Section 9.1.6.4. Rotation images and lunes

Using monochromatic radiation, in order to measure the remaining reflections that do not lie on the surface of the sphere, the crystal must be rotated to bring the reflections into the diffracting condition. If the crystal is rotated about a single axis during sequential exposures, this is known as the rotation method. The rotation axis is, in practice, chosen to be perpendicular to the beam to preserve the symmetry between the two halves of the complete pattern. This is the most commonly applied method of data collection for macromolecular crystals (Arndt & Wonacott, 1977).

If the crystal is rotated during exposure, the ellipses observed on a still image change their position on the detector. In effect, all reflections diffracting during one exposure will be contained within lunes formed between the two limiting positions of each ellipse at the start and end of the given rotation. The width of the lunes in the direction of the crystal rotation, perpendicular to the rotation axis, is proportional to the rotation range per exposure. In contrast, along the rotation axis the width of the lunes is very small, since the intersection of the reciprocal-lattice plane with the Ewald sphere does not change significantly. For crystals of small molecules, the lunes are not pronounced, owing to the sparse population of reciprocal space, but for crystals with large cell dimensions, the lunes are densely populated by diffraction spots and often exhibit clear and well pronounced edges. At high resolution, the mapping of the reciprocal lattice within each lune is distorted, and rows of reflections form hyperbolas. At low diffraction angles, where the surface of the Ewald sphere is approximately flat, this distortion is minimal, and the lunes look like fragments of precession photographs.