GRAVITY+

GRAVITY+ is an upgrade to VLTI and its GRAVITY instrument. It will enable the imaging of fainter and more remote astronomical objects than previously possible, while also improving the high contrast precision on bright objects.

Since 2016, GRAVITY has delivered astonishing optical interferometric results. It has made the deepest observations of stars orbiting the black hole at the centre of the Milky Way, produced the first ever detection of an exoplanet using optical interferometry, and enabled successful tests of Einstein’s Theory of General Relativity near a black hole.

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 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.

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.

The GRAVITY+ upgrade was first suggested to the ESO community at the VLT2023 conference in 2019, dedicated to discussing future science with, and potential development paths for, the VLT. After a recommendation by ESO’s Scientific Technical Committee in 2020, the consortium conducted an early concept study for GRAVITY+, which was completed in the Summer of 2021. The final agreement on its construction was signed in February 2022 between ESO and a consortium led by the Max Planck Institute for Extraterrestrial Physics.

The upgrades to GRAVITY+ are being implemented incrementally, minimising disruption to astronomers (the banner image is a recreation of what the upgrades will look like). The new features will benefit all present and future VLTI instruments and the scientists who use them.