Left: same as left hand panel of Fig. 6, but for all 25 wavelength bins. Clearly the first zero shifts to lower spatial frequencies with increasing wavelength, i.e. the FDD diameter increases with increasing wavelength. Right: visibility as function of position angle for wavelength bin 6 (9.07 μm). Measurements (crosses) are connected with their corresponding circular FDD model points (small open circles) showing that there is a systematic shift in the difference, V_model − V_measurement, by going from 0° to 90°. For data points with low visibilities, corresponding to data in the increasing part of the second lobe, the systematic is inverse. See Sect. 4 for a detailed interpretation.