| Long focal length optics lens have been widely used in high-power laser fusion, astronomical telescopes and other optical system. But its focal length is still difficult to measure. Using Talbot interferometry for measurement of long focal length has been a way to solve this problem for its high precision, simple optical path.The basic theory of long focal length measurement based on Talbot-Moire fringes is analyzed. The relationship between the focal length and the system parameters, which included the angle, spacing of the two gratings and Moire fringes angle, is established. The formula is optimized through the analysis of measurement system’s limit state. The optimized formula not only begs measurement baseline problem, and effectively reduces the influence of collimated wave front distortion, manufacturing errors of the gratings. The system parameters are designed based on moire fringes analog. A measurement system model is built and the results show that the measurement system meet design requirements. Based on the analysis of FFT, A method for obtaining the angle of moire fringe using the sub-pixel method is proposed, and its measurement accuracy can reach2", which is higher than the Moire fringes minimum angle sensitivity14.4"/m. The experimental device is verified and the software is prepared based on VS2008. The experimental results show that the measurement accuracy reach0.41%when the focal length of the test measurement system is59.38m and the system stability is good. |
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