The Visibility of Stars in the Daylight Sky

Abstract

From the theory of the passage of light through a telescope and into the eye, formulas are derived for the improvement in the visual threshold of a point source, as a star, viewed in a field of light, as the sky, in terms of telescope parameters, as magnification, transmission, and entrance and exit pupils. For fields of the brightness of the daylight sky the first parameter is the most important and the others are of less importance. Observations of artificial stars in the laboratory and of real stars in the daylight sky are given, which were in fair accord with the theory.Charts and tables are presented of the visual thresholds of stars in various parts of a clear daylight sky in terms of the stellar magnitude, the magnification of the telescope, the use of a polarizer, the altitude of the observer above the surface, and the altitude of the sun above the horizon.New data are given of extra-foveal thresholds of point sources in a field of brightness 360 ft.-c−2.