ψ(q)

Figure 14.2 shows the Fourier transform of the two relevant functions ψ±1 together

with their phase. For ψ−1 the phase increases along a counterclockwise path

around the centre. For ψ1 the opposite is true. Multiplying with an aperture

function as sketched by the rectangle creates a positive/negative phase ramp along

qx in the two cases.