Very Large Telescope: Powerful Eyes on the Sky

The Very Large Telescope (VLT) in Chile can spot objects that are 4 billion times fainter than what can be seen with the unaided eye.

Seated in the Atacama Desert of Chile, the European Southern Observatory (ESO)’s Very Large Telescope (VLT) consists of four main telescopes and four smaller telescopes that can be used separately or combined into a single larger instrument powerful enough to distinguish two car headlights at the distance of the moon.

The VLT is located at Paranal Observatory in the Atacama Desert. The four Unit Telescopes boast 8.2-meter (27 feet) mirrors. Just one of these instruments can spot objects that are 4 billion times fainter than what can be seen with the unaided eye. According to the ESO’s website, the VLT is “the world’s most advanced optical telescope.”

The first of the four instruments, Unit Telescope 1 (UT1), saw first light on May 25, 1998, and went into scientific operations on April 1, 1999. UT2 saw first light only four days before the observatory’s March 5, 1999, inauguration.

The four Unit Telescopes sit in compact, thermally controlled buildings that rotate with the instruments. These buildings minimize adverse effects, such as turbulence in the telescope tube, on observations.

At the inauguration, the four Unit telescopes were given names in the Mapuche language, from an indigenous people living in the area south of Santiago de Chile. Chile’s schoolchildren participated in the naming, with an essay by then-17-year-old Jorssy Albanez Castilla unanimously selected by the committee.

  • UT1 is known as Antu (an-too), which means the sun.
  • UT2 is Kueyen (quay-yen), or the moon.
  • UT3 is Melipal (me-li-pal), or the Southern Cross).
  • UT4 is Yepun (ye-poon), or the evening star (Venus).

The VLT also contains four moveable 1.8-meter Auxiliary Telescopes. All eight telescopes are currently operational.

Together, the eight telescopes can create a massive interferometer. However, the Unit Telescopes are more often used individually, and are only available to be combined for a limited number of nights each year. The four smaller Auxiliary Telescopes, however, are available to allow the interferometer to operate each night.

In February 2018, the ESPRESSO instrument (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) on the VLT allowed the four Unit Telescopes to combine light from all four telescopes for the first time, making the VLT the largest optical telescope in existence in terms of collecting area. Due to the complexity involved, the light from the four had not been previously combined; only three Unit Telescopes could be combined at once.

“This impressive milestone is the culmination of work by a large team of scientists and engineers over many years,” project scientist Paolo Molaro said in a statement. “It is wonderful to see ESPRESSO working with all four Unit Telescopes, and I look forward to the exciting science results to come.”

A system of mirrors, prisms and lenses transmits the light from each Unit Telescope to the ESPRESSO instrument up to 226 feet (69 meters) away. ESPRESSO can collect the light from all four Unit Telescopes together, or from each one individually.

“ESO has realized a dream that dates back to the time when the VLT was conceived in the 1980s: bringing the light from all four Unit Telescopes on Cerro Paranal together at an incoherent focus to feed a single instrument!” said ESPRESSO instrument scientist Gaspare Lo Curto.

Over nearly two decades, the VLT has made significant contributions to astronomy, snapping the first image of an exoplanet, capturing the first direct measurements of the atmosphere of a super-Earth, and taking the universe’s cosmic temperature. In addition, the observatory’s hotel served as a villain’s hideout in the James Bond flick, “Quantum of Solace.”

In 2004, a team of European and American astronomers studying the TW Hydrae Association, a group of very young stars and other objects, spotted a red speck of light near one of the association’s brown dwarfs. The object was more than 100 times fainter than its parent star. Further observations confirmed that it was an exoplanet orbiting its star at 55 times the Earth-sun distance.

“Our new images show convincingly that this really is a planet, the first planet that has ever been imaged outside of our solar system,” ESO astronomer Gael Chauvin said in a statement.

In 2008, a team of scientists used the VLT to discover and image an object near the star Beta Pictoris. Most directly imaged exoplanets lie far from their stars, past where Neptune would orbit, where stellar light is dimmer. In contrast, the planet Beta Pictoris b lies much closer, where Saturn would orbit.

“Direct imaging of extrasolar planets is necessary to test the various models of formation and evolution of planetary systems,” researcher Daniel Rouan said in a statement. “But such observations are only beginning. Limited today to giant planets around young stars, they will in the future extend to the detection of cooler and older planets, with the forthcoming instruments on the VLT and on the next generation of optical telescopes.”

Researchers also used the VLT to determine how fast Beta Pictoris b is spinning, clocking the massive planet almost 62,000 mph (100,000 km/h) at its equator. In comparison, Earth’s equator spins at only 1,056 mph (1,700 km/h), while Jupiter travels at about 29,000 mph (47,000 km/h). This was the first time an exoplanet’s rotation rate had been determined.

“It is not known why some planets spin fast and others more slowly,” researcher Remco de Kok said in a statement. “But this first measurement of an exoplanet’s rotation shows that the trend seen in the solar system, where the more massive planets spin faster, also holds true for exoplanets. This must be some universal consequence of the way planets form.”

The private organization Breakthrough Initiatives has enlisted the help of the VLT to hunt for planets around Earth’s closest star, Proxima Centauri. After helping to fund an upgrade to an existing instrument on the VLT, Breakthrough Initiatives will receive time for a “careful search” of the Proxima Centauri system for new planets. The improvement in the VLT Imager and Spectrometer for Mid Infrared instrument will equip it with a coronagraph, which blocks much of the light from a star, as well as an adaptive optics system to correct for distortions in starlight caused by Earth’s atmosphere. The upgrade is scheduled to be completed in 2019.

The VLT was also instrumental in revealing a system of seven Earth-sized planets just 40 light-years from Earth. The TRAPPIST-1 system boasts seven worlds, six of which appear to be rocky. All seven could potentially boast liquid water on their surface, though the innermost three appear to be too hot to hold onto it on more than a fraction of their surface. After an intriguing observation on the VLT in early 2016, when the system was first announced, researchers used multiple telescopes, including the VLT, to follow-up and observe the seven worlds.

“This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth!” said researcher Michaël Gillon of the STAR Institute at the University of Liège in Belgium.

The VLT has also been used to probe exoplanet atmospheres. In 2010, it studied the super-Earth GJ1214b and found an atmosphere dominated by thick clouds or hazes.

“This is the first super-Earth to have its atmosphere analyzed. We’ve reached a real milestone on the road toward characterizing these worlds,” researcher Jacob Bean said in a statement.

In 2008, the VLT took the cosmic temperature of the universe. By detecting carbon monoxide molecules in a galaxy located almost 11 billion light-years away, it allowed astronomers to obtain the most precise measurement of the cosmic temperature at such a remote epoch.

“This is the first time that these three molecules have been detected in absorption in front of a quasar, a detection that has remained elusive for more than a quarter century,” Cédric Ledoux, an ESO researcher, said in a statement.

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