Oscillation camera data processing: reflecting range and prediction of partiality. 2. Monochromatized synchrotron X-radiation from a singly bent triangular monochromator

Monochromated synchrotron X-radiation from a singly bent triangular perfect-crystal monochromator system, uniformly adapted at LURE, DORIS and the SRS, offers considerable improvement in exposure times for protein single-crystal oscillation photography with improved resolution of data and better signal to noise. There are important differences in the diffraction geometry with such a system compared to a conventional laboratory experimental arrangement. In particular, an asymmetric beam cross fire, a spectral bandwidth that is variable over a wide magnitude and a correlation between the direction of the incident ray and its photon wavelength. These new considerations considerably affect the data processing considerations in oscillation camera work since they affect the appropriate flagging of partial reflections. As in an earlier treatment for conventional sources [Greenhough & Helliwell (1982). J. Appl. Cryst. 15, 338-351], reflecting-range formulae are derived from first principles, leading to expressions from which partiality can be calculated. It is shown that the general equations for monochromated synchrotron X-radiation reduce to those for conventional sources when the characteristics of the former are removed. The often observed difference between spot sizes on opposite sides of the oscillation film, under certain conditions in the synchrotron case considered, is explained. At the Guinier position the equations reduce to those for a conventional source with asymmetric beam cross fire [Greenhough & Helliwell (1982). J. Appl. Cryst. 15, 338-351] with spots comparably placed on either side of the film showing no size difference due to the incident radiation. In view of the interest in the possibility of polychromatic single-crystal studies [Arndt, Greenhough, Helliwell, Howard, Rule & Thompson (1982). Nature (London). In the press] the vertical-rotation-axis equations are also derived. In both cases large differences in reflecting range, when working away from the Guinier position, are predicted for reflections identically placed except for the sense of horizontal displacement, with the elongated spots containing variable-wavelength information along their length. In order to find suitable values for use in the diffraction equations, the properties of the monochromator are discussed; this also leads to the conclusion that the full sampling of absorption-edge fine structure is feasible as demonstrated by Helliwell et al. [Acta Cryst. (1981). A37, C316].