The value of _diffrn_detector_element.reference_center_fast is the fast index axis beam center position relative to the detector element face in the units specified in the data item '_diffrn_detector_element.reference_center_units' along the fast axis of the detector from the center of the first pixel to the point at which the Z-axis (which should be colinear with the beam) intersects the face of the detector, if in fact is does. At the time of the measurement all settings of the detector positioner should be at their reference settings. If more than one reference setting has been used the value given whould be representive of the beam center as determined from the ensemble of settings. It is important to note that for measurements in millimetres, the sense of the axis is used, rather than the sign of the pixel-to-pixel increments.