Optical Plane Of The Absolute Detection

Methods for the absolute testing of the optical flats are studied using a Fizeau interferometer, including Three-flat testing and Pseudo-shear method. In the Three-flat testing , three flats are combined in pairs in different positions , only two linear profiles are determined with the traditional Three-flat testing. Zernike fitting method and even-odd function method are both extended Three-flat testing methods, which can give full-aperture topographies of the three flats. In the Zernike fitting method, properties of Zernike polynomials are utilized to fit data points from four interferograms. Then the Zernike coefficients of the three flats are solved and the absolute distributions of the three flats are obtained. In the even-odd method, according to the symmetry properties of odd and even functions, the function to describe the surface distribution of a flat is decomposed to four components: even-odd odd-even even-even and odd-odd functions, the absolute distribution of three flats are obtained by solving every component. Compared with the Three-flat testing, only two flats are used in the Pseudo-shear method, one is reference flat, the other is tested flat, three measurements are required to solve for the absolute distribution of the tested flat, and the system error can also be obtainable. All methods above are programmed for simulation and experiment, and experiments are done to achieve high accuracy flatness measurement.