Beam footprint projection on the detector plane. The GID geometry parameters are impinging beam grazing angle α = 0.12°, incident beam size 0.1 × 1.0 mm (vertical × horizontal), exit beam grazing angle β = 0.16°, in-plane scattering angle 2θ = 20°, two-dimensional detector angular position δ = 19°, sample–detector distance L = OD = 394 mm. The point at the centre of the parallelogram (O′) is the projection of the point O (see Fig. 2). The scale at the top of the graph displays the footprint position in terms of scattering angle counted from the coordinate system origin. Note the factor of 10 difference in the aspect ratio between ranges of the ordinate and the abscissa.