semiconductor wafer feature the feature shown in fig6 a 6c for aperture aligning using a cdsem comprising an aperture assembly 30 having the automatically aligning objective aperture of the invention the steps for cd measurement include step 1 the automatic objective alignment method of the invention step 2 conventional rough focus adjustment and step 3 conventional fine focus adjustment astigmatism adjustment note that once the objective aperture has been correctly aligned cd measurements can be taken for many months by merely performing the rough focus adjustment of step 2 and the astigmatism adjustment of step 3 over time however the objective aperture tends to degrade thereby making rough focus and astigmatism adjustment difficult for the operator as the image of feature will tend to move across and or off the display screen during focussing when this occurs the operator must change the objective aperture and perform step 1 according to the earlier described method to properly align the new objective aperture once step 1 has been performed the rough focus method of step 2 is performed if image of the feature still moves across and or off the display screen during rough focussing the objective aperture may have to be aligned again by repeating the automatic objective aperture alignment method of step 1 after rough focus has been achieved fine focussing or astigmatism adjustment is performed if a fine focus is not achieved it may be necessary to repeat step 2 and then perform step 3 again once a fine focus image of the feature is achieved it may be saved to the image controller 50 while the foregoing invention has been described with reference to the above various modifications and changes can be made without departing from the spirit of the invention accordingly all such.