Processing and post-refinement of oscillation camera data

When collecting data using oscillation photography, it is necessary to distinguish between those reflections which have passed completely through the Ewald sphere (`full reflections') and those whose penetration is incomplete (`partial reflections'). Such differentiation requires an accurate knowledge of crystal setting parameters. This is particularly necessary if partial reflections are to be used when adjacent films do not originate from the same crystal. In general this is the case where diffracted intensities are weak, as in crystals of oligomeric proteins and viruses. Estimates of setting parameters can be obtained by comparing a partial reflection on one film with a full observation of the same reflection on the same or another film [Schutt, C. E. & Winkler, F. K. (1977). The Rotation Method in Crystallography, edited by U. W. Arndt & A. J. Wonacott, pp. 173–186. Amsterdam: North-Holland]. A measure of the observed degree of partiality, p_obs, can thus be made and related to the calculated value, p_calc, derived from the crystal setting parameters. Least-squares minimization of E = Σω(p_obs − p_calc)² leads to the refinement of (1) the orientation of each crystal, (2) the crystal cell dimensions and (3) each crystal's effective mosaic spread. Examples of this technique are provided from the processing of high-resolution southern bean mosaic virus data.