**Figure 1**
The Ewald sphere construction. The crystal is located at the centre of the Ewald sphere (*O*), the X-ray beam is along the *X* axis and the origin of the reciprocal lattice (*Q*) is at the point where the X-ray beam exits the Ewald sphere. The laboratory coordinate frame is defined by the orthogonal set of axes denoted *X*, *Y*, *Z* and the spindle axis is parallel to *Z*. A set of six reciprocal-lattice points lying in a plane parallel to the *YZ* plane are shown as lying on the surface of the Ewald sphere, giving rise to a set of diffracted beams (dotted lines) that result in a set of spots that lie on a circle on the planar detector. *X*_{d}, *Y*_{d} define the detector coordinate frame. The scattering vector for one of the reciprocal-lattice points is shown as a bold line. |