The value of _diffrn_data_frame.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_data_frame.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 the current setting of detector positioner given frame are used. 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.