Abstract
An improved objective procedure has been developed for focusing a spectrograph. A spectral line is selected that has a ratio of spectral line intensity to background intensity of about three. Microphotometer measurements are made of the optical density of this spectral line and of the adjacent background, and the intensity ratio is calculated from these densities for each slit-to-grating distance. In order to select the optimum focal position, a cumulative distribution is calculated from these intensity-ratio data and plotted against slit-to-grating distance on probability graph paper. The resulting graph is linear except at its extreme ends. The slit-to-grating distance corresponding to the cumulative distribution at 50 per cent is selected as the optimum focal position.
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