MEASUREMENT OF JUPITER'S SATELLITES BY INTERFERENCE

Abstract

IT has long been known that even in a telescope which is theoretically perfect, the image of a luminous point is composed of a series of concentric circles with a bright patch of light at the common centre. This system of circles can easily be observed by examining any bright star with a telescope provided with a circular diaphragm which diminishes the effective aperture. The appearance of the image is shown in Fig. I, a. In the case of an object of finite angular magnitude the image could be constructed by drawing a system of such rings about every point in the geometrical image. The result for a small disk (corresponding to the appearance of one of the satellites of Jupiter as seen with a 12-inch telescope whose effective aperture has been reduced to six inches) is given in Fig. 1, b; the chief points of difference between this and Fig. 1, a, being the greater size of the bright central disk, and the lesser clearness of the surrounding rings. The larger the disk the more nearly will the appearance of the image correspond to that of the object; and the smaller the object the more nearly does it correspond with Fig. 1, a, and the more difficult will be the measurement of its actual size. Thus, in the case just cited, the actual angular diameter is about one second of are, and the uncertainty may amount to half this value or even more.