Introduction

The atmosphere behaves like a very thick bad piece of glass in front of your telescope, a piece which is constantly changing. The result of this bad optical element is that the image of a point star is not what the simple physics would lead us to expect, namely the diffraction pattern of the geometrical entrance aperture, but a much more complicated and diffuse image. The image has two general properties: an envelope, which is the image recorded in a long-exposure photograph (longer than, say, one second) and, within it, an internal speckle structure which is continuously and rapidly changing and can only be photographed using a very short exposure (less than about 5 ms, as in figure 5.1). The angular diameter of the envelope, which is called the “seeing,” has a value between 0.5 and 2 arcsec at a good observing site; this is determined by the averaged properties of the atmosphere, which are the subject of this chapter. On the other hand, although the speckle structure is changing continuously, the angular diameter of its smallest distinguishable features correspond to the diffraction limit of the complete telescope aperture. For comparison, figure 5.2 shows the same effect in the laboratory when imaging a point source through the bad optics of a polythene bag.

Michelson (1927), in his book Studies in Optics, refers to the effect of the “seeing” on his 1890 stellar interferometer.