Very Large Telescope - General Information

General Information

The VLT consists of an arrangement of four large (8.2 meter diameter) telescopes (called Unit Telescopes or UTs) with optical elements that 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 first of the UTs started operating in May 1998 and was offered to the astronomical community on 1 April 1999. The other telescopes followed suit in 1999 and 2000, thus making the VLT fully operational. Four 1.8-metre Auxiliary Telescopes (ATs) have been added to the VLTI to make it available when the UTs are being used for other projects. These ATs were installed between 2004 and 2007. Today, all four Unit Telescopes and all four Auxiliary Telescopes are operational.

The VLT's 8.2-meter telescopes were originally designed to operate in three modes:

• as a set of four independent telescopes (this is the primary mode of operation).
• 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. 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.