Results of the FFTs applied to the BS dispersion maps σ_bs, as built from disc models projected at 30° (top row) and 70° (bottom row). Left panel: map of σ_bs. The synthesised beam adopted in the toy model is shown on the bottom left corner. Middle panel: amplitudes of the Fourier modes for orders k = 0 to k = 4. Right panel: phase angles of the Fourier modes. A horizontal dashed line shows the half period of the Fourier modes. The disc scale-length is shown as a white contour (left) and as a vertical dashed line (middle and right).