The second generation VLTI instrument for precision narrow-angle astrometry and interferometric imaging

GRAVITY is a second generation instrument on the Very Large Telescope Interferometer (VLTI) and represents a huge step forward for both interferometry and for the Very Large Telescope (VLT). GRAVITY is almost an all-in-one device: is a four-beam combiner, near-infrared, adaptive-optics-assisted, fringe-tracking, astrometric and imaging instrument.

GRAVITY is born from the desire to observe very small details on faint objects like the centres of galaxies. With its high sensitivity performance, GRAVITY is going to reveal a whole new world of planets, stars and galactic centres that were previously out of reach because they were too faint for the previous instruments,” explains Xavier Haubois, ESO instrument fellow of GRAVITY.

The second important feature of GRAVITY is its astrometric capacity, which is its ability to measure distances in the sky to very high accuracy. “Measuring distances so precisely means that we will detect the movements of objects that are very far away in just one night. With GRAVITY, we will see the Universe in motion!” explains Xavier Haubois.

GRAVITY is able to observe the immediate surroundings of SgrA*, the supermassive black hole at the centre of our galaxy. “These observations are extremely difficult because SgrA* is far away and obscured by all the gas and dust there is between Earth and the centre of the Milky Way. Observing SgrA* will allow us to understand the properties of black holes located at the centres of galaxies and how they affect the gas and the formation and evolution of stars orbiting close to it,” says Xavier Haubois.

Moreover, observing how light propagates around such a massive object allows us to test Einstein’s theory of General Relativity for the first time in the strong gravity regime.

GRAVITY is an interferometer. “It combines the light from four telescopes (the four UTs or the four ATs) and achieves the spatial resolution of an equivalent telescope of up to 100 metres in diameter when looking in certain directions in the sky. But more than that, GRAVITY can be seen as two interferometers in one,” says Haubois. One part of GRAVITY makes sure the light is stabilised by observing a bright star. That star can also act as a reference position in the sky, like a beacon for a boat. “Thanks to this stabilisation, the second part of the instrument can observe the object in question with long-exposure times. This is why GRAVITY can observe faint targets,” explains Haubois.

In order to measure extremely small angles in the sky, equivalent to picking out a one-euro coin on the surface of the Moon, the distance travelled by the light within GRAVITY must be controlled in real time with extremely high accuracy. This is done using lasers and stretchable optic fibers.

To provide the most accurate measurements, “GRAVITY constantly readjusts itself at many different levels, a bit like a living organism!” concluded Xavier Haubois.


The authoritative technical specifications as offered for astronomical observations are available from the Science Operation page.

Site: Cerro Paranal
Telescope: Very Large Telescope
Focus: Interferometric
Type: Four-beam combiner, near-infrared, AO-assisted, fringe tracking, astrometric and imaging
Wavelength coverage: 2.0–2.4 micrometres (K-band)
Spatial resolution: Up to 3 milliarcseconds
Spectral resolution: 22, 500, 4500
First light date: 2015
Images taken with the instrument: Link
Images of the instrument: Link
Press Releases with the instrument: Link
Data papers: Link
Science goals: Galactic Centre, black holes, Sgr A* flares, X-ray binaries, planet detection and formation in circumstellar discs and evolved stars.


Max-Planck-Institut für extraterrestrische Physik (Garching).
Max-Planck-Institut für Astronomie (Heidelberg).
I. Physikalisches Institut, Universität zu Köln.

LESIA, Observatoire de Paris, Section de Meudon.
Laboratoire d'Astrophysique, Observatoire de Grenoble.

SIM, Faculdade de Ciências da Universidade de Lisboa

European Southern Observatory (ESO).