Astronomical Seeing
Astronomical seeing is the blurring and distorting effect of viewing astronomical objects through the Earth's atmosphere. Density variations, wind, and convection in the atmosphere cause an ever-changing refraction of the light passing through it, much like the shimmering heat waves seen rising from the ground on a hot, sunny day.
Over very short image exposures, the effects of astronomical seeing distort a point source of light into a randomly varying blob of speckles. Over longer image exposures, these short-term randomly varying speckles average out to be an approximately circular blurring.
Astronomical seeing is always changing, sometimes fairly significantly over short periods of time. Thus, the amount of its blurring effect is also constantly changing.
Theories exist for the analysis of astronomical seeing, speckle processing of images, and compensation using adaptive optics. For the purpose of focus, it is sufficient to consider the long-exposure behavior of astronomical seeing which is well-approximated by a Gaussian blur with a randomly changing standard deviation or variance.
The net effect of astronomical seeing is to spread a point source of light, such as a star, into a larger circular shape.
