Schematic model of the VLTI light path. The light path is shown for two UTs, including GRAVITY, in red and blue. The rounded arrows indicate the possible rotations in the VLTI – for changes in azimuth (Az), elevation (El), and parallactic angle (Pa) – and in GRAVITY, at the field de-rotator and the half-wave plate (HWP). The straight arrows indicate the movement of the delay lines. The location of the star separator is only shown for the blue beam but is in the same place for both telescopes. The fiber optics and elements within the GRAVITY beam combiner are only indicated here and further explained in Sect. 5.