Back-reflection precession camera, a new instrument in reciprocal-lattice photography

Abstract

Abstract The precession method is extended to the back-reflection region and used to record distant upper levels. Nets fulfilling the equi-inclination condition ζ = 2 cos [unk] are photographed without a central blind spot. The back-reflection cone-axis technique records nets with ζ [unk] 2 cos [unk]. Due to the mechanical design of the camera, cone-axis patterns are taken by an uninterrupted precession motion; isolated nets are taken by precession oscillations. The crystal can be rotated about the dial axis and set at will as in frontreflection cameras. In addition, a 90° auxiliary arc permits the dial axis to be tilted by as much as 90° in a plane parallel to the precession axis. Consequently back-reflection photographs of isolated nets and cone-axis photographs can be taken in any direction using a single mounting of the crystal. Also, the instrument is easily converted to operate as a frontreflection precession camera. Distortions of back-reflection cone-axis rings may be used to orient the crystal. Computer programs are available for (i) correcting misorientations, (ii) calculation of cell parameters and calibration of the camera, and (iii) indexing cone-axis patterns.