Fig. 9.

Observation of star GJ 65 A on 21 November 2018. This example shows the case of a glitch on the AT2 adaptive optics system that resulted in the loss followed by the recovery of the fringe by the fringe tracker. Upper two panels: fringe tracker phase Φi, j and group delay Ψi, j estimators. The color lines correspond to each of the six baselines. The π/2 phase jumps at −7 and −1.5 s are normal and correspond to the modulation synchronized with the 5 s DIT science camera (the high spectral resolution detector). The gray areas correspond to detection of low S/N fringes by the controller. Lower left panels: S/N for each of the baselines calculated as the inverse of the square root of the phase variance (). The horizontal red lines correspond to the group-delay threshold (
). If the black line dips below the red line, the FT stops tracking on that baseline. The horizontal purple lines correspond to the phase-delay threshold (
). Lower center panel: flux Fi. Similarly, the red line is a threshold made by a moving average of all flux, which is used to detect the loss of a telescope. Lower right panels: commands to the piezo-actuators and VLTI delay line actuators. The piezo-actuators take care of the fast control signal, while the VLTI delay lines are used to offload and to search for fringes over large distances.
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