(Top) Contour plots of 2Θ versus pixel column as the detector arm is scanned through the LaB6 100 peak: (a) the raw data (cf. Fig. 1), with the horizontal dashed lines indicating the angular range shown in (b), corresponding to the values after correction. (Middle) Fit of a simple pseudo-Voigt function to the corrected values, showing (c) the fitted positions and (d) the FWHM and, in red on the same scale, the difference between the fitted position and the average fitted position. The corrected positions cover a very small angular range compared with the FWHM values, though there are perhaps indications of a residual systematic trend. (Bottom) Fitted positions of the pseudo-Voigt function (e) to the 100 peak (raw data in black and corrected data in red) and (f) to the 421 peak. Note that the maximum in peak position for the raw data moves axially with angle because of the roll of the crystal.