GRAVITY+: towards faint science, all sky milli arcsecond optical interferometric imaging

Abstract

GRAVITY and the VLTI have transformed optical interferometry with ground-breaking results on the Galactic Center, Active Galactic Nuclei, and Exoplanets. With modest upgrades to GRAVITY+, the Paranal Observatory will open up the extragalactic sky for milli-arcsec resolution interferometric imaging, and give access to galactic targets as faint as 22 mag. GRAVITY+ will measure the black hole masses of active galactic nuclei across cosmic times, establish whether globular clusters harbor intermediate-mass black holes, and obtain high-quality exoplanet spectra and orbits. The straightforward upgrade consists of implementing off-axis fringe tracking, an improved throughput, and laser-guide star adaptive optics for all 8m telescopes and can be divided up into several phases. Most of the upgrades will be beneficial to all VLTI instruments. Here we present a few selected science cases and outline the project, which could be implemented within a few years.