In transmission geometry the focusing properties are mainly affected by the spreading over the Borrmann triangle. However, using typical dynamical diffraction effects, focusing of the beam can be obtained even without any bending, using the curvature of the dispersion surfaces [1]. This effect is counterintuitive, when using a geometrical approach, but can be combined with the usual geometrical focus of a bent polychromator, thus widely improving the focusing performances [2].Apart from the technological considerations related to surface and interface roughness, the focusing performances in reflection geometry are typically limited by the penetration length of the wave. Indeed, both using Bragg curved crystals or elliptically shaped multilayers, the beam penetration cannot be easily neglected, in particular on a sub-micrometer scale [3]. Analytical derivations of the caustic shape, using a simplified model combining …