The Hertzsprung Multiple Exposure Technique and its Application to 61 Cygni

E. Hertzsprung developed the multiple exposure technique for observing double stars photographically around 1914 at Potsdam (Hertzsprung, 1920). The technique consists in taking one or more rows of 17 to 35 exposures of a small field which normally includes only the double star itself. The total number of exposures on a plate can vary from 17 to 140, depending primarily on the separation of the double star and the anticipated systematic effects in the observational system. An important feature of the technique is the use of objective gratings to substantially reduce the problem of magnitude error. The large number of exposures serve to reduce the random error, while the objective gratings, together with other innovations introduced by Hertzsprung, help to minimize the systematic errors. In this way Hertzsprung provided us with an observational technique of considerably higher accuracy than the traditional visual micrometer work.