Analysis of Effects of Errors of the Absolute Difference Algorithm on Day-time Seeing Measurement

A Solar Differential Image Motion Monitor (S-DIMM) is an important instrument to measure the day-time seeing r₀. S-DIMMs are widely used in solar telescope site testings. An S-DIMM yields the day-time seeing by measuring fluctuations of the images of the solar edge. In this paper, we adopt the Absolute Difference Algorithm for solar-edge detection to measure the day-time seeing, with the second-order fitting used to improve the accuracy on the sub-pixel level. This not only solves problems caused by the small field of view of an S-DIMM, which leads to instability in observations in addition to atmospheric effects, but also helps long and steady observations with an S-DIMM. We also analyze possible factors (such as noise and aberration) influencing the Absolute Difference Algorithm with the aid of numerical simulations. The simulations show that the Absolute Difference Algorithm can be used to undertake high-precision observations, with the accuracy of the algorithm better than 0.1". Finally, we apply the algorithm to real measurement of seeing, and present some preliminary results.