A real-time interactive graphics program to determine crystal orientation for the analysis of oscillation diffraction photographs

This paper describes a powerful preliminary to various least-squares programs for finding the exact crystal orientation for the oscillation method. It is well suited to cases of large initial misorientation. It uses the real-time rotation and clipping capabilities of the high-performance graphics system PS300 from Evans & Sutherland. A program has been written that performs a geometrical inversion of reciprocal space through its origin and manipulates this transformed image in place of the direct one. The Ewald sphere is thus transformed into a plane that can be superimposed on the film. The crossing of any reciprocal-lattice point through the Ewald sphere is then replaced by the crossing of its transform through the film plane. Clipping allows elimination of all the points too far away from this plane, i.e. those that are not in a diffraction condition. Determination of the correct orientation is achieved by superimposing the inverted reciprocal-lattice image on the corresponding transformed image of the observed film.