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Fig. 2.

image

Block diagram of the GRAVITY fringe-tracking controller. The gray area corresponds to the GRAVITY open-loop transfer function, and the rest corresponds to the two controllers. The group-delay integrator controller is shown in blue, the phase-delay state controller in red, and the actuators predictive model is plotted in green. The group-delay controller is the main controller: it continues to track the fringes even if the instantaneous S/N is too low for phase-delay tracking. The phase-delay state controller is a closed-loop system that determines the atmospheric perturbations . A proportional controller (K) corrects for the effect of the atmosphere. The last block of the group-delay controller, the quantization step, ensures that the group-delay control signal is always a multiple of 2π: the change in OPD caused by the group-delay controller is not seen by the phase-delay controller.

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