The Very Large Telescope Interferometer

ESO’s Very Large Telescope Interferometer (VLTI) uses interferometry to combine the light collected by the four VLT 8.2-metre Unit Telescopes (UT) or the four movable 1.8-metre Auxiliary Telescopes (ATs). This creates a larger “virtual” telescope with a diameter equivalent to the distance between the individual telescopes (called “baseline”), allowing the VLTI to pick up much finer details of the cosmos than would be possible with the ATs or the UTs alone.

The possibility to combine light using either the UTs or the ATs makes the VLTI particularly special. The UTs, with their larger diameters, have powerful light collecting ability and can form six baselines, all with different lengths and orientations. This yields a maximum possible resolution equivalent to a telescope 130 metres in diameter. The ATs can be moved to 30 different locations along the VLT platform to obtain more information about the observed cosmic objects. This provides a much larger number of possible baselines and enables the VLTI to achieve a maximum possible resolution equivalent to a telescope of 200 metres in diameter.

At present, the VLTI is operated with baselines of up to 140 metres, depending on the position of the ATs. Thanks to this large diameter, astronomers can see details up to about 17 times finer than with a single UT, a sharpness equivalent to distinguishing the front and rear lights of a car, as viewed on its side, parked on the Moon.

The light beams collected by the individual telescopes are brought together in the VLTI using a complex system of mirrors in underground tunnels. These guide the beams towards an impressive suite of VLTI instruments, where they are combined and are finally ready for scientific analysis. To successfully combine the light, the beam’s path difference from each telescope must be kept within a precision of one thousandth of a millimetre.

Science with the VLTI

Thanks to its exquisite sharpness, the VLTI can image a wide range of objects in unprecedented detail, shedding new light on many research fields in astronomy. Examples include the search for planets outside the Solar System, the observation of both young and old stars, the study of the immediate surroundings of the supermassive black hole at the centre of the Milky Way, and the investigation of cosmic objects far away from our galaxy such as active galactic nuclei, one of the most energetic and mysterious phenomena in the Universe.

Some of the VLTI’s most outstanding results include:


The following instruments are currently operational at the VLTI, all active at near and mid-infrared wavelengths:

The following instruments have been used during the first years of the VLTI and are no longer operational:

Very Large Telescope Interferometer

Name: Very Large Telescope Interferometer
Site: Cerro Paranal
Altitude: 2635 m
Type: Infrared interferometer
First Light date: 29 October 2001 (two UTs combined)
Interferometry: UTs: maximum 130 m baseline. ATs: maximum 200 m baseline.
Images taken with VLTI: Link
Images of VLTI: Link
Videos of VLTI: Link
Press Releases with VLTI: Link