Abstract: A cross hair assembly has an adjustable cross hair intersection point to facilitate calibration of the cross hair assembly relative to the optical axis of an associated x-ray radiation source. Each cross hair has a fixed anchorage on a mounting frame, and an adjustable anchorage on the mounting frame. By selectively adjusting the adjustable anchorages it is possible to change the cross hair intersection point. Each cross hair is a steel wire having a relatively small diameter; a coil spring is included in each fixed anchorage for holding the wire in a taut condition free from sagging or bending.