The value of _diffrn_data_frame.center_fast is the fast index axis beam centre 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 centre of the first pixel to the point at which the Z axis (which should be collinear with the beam) intersects the face of the detector, if in fact it does. At the time of the measurement the current settings of the detector positioner for the given frame are used. It is important to note that for measurements in mm, the sense of the axis is used, rather than the sign of the pixel-to-pixel increments.