Very Large Telescope
The Very Large Telescope is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with a primary mirror that measures in diameter. These optical telescopes, named Antu, Kueyen, Melipal, and Yepun, are generally used separately but can be combined to achieve a very high angular resolution. The VLT array is also complemented by four movable Auxiliary Telescopes with apertures.
The VLT is capable of observing both visible and infrared wavelengths. Each individual telescope can detect objects that are roughly four billion times fainter than what can be seen with the naked eye. When all the telescopes are combined, the facility can achieve an angular resolution of approximately 0.002 arcsecond. In single telescope mode, the angular resolution is about 0.05 arcseconds.
The VLT is one of the most productive facilities for astronomy, second only to the Hubble Space Telescope in terms of the number of scientific papers produced from facilities operating at visible wavelengths. Some of the pioneering observations made using the VLT include the first direct image of an exoplanet, the tracking of stars orbiting around the supermassive black hole at the centre of the Milky Way, and observations of the afterglow of the furthest known gamma-ray burst.
General information
The VLT consists of an arrangement of four large, diameter telescopes with optical elements that can combine them into an astronomical interferometer, which is used to resolve small objects. The interferometer also includes a set of four 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 became operational in 1999 and 2000, enabling multi-telescope VLT capability. Four 1.8-metre Auxiliary Telescopes have been added to the VLTI to make it available when the UTs are being used for other projects. These ATs were installed and became operational between 2004 and 2007.The VLT's 8.2-meter telescopes were originally designed to operate in three modes:
- as a set of four independent telescopes.
- as a single large coherent interferometric instrument, for extra resolution. This mode is used for observations of relatively bright sources with a small angular extent.
- as a single large incoherent instrument, for extra light-gathering capacity. The instrumentation required to obtain a combined incoherent focus was not originally built. In 2009, new instrumentation proposals were put forward to potentially make that 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.
Unit telescopes
The primary mirrors of the UTs are in diameter but, in practice, the pupil of the telescopes is defined by their secondary mirrors, effectively reducing the usable diameter to at the Nasmyth focus and at the Cassegrain focus.
The 8.2-metre-diameter telescopes are housed in compact, thermally controlled buildings, which rotate synchronously with the telescopes. This design minimises any adverse effects on the observing conditions, for instance from air turbulence in the telescope tube, which might otherwise occur due to variations in the temperature and wind flow.
The principal role of the main VLT telescopes is to operate as four independent telescopes. The interferometry is used about 20 percent of the time for very high-resolution on bright objects, for example, on Betelgeuse. This mode allows astronomers to see details up to 25 times finer than with individual telescopes. The light beams are combined in the VLTI using a complex system of mirrors in tunnels where the light paths must be kept equal within differences of less than 1 μm over a light path of a hundred metres. With this kind of precision, the VLTI can reconstruct images with an angular resolution of milliarcseconds.
Mapuche names for the Unit Telescopes
It had long been ESO's intention to provide "real" names to the four VLT Unit Telescopes, to replace the original technical designations of UT1 to UT4. In March 1999, at the time of the Paranal inauguration, four meaningful names of objects in the sky in the Mapuche language were chosen. These indigenous people live mostly south of Santiago de Chile.An essay contest was arranged in this connection among schoolchildren of the Chilean II Region of which Antofagasta is the capital to write about the implications of these names. It drew many entries dealing with the cultural heritage of ESO's host country.
The winning essay was submitted by 17-year-old Jorssy Albanez Castilla from Chuquicamata near the city of Calama. She received the prize, an amateur telescope, during the inauguration of the Paranal site.
Unit Telescopes 1–4 are since known as Antu, Kueyen, Melipal, and Yepun, respectively. Originally there was some confusion as to whether Yepun actually stands for the evening star Venus, because a Spanish-Mapuche dictionary from the 1940s wrongly translated Yepun as "Sirius".
Auxiliary telescopes
Although the four 8.2-metre Unit Telescopes can be combined in the VLTI, their observation time is spent mostly on individual observations, and are used for interferometric observations for a limited number of nights every year. However, the four smaller 1.8-metre ATs are available and dedicated to interferometry to allow the VLTI to operate every night.The top part of each AT is a round enclosure, made from two sets of three segments, which open and close. Its job is to protect the delicate 1.8-metre telescope from desert conditions. The enclosure is supported by the boxy transporter section, which also contains electronics cabinets, liquid cooling systems, air-conditioning units, power supplies, and more. During astronomical observations the enclosure and transporter are mechanically isolated from the telescope, to ensure that no vibrations compromise the data collected.
The transporter section runs on tracks, so the ATs can be moved to 30 different observing locations. As the VLTI acts rather like a single telescope as large as the group of telescopes combined, changing the positions of the ATs means that the VLTI can be adjusted according to the needs of the observing project. The reconfigurable nature of the VLTI is similar to that of the Very Large Array.
Scientific results
Results from the VLT have led to the publication of an average of more than one peer-reviewed scientific paper per day. For instance in 2017, over 600 refereed scientific papers were published based on VLT data. The telescope's scientific discoveries include direct imaging of Beta Pictoris b, the first extrasolar planet so imaged, tracking individual stars moving around the supermassive black hole at the centre of the Milky Way, and observing the afterglow of the furthest known gamma-ray burst.In 2018, the VLT helped to perform the first successful test of Albert Einstein's General Relativity on the motion of a star passing through the extreme gravitational field near the supermassive black hole, that is the gravitational redshift. In fact, the observation has been conducted for over 26 years with the SINFONI and NACO adaptive optics instruments in the VLT while the new approach in 2018 also used the beam-combiner instrument GRAVITY. The Galactic Centre team at the Max Planck Institute for Extraterrestrial Physics used these observations to reveal these effects for the first time.
Other discoveries with VLT's signature include the detection of carbon monoxide molecules in a galaxy located almost 11 billion light-years away for the first time, a feat that had remained elusive for 25 years. This has allowed astronomers to obtain the most precise measurement of the cosmic temperature at such a remote epoch. Another important study was that of the violent flares from the supermassive black hole at the centre of the Milky Way. The VLT and APEX teamed up to reveal material being stretched out as it orbits in the intense gravity close to the central black hole.
Using the VLT, astronomers have also estimated the age of extremely old stars in the NGC 6397 cluster. Based on stellar evolution models, two stars were found to be 13.4 ± 0.8 billion years old, that is, they are from the earliest era of star formation in the Universe. They have also analysed the atmosphere around a super-Earth exoplanet for the first time using the VLT. The planet, which is known as GJ 1214b, was studied as it passed in front of its parent star and some of the starlight passed through the planet's atmosphere.
In all, of the top 10 discoveries done at ESO's observatories, seven made use of the VLT.
Technical details
Telescopes
Each Unit Telescope is a Ritchey-Chretien Cassegrain telescope with a 22-tonne 8.2-metre Zerodur primary mirror of 14.4-metre focal length, and a 1.1-metre lightweight beryllium secondary mirror. A flat tertiary mirror diverts the light to one of two instruments at the f/15 Nasmyth foci on either side, with a system focal length of 120 metres, or the tertiary tilts aside to allow light through the primary mirror central hole to a third instrument at the Cassegrain focus. This allows switching between any of the three instruments within five minutes, to match observing conditions. Additional mirrors can send the light via tunnels to the central VLTI beam-combiners. The maximum field-of-view is around 27 arcminutes in diameter, slightly smaller than the full moon, though most instruments view a narrower field.Each telescope has an alt-azimuth mount with total mass around 350 tonnes, and uses active optics with 150 supports on the back of the primary mirror to control the shape of the thin mirror by computers.