A method for measuring the flatness of mounted stage mirrors is presented. This is the first part of an effort to map stage mirrors mounted on an X-Y stage for the purpose of subtracting mirror flatness errors from distance measurements. The absolute surface flatness of an optical straight edge is determined using an interferometric three flat test. This calibrated mirror is measured against the stage mirror using a displacement measuring interferometer (DMI) system. A sample stage mirror is measured using a three flat test. The measured flatness is compared against that obtained using the DMI system. Results to date are presented. The accuracy of the system is currently limited by the accuracy of the three flat test, environmental factors, and second order rotation sensitivity of the interferometer.
"Flatness Measurements Of Mounted Stage Mirrors", Proc. SPIE 1087, Integrated Circuit Metrology, Inspection, and Process Control III, (19 July 1989); doi: 10.1117/12.953114; https://doi.org/10.1117/12.953114