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

GRAVITY+ is the upgrade for GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8 m Unit Telescopes (UTs) to enable ever-fainter, all-sky, high-contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, known as GRAVITY Wide. GRAVITY Wide combines the dual-beam capabilities of the VLTI and the GRAVITY instrument to increase the maximum separation between the science target and the reference star from 2 arcseconds with the 8 m UTs up to several 10 arcseconds, limited only by the Earth's turbulent atmosphere. This increases the sky-coverage of GRAVITY by two orders of magnitude, opening up milliarcsecond resolution observations of faint objects and, in particular, the extragalactic sky. The first observations in 2019-2022 include the first infrared interferometry of two redshift z similar to 2 quasars, interferometric imaging of the binary system HD 105913A, and repeat observations of multiple star systems in the Orion Trapezium Cluster. We find the coherence loss between the science object and fringe-tracking reference star well described by the turbulence of the Earth's atmosphere. We confirm that the larger apertures of the UTs result in higher visibilities for a given separation due to the broader overlap of the projected pupils on the sky and provide predictions for visibility loss as a function of separation to be used for future planning.

Automated summary

GRAVITY+ is an upgraded version of GRAVITY and VLTI that enables fainter, all-sky, high-contrast, milliarcsecond interferometry with wide-separation fringe tracking, new adaptive optics, and laser guide stars. GRAVITY Wide, the first phase of GRAVITY+, increases the maximum separation between the science target and the reference star, resulting in a significant increase in the sky-coverage and enabling observations of faint objects and extragalactic sky. The initial observations have provided valuable insights and predictions for future planning.