Image motion as a measure of seeing quality

Relations between image centroid motion and large-telescope long-exposure image size are given for the cases of absolute motion measured with a single aperture and differential motion measured with two apertures. The effect of a finite exposure time in the measurement of image positions is included in the analysis, and is shown to be of crucial importance, especially in the measurement of differential image motion. For example, the contribution of the free atmospherre to the mean square differential motion can be underestimated by more than a factor of ten in realistic circumstances if an exposure time of 1/30 s is used.