A Simulation of the Absolute Flatness Measurement Based on the Two Flats Test

Large optical flat has been applied more and more extensively, but its accuracy is limited by the accuracy of the reference flat installed in an interferometer. To solve this problem, a Two Flats Test is used to get the absolute surface deviation distribution of the reference flat, which can eliminate the effect caused by the reference flat. During the calibration process, the Two Flats Test simply needs two flats-which serves as its most remarkable advantage-and it does not require changing the reference flat of the interferometer; moreover, it fits the absolute measurement of the large optical flat. By simulating the mathematical model of the Two Flats Test, we verify its validity and analyze the error. This research solves the bottleneck problem at present that the accuracy of the interference test is limited by the reference flat. At the same time, it provides technological support for calibrating surface deviation distribution of the reference flat of large flat interferometers in the future.