The pixel-centre-to-pixel-centre distance for a one-step
               change in the setting of the specified axis in millimetres.
               This is meaningful only for 'constant velocity' spiral scans
               or for uncoupled angular scans at a constant radius
               (cylindrical scans) and should not be specified for cases
               in which the angle between pixels (rather than the distance
               between pixels) is uniform.
               See _array_structure_list_axis.angle_increment.