The setting of the specified axis in degrees
              against which measurements of the reference beam centre
              and reference detector distance should be made.
              In general, this will agree with
              _diffrn_scan_frame_axis.reference_angle.
              If the individual frame values vary, then the value of
              _diffrn_scan_axis.reference_angle  will be
              representative of the ensemble of values of
              _diffrn_scan_frame_axis.reference_angle  (e.g.
              the mean).
              If not specified, the value defaults to zero.