With the ever increasing need for higher resolution and placement accuracy in mask making, the requirement for eliminating position shifts due to target height variations is becoming a critical factor in the overall image placement error budget. In theory this problem has been eliminated with the IBM patented variable axis immersion lens which can achieve perpendicular landing in all parts of the major magnetic field as well as in the electrostatically deflected subfield. The initial setup and periodic checks, however, require very high accuracy measurements. In the past we have relied on a tedious method which involves exposure and detection of registration marks at various heights, in conjunction with a very accurately calibrated laser stage, followed by position measurements with a state-of-the-art metrology tool. In this article we describe a method which makes this measurement with high accuracy, in real time, using a standard oscilloscope display.