Degradation of stellar images is the most familiar example of propagation through random media and is visible to the naked eye. When a star is viewed through a telescope this degradation manifests itself in three ways: (a) as a variation of the image intensity, (b) as image broadening and (c) as wandering of the centroid of the image. This chapter is devoted to the third effect, which has also been called quivering, dancing and jitter. Image wandering is influenced primarily by large irregularities in the lower atmosphere for which ray theory is a good description. Image motion and angle-of-arrival fluctuations are different manifestations of the same random ray bending by atmospheric irregularities.

Image motion is readily observed in photographic plates placed at the focal plane of a stationary telescope. If there were no atmosphere, the stellar source would trace a smooth star trail on the plate as the earth and telescope turn together. Actual star trails exhibit random angular fluctuations about this nominal trajectory of 1 or 2 arc seconds as indicated in Figure 7.1. This random motion is observed in all astronomical measurements, although the magnitude varies with time, altitude and location. The error ranges from 0.5 to 2.0 arc seconds at sea level. It decreases with altitude and is usually 0.5 arc seconds on Mauna Kea (14 000 ft) but is sometimes as small as 0.25 arc seconds. Geometrical optics provides a valid description for astronomical quivering over a wide range of applications [1][2][3][4].