Accurate cells from area-detector images

A procedure is described, for an Eulerian or Kappa goniostat, to determine the exact spindle value phi at the moment of impact of a reflection on an a rea 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 tw o related images. One image is obtained by spindle rotation over a few degr ees ('phi scan') and the other by the same phi rotation, but with a superpo sed rotation (chi) perpendicular to the first ('phi/chi scan'). In both cas es, the spindle is kept perpendicular to the primary beam, i.e. omega = 0 o r 180 degrees. 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 mo ment of impact (phi value) is calculated. The method is especially useful i n small-molecule work, where rather wide scans are required for a reasonabl e number of reflections in one image. It is shown that for our purpose the Eulerian phi/chi scan can be simulated practically exactly by simultaneous uniform rotations of the axes omega(K), kappa and phi(K) of the Kappa gonio stat.