An experimental absorption-extinction correction technique

An extension of the experimental method of absorption correction of Flack [Acta Cryst. (1974), A30, 569-573; J. Appl. Cryst. (1975), 8, 520-521] is presented. The technique may now be applied to crystals of any symmetry and may also correct for extinction. The correction is based on intensity data of all forms of a few reflexions measured at varying values of the azimuthal angle. The correction is written as a polynomial-Fourier series, polynomial in intensity and trigonometric in the four diffractometer setting angles. The coefficients of this series are determined by least squares with the condition that all corrected intensity measurements of a form of reflexions should be 'as nearly equal as possible'. The least squares is implemented with the (modified) Gram-Schmidt transformations in a Fortran program called CAMEL JOCKEY WITH THREE HUMPS. The extended method has been tested on six crystals, some without and some with extinction. In one case it is shown that the method may also be used as a misalignment correction.