Seeing: its Cause and Cure

Following Fried, the statistics of images formed by an annular telescope aperture looking through the turbulent atmosphere are calculated; the deleterious effect of a central obscuration is much larger in the presence of seeing than for the telescope alone. A method of optimizing resolution through the atmosphere is explained in detail, with particular attention to the requirements for image-motion compensation; under ideal conditions, about a factor-of-four increase in effective resolution over conventional photography can be obtained. The effective resolution can be large or smaller than the "seeing disk," depending on the recorded signal-to-noise ratio. Some common misconceptions about image structure and motion, and various unsound proposals for beating the seeing, are criticized. For the range of turbulent spectra actually observed in the atmosphere, no large improvement in resolution can be obtained at infrared wavelengths. Furthermore, the optimum telescope aperture is rather small; the apparent advantages of large telescopes are due solely to a good signal-to-noise ratio, not good resolution. Subject headings: atmospheres, terrestrial - image processing - turbulence.