The invention relates to a mounting device for mounting a beam manipulating device of an X-ray apparatus to a radiation source of the X-ray apparatus, said mounting device comprising a first mounting flange (10) associated with the radiation source, a second mounting flange (20) associated with the beam manipulating device, and fastening means (60) for fixing the first mounting flange (10) and the second mounting flange (20) to each other. It is an object of the invention to facilitate the alignment of the X-ray beam manipulating device with respect to an X-ray beam path or an X-ray beam focus of the radiation source. To achieve this, the alignment means (30) are adapted to be fastened to the first mounting flange (10), the first mounting flange having a first reference surface (31) which is in a predetermined spaced relationship with respect to the X-ray beam path. The first reference surface (31) corresponds with a second reference surface (34) of the second mounting flange (20). Furthermore, the invention provides a method for mounting a beam manipulating device to a radiation source of an X-ray apparatus.