Atmospheric turbulence is the major reason of the degradation of image quality of the ground-based telescope, the descriptive parameter to the degradation of the astronomical image quality is the seeing parameter r0. In this text, the importance of seeing parameter in the site-testing is introduced, the methods used to measure this parameter and its history, and actuality about development are also introduced. Among the methods, S-DIMM is the more important method to measure the day-time seeing parameter r0, and now it is broadly applied in the solar telescope site-testing, for example, site-testing for ATST, EST, and YNST. S-DIMM measures the day-time seeing by measuring fluctuation variance of the solar edge. In this text, using the absolute difference algorithm to detect the fluctuation variance of solar edge and using the second-order fitting to get the sub-pixel accuracy, it not only solves the small field of view of S-DIMM, which is vulnerable to the non-atmosphere effect in the observation, but also it can help S-DIMM to perform long and steady observation. And in the text, the possible influence factors on the observation are analysed and numeral simulation is conducted. The simulation shows that the detecting precision better than 0.1", so the Absolute Difference Algorithm can undertake high-precision observation. Finally, the algorithm is applied to measure the seeing, and preliminary results are given.
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