Section 3.4.2.2. Preliminary considerations

Prior to the commencement of the setting process, it is useful to align the crystal optically so that prominent morphological features bear fixed geometrical relationships with the component parts of the goniometer head. Thus, a prismatic crystal could be aligned with its longest axis parallel to the mount, but in addition with a face perpendicular to the rotation axis of one of the goniometer arcs. Many modern goniometer heads have a rotatable component to which the crystal mount can be fixed, and judicious use of this facility can considerably simplify the setting process. This may be particularly important for crystals that are very sensitive to X-radiation. It is also useful if the arc readings on the goniometer head are equal or close to zero. Large deviations away from the zero positions can lead to mechanical collision with other parts of the camera (e.g. the layer-line screens of a Weissenberg camera), or in extreme cases to interference with the primary or diffracted X-ray beams. If the crystal mount is fixed to the goniometer head with wax or plasticine, this can often be achieved by manual manipulation of the mount and the wax. The use of less-pliable adhesives requires careful monitoring during the hardening process. Although the detailed alignment will depend on the geometry of the recording device, care taken at the mounting stage will always result in increased efficiency in setting. Sensible orientation of the goniometer head on the camera may also lead to increased efficiency, and it is often useful to start with the axes of the goniometer arcs perpendicular and parallel to the X-ray beam.