Gravity+: VLTI Infrastructure Upgrade

The Gravity upgrade project, Gravity+, will upgrade GRAVITY and the VLTI in order to open up the extragalactic sky for milliarcsecond-resolution interferometric imaging, and give access to targets as faint as K = 22 mag. GRAVITY+ will measure the black hole masses of active galactic nuclei across cosmic time, and obtain high-quality exoplanet spectra and orbits.




The Gravity+ consortium:

Max Planck Institute for Extraterrestrial Physics (lead);
Max Planck Institute for Astronomy;
University of Cologne

Institut National des Sciences de l'Univers, French National Center for Scientific Research;
Institut de Planétologie et d'Astrophysique de Grenoble;
Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique;
Lagrange Laboratory;
Centre de Recherche Astrophysique de Lyon

Instituto Superior Técnico’s Centre for Astrophysics and Gravitation;
University of Lisbon;
University of Porto

University of Southampton

Katholieke Universiteit Leuven


Principal Investigator Frank Eisenhauer MPE

Thibaut Paumard: LESIA

Jean-Baptiste Le Bouquin: IPAG

Laura Kreiberg MPIA

Paulo Garcia: CENTRA

Sebastian Hoenig: Southampton U.

Christian Straubmeier: UoC

Project Manager ESO
Frederic Gonte
Project Scientist
Julien Woillez
Project Engineer Sylvain Oberti
Project Approval February 2022
Project Status AIV?
Location Entire VLTI facility

Baseline Specifications

Spatial Resolution milliarcsecond
Limiting magnitude K=22
Wavelength coverage   K-band
Spectral resolution
(HR grating) 
R~15,000 centred
at λ~2.166 μm (HI Brγ line)

Status and Timeline


  • The GRAVITY+ upgrade was first suggested to the ESO community at the VLT2023 conference in 2019
  • Recommendation by ESO’s Scientific Technical Committee in 2020
  • Early concept study for GRAVITY+ completed in the Summer of 2021.
  • Project approval: December 2021
  • Construction agreement signed in February 2022 


The upgrades to GRAVITY+ are being implemented incrementally, minimising disruption to astronomers.

Scientific Objectives

  • Galactic Centre
  • Active Galactic Nuclei and their coevolution with galaxies
  • Masses of black holes including intermediate-mass and supermassive black holes
  • Characterization of exoplanets
  • Understanding young stars and their planet-forming disks.


Astronomers will be able to use GRAVITY+ to investigate new science, delving to previously unobtainable depths. The upgrade will enable the discovery and characterisation of exoplanets, the imaging of young stars and their protoplanetary disks, and the search for intermediate mass black holes. It will also take us deeper and closer to Sagittarius A*, the black hole at the centre of the Milky Way, providing us with a better understanding of our galactic centre.  

Summary of key GRAVITY+ capabilities
Summary of key GRAVITY+ capabilities

Facility Upgrade Description

GRAVITY+ combines improvements of the GRAVITY instrument with infrastructure upgrades for the VLTI. Like its predecessor, GRAVITY+ combines the light of the VLT’s 8 m Unit Telescopes using interferometry, but it will be assisted by upgraded adaptive optics technology that will enable better correction of the blur caused by the Earth’s atmosphere and improve the contrast of observations. GRAVITY+ will also implement one new laser guide star on each of Unit Telescopes 1-3, and will make use of one of the lasers currently installed on Unit Telescope 4, to improve both the imaging sensitivity.

GRAVITY+ will also upgrade the VLTI’s fringe tracking capabilities. Fringe tracking allows for further correction of the effects of atmospheric turbulence, this time at the level of the 130-metre-equivalent telescope achieved by the interferometric operations of all four telescopes. This will improve the instrument’s resolution to just milli-arcseconds, meaning that the cosmic objects will be imaged in unprecedented detail.

The new features will benefit all present and future VLTI instruments and the scientists that use them.

Overview of the Gravity+ project