Visualisation of the modulus of the Fourier transform of the different influence functions before , left side of the panels) and after , right side of the panels) orthogonalisation for the different proflies. The 2D maps were normalised to their maximal value. The grey squares represent the cut-off frequency |k_i∊{1,2}| < (2Δ)⁻¹. Panel a: 2D local piston. Panel b: 2D pyramid. Panel c: 2D axisymmetric Gaussian profile. Panel d: 2D axisymmetric ALPAO profile. Panel e: 2D sinc function.