OPD residuals as a function of magnitude for three different vibration levels. Top: no vibration; middle: low vibration level with 150 nm rms OPD; bottom: high vibration level from 240 to 380 nm rms OPD. The atmospheric turbulence is 10 μm rms OPD and, and the residual tip-tilt of the beams is 15 mas rms. The dashed lines correspond to the specification of the fringe tracker of GRAVITY, which is to stabilize the fringes down to 350 nm rms on a reference star of magnitude K = 10.