Two decades of discoveries with ESO’s Very Large Telescope Interferometer: happy birthday VLTI!

Twenty years ago today, light from two of the 8.2-metre Unit Telescopes (UTs) of ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile was combined for the first time. The event marked the historic moment when these telescopes first operated as a true interferometer, sparking the beginning of a new era for astronomy.

When the light beams of the two UTs were combined on the night of 29 October 2001, the VLTI did not only successfully start its operations, but also delivered its first scientific results: the measurement of the size of the southern star Achernar, which was found to be approximately ten times larger than our Sun. This was followed by three more nights of scientific observations, where the VLTI showcased its excellent potential by measuring ten more stars, and notably performing the first interferometric observation of the intriguing Eta Carinae.  

The successful concept of the VLTI had been demonstrated some months earlier, on 17 March 2001. That night, light from another star, the bright Sirius, was collected and combined with two small telescopes (called siderostats), specially constructed for the early VLTI test stages.

The combination of light from the siderostats, and later from the UTs, was just the beginning of the journey for the interferometer. The VLTI can also function using four movable 1.8-metre diameter Auxiliary Telescopes (AT), which were completed between 2004 and 2006. In February 2005, light from two ATs was combined for the first time, marking another important milestone in the interferometer’s life. The ATs combine their light in a similar fashion to the UTs, with the difference that the ATs can be moved up to 30 different locations along the Paranal platform. This offers a whole new set of interferometric configurations, allowing  the VLTI to reach its full observational potential.

Integrating the light collected by the UTs or by the ATs allows the VLTI to act as one “virtual” instrument with a diameter as large as the distance between the individual telescopes (called “baseline”). The interferometer is currently operated with baselines of up to 140 metres, depending on the position of the ATs. When the light from two UTs was first combined in October 2001, the baseline was around 102 metres, which allowed the VLTI to see details on the surface of Achernar with a resolution equivalent to seeing a 4-metre-long truck on the surface of the moon. The VLTI’s resolution, together with the individual telescopes’ ability to gather light, make the VLTI one of the most powerful optical interferometers on Earth.

Thanks to this exquisite level of detail, ESO’s VLTI has helped shed new light on many research fields in astronomy over its two decades of activity. Research carried out on VLTI data has contributed to more than 450 scientific publications since 2002. These include groundbreaking results, such as the 2020 Nobel-Prize-winning observations of stars orbiting the supermassive black hole at the centre of the Milky Way, the ​​first direct observation of an exoplanet using optical interferometry, the highest-resolution image of the Eta Carinae star system, the detection of exozodiacal light, the best ever image of a star’s surface and atmosphere and the sharpest view of a dusty disc around an ageing star.

ESO’s VLTI could not have achieved all these stunning results without its world-class suite of instruments, including those currently in operation — PIONIER, GRAVITY and MATISSE — which saw first light between 2010 and 2018.

Courtesy of European Southern Observatory