Seeing values are calculated from image motions by the following procedures. An image motion is defined to be a variance of image centroids. The centroids are obtained in every captured frame and accumulated up to 60 data points to calculate the variance σ². The relation between the Fried's seeing parameter r₀ and the variance of the differential longitudinal image motion σ² is given by

σ²_l = 2λ²r₀^-5/3[ 0.179 D^-1/3 - 0.0968 d^-1/3]

Also the relation between r₀ and the variance of the differential transverse image motion σ²_t is given by

σ²_t = 2λ²r₀^-5/3[ 0.179 D^-1/3 - 0.145 d^-1/3]

Here, λ is wavelength, D is a diameter of sub apertures and d is a distance between sub apertures. In case of our system, 0.5um for λ, 7.4cm for D and 20.5cm for d are adopted. FWHM θ is estimated numerically with λ and r₀,

θ = 0.98 x λ/r₀,

Finally, the zenith distance correction is applied,

θ(0) = θ(z) x cos(z)^3/5,

where z is a zenith distance, θ(z) is the observed seeing and θ(0) is the seeing at zenith.