Optical set-up displaying X-ray radiation with wavelength ? consists of monocrystal (1) with atomic planes (2) in parallel with optical axis (3). The mutual distance of the atomic planes in resting state without the force is d0. Cross section (S) of the monocrystal (1) is variable. With respect to the optical axis (3) the farther and/or closer side of monocrystal (1), which is orthogonal to this optical axis (3), is equipped with a device creating and maintaining pull or push force (F) in direction orthogonal to atomic planes (2) of such monocrystal (1).