# Very Large Telescope

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The Very Large Telescope (VLT) is made up of four separate optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal telescope, and the Yepun telescope) organized in an array formation, built and operated by the European Southern Observatory (ESO) at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama Desert in northern Chile. Each telescope has an 8.2 m aperture. The array is complemented by four movable Auxiliary Telescopes (ATs) of 1.8 m aperture. Working together in interferometric mode, the telescopes can achieve an angular resolution of around 1 milliarcsecond, meaning it could distinguish the gap between the headlights of a car located on the Moon.[1]

## Contents

### General information

The VLT consists of an arrangement of four large (8.2 meter diameter) telescopes, and optical elements which can combine them into an astronomical interferometer (VLTI) which is used to resolve small objects. The interferometer also includes a set of four 1.8 meter diameter movable telescopes dedicated to interferometric observations. The 8.2 meter telescopes have been named after some astronomical objects in the local mapudungun language: Antu (The Sun), Kueyen (The Moon), Melipal (The Southern Cross), and Yepun (Venus).

The VLT 8.2 meter telescopes was originally designed to be operated in three modes:[2]

• as a set of four independent telescopes (this is the primary mode of operation). 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 times fainter than what can be seen with the unaided eye.
• as a single large coherent interferometric instrument (the VLT Interferometer or VLTI), for extra resolution. This mode is occasionally used, only for observations of relatively bright sources with small angular extent.
• as a single large incoherent instrument, for extra light-gathering capacity. The instrumentation required to bring the light to a combined incoherent focus was not built. Recently, new instrumentation proposals have been put forward for making this observing mode available.[3] Multiple telescopes are sometimes independently pointed at the same object, either to increase the total light-gathering power, or to provide simultaneous observations with complementary instruments.

The VLTs are equipped with a large set of instruments permitting observations to be performed from the near-UV to the mid-IR (i.e. a large fraction of the light wavelengths accessible from the surface of the Earth), with the full range of techniques including high-resolution spectroscopy, multi-object spectroscopy, imaging, and high-resolution imaging. In particular, the VLT has several Adaptive optics systems, which at infrared wavelengths correct for the effects of the atmospheric turbulence, providing images almost as sharp as if the telescope were in space. In the near-IR, the Adaptive Optics images of the VLT are up to three times sharper than those of the Hubble Space Telescope, and the spectroscopic resolution is many times better than Hubble. The VLTs are noted for their high level of observing efficiency and automation.