Geometrical equatorial aberrations in a Bragg-Brentano powder diffractometer with a linear position-sensitive detector

The use of a position-sensitive detector with a Bragg-Brentano diffractomet er implies the registration of the diffracted beam at a finite distance fro m the true focusing point. This gives rise to an aberration which is charac terized by the peak intensity profile I(y) and its parameters, the shift of the centre of gravity (y) and the profile variation W. These are calculate d in terms of exact expressions and approximate results. The obtained close d formulae for (y) and W augment analogous calculations for other aberratio ns, tabulated by Parrish and Wilson in the International Tables for X-ray C rystallography (Vol. III, 1995). Calculation of the peak profiles is verifi ed by experiment.