Some of the more common modes of spectrograph slit irradiation have been studied experimentally. Particular attention has been paid to line shape and its effect on resolution. The noncoherent mode is seldom if ever employed because of practical limitations. The lens and broad source modes offer a means of realizing high flux transmission, but the consequent line resolution may be poor. The resolution can be improved by reducing either the relative aperture of the condenser lens or the angle subtended by the broad source at the slit. The limit in either case is the coherent mode with which very sharp, well-resolved lines can be obtained. The closest approach to flat top line shape is realized through the use of a wide slit when the departure from noncoherent irradiation is not very great.
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