Environmental disturbance caused by mechanical vibration, air turbulence and thermal stratification makes a great impact on the accuracy of absolute flatness measurement, especially for the optical elements with the aperture larger than 600mm. Simplification of the measurement procedure is crucial for ensuring high precision and repeatability measurements. In this paper, the absolute flatness measurement approach combing the shift-rotation method and the second derivative method in polar coordinates is proposed to improve the stability and error tolerance of absolute measurement. The second derivative of the azimuth angle is achieved by rotating the tested flat, while that of the radial direction is achieved by translating high-order Zernike fitting. The absolute surface shape can be calculated simply by integration. The experiment procedure is simplified to two rotations and two translations with two involved mirrors. Furthermore, expanding the wavefront error to second-order can reduce the influence of environmental disturbance and make the measurement more robust. This study may provide technical support on the absolute flatness measurement for large aperture.
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