The quality of astronomical viewing can vary tremendously from day to day, hour to hour and even over fractions of a second. This rapid change in "seeing" is illustrated by the following sequence of images captured from video only one second apart. The atmosphere was relatively stable when these images were captured - a front had moved through the area some hours before.
The vast majority of individual images captured in this way are of low to mediocre quality. By searching through a number of them, the moments of best seeing can be isolated and the high quality images extracted from a video file or video tape.