First light for GRAVITY Wide. Large separation fringe tracking for the Very Large Telescope Interferometer

Vol. 665
13. Astronomical instrumentation

First light for GRAVITY Wide. Large separation fringe tracking for the Very Large Telescope Interferometer

ESO/VLTI and its K-band instrument GRAVITY offer interferometric imaging capabilities at milliarcsecond resolution to a wide community. This paper demonstrates the operationality of the new observing mode “GRAVITY Wide.” Opening this mode is an important part of a larger upgrade, VLTI/GRAVITY+, which aims at widely opening the milliarcsecond interferometric imaging window to faint all-sky targets. This first step consists in extending the off-axis fringe tracking separation from 2 to several tens of arcseconds, a radius that reaches the fundamental limit of the Earth turbulence angular decorrelation. Fringe tracking on a moderately bright star, and possibly including spectroscopic resolution, is mandatory for deep interferometric integrations on faint targets. The accessible sky fraction is now increased by two orders of magnitude. This new high angular resolution window to the faint extragalactic sky is illustrated by the observation of the H alpha broad line region emission of two redshift z~2 quasars.