On the nature of the diffraction figures due to the heliometer

Abstract

1. In investigating the influence of personal equation in connection with double star measurements, the actual values of distance and position angle must be known, and hence the only possible method is to use an artificial star. The method adopted in the Students’ Observatory at University College, in order to obtain a known separation and direction, is to use a weak heliometer lens in front of the equatorial and a spark between cadmium electrodes some distance away. The direction of the line joining the two “star” images and the distance between them can be measured by a divided circle and micrometer screw respectively, while the intensity of the stars can be altered by using various stops. It was found that when a moderately small stop was used, the diffraction images became quite perceptible, and hence, before any further progress could be made in the determination of personal equation, it was necessary to investigate these diffraction images. 2. On consulting the literature of the subject, I found that the earliest papers dealing with the diffraction figure of the heliometer are by Bessel. He observed ( a ) that two opposite brushes of light emerged from the central disc perpendicularly to the line of separation; ( b ) that when the halves of the lens were separated, the disc was lengthened in the direction perpendicular to the line of separation; and ( c ) that the brushes of light were of equal length when the halves were united; but that, when the halves were separated, the brush of light on the same side of the image as the half-lens forming it was longer than the one on the opposite side; and he attributed the lengthening of the disc formed by the half-lens to the fact that the lens was corrected for spherical aberration for the whole lens, and not for the two halves separately.