Simplified version of the VLTI light path from Fig. 2 to show the modeling and the experimental setup. The black rectangles show where the laser is launched and where the polarimeter is mounted. The names of the mirrors used in the text are given. The color of the mirror number shows the grouping that was used for the fitting. Gray mirrors are not fitted in our calibration model. Each colored group is located in one common plane: M4−M8 are in one vertical plane, and M10−M18 are in one horizontal plane.