arrays t low f and t high f will contain the true alignment information of the system e the phase information of phase t low f and phase t high f could potentially be used directly alternatively as illustrated here the possibility of using the reverse bump test to compute the alignment change between two reverse bump tests that are performed perhaps days weeks months apart was considered in this case the alignment change between the alignment at the time of an u201c old u201d reverse relative to the alignment at the time of a u201c new u201d reverse bump test is computed as follows then the phase information phase h low f and phase h high f are plotted with respect to the spatial frequency v as shown in fig8 b and 8d respectively g a straight line through the low frequency components of phase h low f and phase h high f is fitted through the low frequency components of the two plots of fig8 b and 8d respectively for the example illustrated in fig8 the low side phase fig8 b has a slope of 29 5 engineering units at zero frequency since the simulation used millimeters the slope is 29 5 mm the high side phase fig8 d.