The device enables the detection of positions of the navigation marking elements on a reference star by way of at least two projection images taken by an X-ray system at different pivoting angles of a C-arm. The positions of the navigation marking elements on the reference star are also calculated by a navigation system tracking camera. The transformation from the coordinate system of the navigation system into the coordinate system of the X-ray system is calculated from the position of the reference star detected by the X-ray system and the position of the reference star detected by the tracking camera. The navigation marking elements are located outside of the volume which is reconstructed by way of the imaging process in order to prepare tomographic images. The positions of the navigation marking elements are determined from two-dimensional projections. All the navigation marking elements need only be detected by at least two projection images.