The dxtbx uses the concept of experimental models to encapsulate certain aspects of the experimental description that are separable with respect to one another. The experimental models are encoded in four container classes: the beam, goniometer, detector and scan. These contain information about the source wavelength and direction, the axis about which the crystal is rotated (for rotation data), the instrument performing the measurements, and the relationship between the image frames and any rotation, respectively. In the context of single-crystal X-ray diffraction, the models are completely general with respect to experimental technique and beamline hardware. This is achieved by employing a fully vectorial description that expresses only the abstract geometry of the experiment and not other properties. No assumptions are made about the geometry besides the intersection of the beam with the crystal and the rotation axis. In particular, the rotation axis is not assumed to be orthogonal to the direction of the beam in the representation of a rotation method scan. As the models consist of vector descriptions, in principle, their components may be expressed in any chosen coordinate system; however, within the dxtbx, the geometry is expressed using the standard imgCIF conventions (Bernstein & Hammersley, 2005). We take many ideas from the proposals described in the EEC Cooperative Programming Workshop on Position-Sensitive Detector Software (Bricogne, 1987). In particular, we adopt the scheme for `virtualization' discussed therein, which involves forming an abstract and general definition for every component of the diffraction experiment. The dxtbx forms the basis of the `instrument definition language' outlined at that workshop, by which actual beamline hardware is mapped to its abstract model representation for any particular experiment.
The majority of ADSC CCD detectors are mounted on simple translation stages: given the size and weight of these devices there are rarely circumstances where more complex axes are needed. However, at ALS beamline 8.3.1 the Quantum 315 detector is mounted on a 2θ arm, which must be taken into consideration when processing the data. Here the beam centre recorded in the image header corresponds to the 2θ offset value rather than the position where the 2θ angle is 0 (James Holton, private communication). The Format class to support this, included in Appendix B, replaces the detector definition to account for the shift in the detector origin and the changes in the vectors defining the detector plane resulting from the offset in 2θ. It is important to note that the changes are limited to the detector geometry, simplifying implementation for a beamline scientist, and will only affect detectors with a particular serial number (shown in the source code).
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