Very Large Telescope , Paranal

A mountain and four mirrors

The European Southern Observatory chose Cerro Paranal, at 2,635 metres in the Atacama Desert, in 1990 for its dark, dry and stable air, and levelled the summit for the observatory. The first Unit Telescope, Antu, saw first light in May 1998 and was offered to the astronomical community on 1 April 1999. Three more followed, named in the Mapuche language Kueyen, Melipal and Yepun.

Each of the four 8.2-metre mirrors is a thin meniscus of glass, kept in shape by an active optics system of actuators, which corrects distortions from gravity and temperature during observation.

Combining the telescopes

In 2011 the four Unit Telescopes and four 1.8-metre Auxiliary Telescopes were combined for the first time in the VLT Interferometer. Light from the telescopes is brought together through underground tunnels, with the path lengths matched to a fraction of a wavelength, so that the array has the resolution of a much larger telescope.

In 2004 the VLT took the first direct image of an exoplanet, and in 2018 the GRAVITY instrument measured the gravitational redshift of the star S2 as it passed close to the black hole at the centre of the Milky Way, a test of Einstein's theory. Reinhard Genzel shared the 2020 Nobel Prize in Physics for work on the Galactic Centre using ESO telescopes.

The next step

ESO is building the Extremely Large Telescope, a 39-metre telescope on Cerro Armazones near Paranal, to follow the VLT. Paranal's skies are protected by Chilean regulations on light pollution, and ESO engages in the debate on industrial projects planned near the site. The VLT's data are archived and become public after a year.

A telescope and its neighbourhood

The VLT's performance depends on the air above Paranal and on the darkness of the desert around it. ESO has long worked with Chilean authorities and neighbours on protecting the site, including a regulation on light pollution and attention to the plans of industrial projects in the vicinity. The Extremely Large Telescope at nearby Armazones will extend the observatory's reach and will share the infrastructure. The VLT is thus a reminder that an observatory is a property right over an invisible resource, the clarity and darkness of the sky, which must be defended as carefully as a building.

Instruments and technologies

• Unit Telescopes Four 8.2-metre altazimuth reflectors with active optics, each with Nasmyth and Cassegrain foci, hosting a suite of instruments.
• VLT Interferometer Combines light from up to four telescopes through delay lines, with baselines up to 130 metres.
• GRAVITY An instrument that combines four telescopes and measures positions of stars near the Galactic Centre.

The people

• Reinhard Genzel Astrophysicist; shared the 2020 Nobel Prize in Physics for work on the Galactic Centre.

Timeline

• 1998 Antu, the first Unit Telescope, sees first light in May.
• 1999 Offered to the astronomical community on 1 April.
• 2011 All four Unit and four Auxiliary Telescopes combined as the VLTI for the first time.
• 2018 Gravitational redshift measured in the orbit of the star S2 around the Galactic Centre black hole.
• 2020 Reinhard Genzel shares the Nobel Prize in Physics for Galactic Centre work using ESO telescopes.

Key lessons for observation today

• Active control makes thin mirrors work Actuators keep the mirror's shape. Active control lets structures be lighter and larger.
• Path lengths must be matched The interferometer works only if timing is controlled to a fraction of a wavelength. Precise timing is the basis of combining sensors.
• Protect the dark sky The site's value depends on its surroundings. Environmental protection of a site needs legal and political work.

Sources

• Wikipedia, Very Large Telescope
• ESO, ANTU enclosure – 1998
• ESO, Nobel Prize in Physics 2020 press release