The calculated distribution of intensity impinging on the detector at the direct beam position (2θ = 0°) for the configuration of Fig. 3(a); this can be compared with Fig. 5 to illustrate the very small divergence in the beam (continuous line). These calculations are for different pixel strip dimensions: dash–dotted line represents the data from a 55 µm pixel (FWHM = 0.101° 2θ) [this pixel dimension (grey region) represents a width of 0.0547° 2θ for the minimum radius used of 55 mm, i.e. the most extreme case], dotted line 27.5 µm pixel (FWHM = 0.0535° 2θ), dashed line 13.75 µm pixel (FWHM = 0.0371° 2θ), solid line 6.875 µm pixel (FWHM = 0.0334° 2θ).