Photographic Positions

Abstract

Measuring the relative positions of double stars on photographic plates was advanced, after earlier attempts, to a high-accuracy standard method by Hertzsprung (1921). Though the mean error of one measured image is of the order of 0"1 in each coordinate, one plate can accomodate many exposures obtainable in a short time, and an internal plate error of only 0"02 to 0"01 can be reached from 25 to 100 images. The actual errors are somewhat larger, owing to systematic effects of the optical system and their changes, but the improvement over the visual accuracy, both in random and systematic errors, is distinct. The study of how to avoid certain systematic errors was crucial to the accuracy achieved by Hertzsprung.