NBS Internal Report 75-975; 39p. December 1975.
Calibration; diameter; gravitational bending; interferometry; least squares line; linear model; optical flat; profile; standard deviation; uncertainty.
Optical flats are normally calibrated by one of two methods at the National Bureau of Standards. The most frequently used method involves a direct interferometric comparison of a test flat to a master flat. The purpose of this paper is to present a detailed description of this measurement process. Some of the topics discussed are methods of supporting the flats, effects of gravitational bending, measurement of interference fringes, formulation and solution of the linear least squares model, analysis of errors, choice of profile reference line, and display of profiles. An example is worked out in a step-by-step fashion in order to illustrate the process, and a statistical test is incorporated to check whether the process is in control.