Doubly curved crystal point-focusing x-ray monochromators: geometrical and practical optics

The geometrical optics of symmetrical point-focusing x-ray monochromators made with elastically bent ideal monocrystalline wafers in the geometry employed by Despujols and Berreman are reinvestigated. Sharpness of focus is limited mainly by accuracy of curvature of the crystal wafer and by thermal diffuse scattering. Monochromaticity is limited by these factors and by source and crystal dimensions in the plane of reflection and rarely by the inherent width of the reflection band of the curved crystal. Two existing monochromators reflect about 2-3% of the incident CuKalpha(1) radiation.