test object actuator 150 and moving the entire mirau assembly 110 along the same axis 170 using a mirau assembly actuator 160 both types of opd scans facilitate clear identification of the zero opd position corresponding to the peak of the coherence envelope by allowing it to remain in the focal plane of the microscope however in both cases the test surface 120 goes in and out of focus when the opd scan length exceeds the dof of the imaging system the loss of focus of the test surface 120 is illustrated in fig2 which shows two sketches indicated by reference numerals 210 and 220 of paths taken by light rays through a mirau assembly with a test object at two different locations relative to the mirau assembly along an opd scan in sketch 210 the paths of a reference ray 200 and a test ray 201 interfere on the detector not shown at a location corresponding to the image of the in focus test point 202 both rays 200 and 201 come from the same point on the illumination source in the course of an opd scan the test object or mirau assembly is displaced continuously resulting in the situation sketched in 220 the test object surface is now out of focus consequently the test ray 205 that appears to come from the point in focus 204 no longer reflects from the original point 202 on the.