Recently most of modern absolute measurement rotation the flats or spheres in the interferometer. We review traditional absolute testing of flats methods and emphasize the method of reconstruction of rotational shear. The rotation of the lens can lead to some errors such as angle rotation error, center excursion error and other coordinate system motion error. 36 Zernike polynomials are used to generate 3 flats and Matlab are used to simulate the arithmetic. There have some phase problems on FFT arithmetic in Matlab which can cause some errors. Results are shown about how these errors influence the accuracy. The analysis results can also be used in other interferometer systems which have the motion of the coordinate system.