Abstract: The invention relates to an optical device intended to treat an incident X-ray beam. The optical device comprises a monochromator and an optical element for conditioning the incident beam. The reflective surface of the optical element is able to produce a two-dimensional optical effect in order to adapt a beam in destination of the monochromator. The reflective surface of the optical element comprises a multilayer structure type surface that is reflective to X-rays. In particular, the reflective surface consists of a single surface shaped according to two curvatures corresponding to two different directions.