OPD residuals as a function of magnitude, with flux variations due to 20 mas rms residual tip-tilt jitter. The OPD disturbances are 10 μm rms atmospheric OPD and 150 nm rms vibration OPD (low vibration level). The dashed lines correspond to the specification for the fringe tracker of GRAVITY, which is to stabilize the fringes below 350 nm rms on a reference star of magnitude K = 10.