Comparisons of amplitudes and phase angles between the Fourier k = 1, 2 and 4 orders, shown as differences of amplitudes versus ϕ₂/T₂ (upper row) and histograms of phase angles difference normalised to π (bottom row). The comparisons between the orders k = 1 and k = 2, k = 1 and k = 4, and k = 2 and k = 4 are shown in the left, middle, and right columns, respectively. In the upper row, the amplitude difference is colour coded with the logarithmic number of the tilted rings.