The performance of a plane, unblazed grating (600 lines/mm) as a predisperser has been investigated. Placed 7 cm in front of the primary slit of a 2-m grazing incidence vacuum spectrograph, the foregrating allowed passage of uv (100 to 500 Å) into the spectrograph in 50 to 150 Å bands. The spurious background observed when radiation passed directly through the primary slit was found to be absent from the predispersed spectra, which have been observed by both photographic and photoelectric methods. These spectra, free from scattered background and second-order images, exhibited measurable line displacement, up to 0.3 mm from the line image positions of the direct spectra. Such displacement, occuring naturally with the plane foregrating and shown to be in agreement with theoretical predictions, are sometimes found as a result of source misalignment with grazing incidence instruments. Use was made of the foregrating for isolation of true continuum radiation for the Vodar vacuum sliding spark and linear pinch sources.
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