Ray-tracing and Spot Diagrams for a Spherical Concave Grating in Seya-Namioka Monochromator

A spherical concave grating in a Seya-Namioka monochromator is supposed divided into many small elementary gratings. Computation enables the projection of diffracted rays through the elementary gratings for incident rays emitted from a point light source situated in the plane of projection in the middle of the entrance slit, in the plane of symmetry perpendicular to the grooves of the grating. The intersection points of diffracted rays with a plane perpendicular to the diffracted ray through the vertex of the grating are determined, which give a ‘spot diagram’ in this plane. Curves of equal aberration are defined and traced on the grating. The astigmatism, the shape, and the dimensions of the geometrical image are discussed. The advantage to be obtained by the use of dissymmetrical gratings is demonstrated.