Direct Measurement of Intensity Distributions in Star Images

Abstract

MEASUREMENTS of the distribution of light intensity in the images of a star, formed by lenses, may be made with photomultiplier techniques. The experimental evidence so provided is of immediate application to the theory of the resolution of optical instruments, permitting confirmation of rigorous calculations, for example, those of Airy¹ for the aberration-free axial case, of Martin² for the axial case with aberration and of Hopkins³ for the general case at a position in the field of a system with aberrations. Another use for these measurements is in checking the accuracy of approximate methods of image calculation⁴. Apparatus based on photomultipliers has generally been designed to employ a slit probe, for facility of image location and in order to keep the incident flux at a convenient level. Results have been obtained by Ingelstam⁵, Schade⁶, Lindberg⁷ and Sprosen⁸ in this way, and R. E. Hopkins⁹ has used a circular aperture to measure the proportion of the total energy which falls within a chosen diameter of the image. The measurements made by these means need to be converted by the appropriate mathematical techniques in order to arrive at the requisite distributions. Direct intensity distribution measurements are evidently desirable provided the experimental difficulties of image location and low light level can be overcome.