Journal of Applied Crystallography

Volume 33, Part 3 Number 2 (June 2000)

research papers

J. Appl. Cryst. (2000). 33, 893-898 [ doi:10.1107/S0021889800002363 ]

Accurate cells from area-detector images

A. J. M. Duisenberg, R. W. W. Hooft, A. M. M. Schreurs and J. Kroon

Abstract: A procedure is described, for an Eulerian or Kappa goniostat, to determine the exact spindle value [varphi] at the moment of impact of a reflection on an area detector; such information is essential for the calculation of accurate reflection vectors and, eventually, the unit-cell and lattice orientation. The method is based on a comparison of reflection impact coordinates on two related images. One image is obtained by spindle rotation over a few degrees (` [varphi] scan') and the other by the same rotation, but with a superposed rotation ( [chi] ) perpendicular to the first (`/ [chi] scan'). In both cases, the spindle is kept perpendicular to the primary beam, i.e. [omega] = 0 or 180°. Therefore, on the second image, exactly the same reflections as on the first will appear, but on a different spot on the detector. From the tangential separation between corresponding reflection impacts, the moment of impact ( [varphi] value) is calculated. The method is especially useful in small-molecule work, where rather wide scans are required for a reasonable number of reflections in one image. It is shown that for our purpose the Eulerian / [chi] scan can be simulated practically exactly by simultaneous uniform rotations of the axes [omega] _K, [kappa] and [varphi] _K of the Kappa goniostat.