Optical “Seeing” over Level Terrain

This work is aimed at understanding the phenomena which affect the sharpness and stability of optical images transmitted through the atmosphere. A brief description is given of the results obtained to date from an experimental field-study directed at establishing a connection between optics and meteorology. Most of the work has, so far, been conducted in daytime because our main application is to solar astronomy; other fields such as surveying, aerial reconnaissance and so on can also benefit from basic research in atmospheric optics.A horizontal line of sight over uniform, level terrain presents the simplest case for study. The optical transfer function (OTF) of the system comprising an air-path and telescope objective has been measured with an image analyser which is a modified form of stellar seeing-monitor. The outputs of this analyser are proportional to the modulus and argument of the OTF. The modulus, or modulation transfer function (MTF), is a measure of image contrast; the fluctuations of the argument measure image movement, or “dancing”. The OTF is a function of spatial frequency f (cycles per unit distance) which may be altered by radial movement of the scanning graticule of the image-analvser.