute of Technology (Caltech). The Keck I telescope began operations in May 1993, while Keck II began observing in October 1996.

Other telescopes which are situated on the Mauna Kea site are listed in Table 7.2 and of particular note are the Japanese Subaru 8.2m telescope, the Gemini North 8.1 m telescope, the NASA Infrared Telescope Facility (IRTF), the Canada-France-

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