Calibration of systematic optical path differences for the Very Large Telescope Interferometer (VLTI)

In stellar interferometry, the precise knowledge of systematic optical path differences is essential because it directly influences the ability to rapidly acquire fringes. Besides, when an object is too faint to provide a direct measurement of the fringe position error, the VLTI will provide the possibility to blindly track the fringes in open loop, according to the precalculated evolution of the OPD over time. In this case, a poor estimation of the OPD implies that the delay lines follow a trajectory, which progressively introduces residual OPD eros larger than the coherence length, and thus prevents further fringe observation.