$\bullet$

Seeing. Positional changes and image quality changes. The effect of seeing depends on aperture size: for small apertures, one sees a diffraction pattern moving around, while for large apertures, one sees a set of diffraction patterns (speckles) moving around on scale of $\sim$ 1 arcsec. These observations imply local wavefront curvatures flat on scales of small apertures, and instantaneous slopes that vary for about an arcsec. The time variation scales are several milliseconds and up. The effect of seeing can be derived from theories of atmospheric turbulence, worked out originally by Kolmogorov, Tatarski, Fried. Here, I'll quote some pertinent results, without derivation.