Problems relating to the design of a far infrared spectrograph are discussed. The design of a spectrograph which was constructed is described. A platinum strip coated with thorium-oxide is used as a source. Discrimination against short-wave radiation is provided by quartz, paraffin, turpentine soot, a compensated potassium bromide chopper, reststrahlen plates, and a grating (used as a reflection filter) with groove separation somewhat less than the wavelength being investigated. As a dispersing device, an echelette grating ruled with 180 lines per inch is used. The detector is a Golay pneumatic cell. A low noise vacuum-tube amplifier renders the signal from the detector suitable for operation of a strip-chart recorder. The spectrograph is evacuable.
Spectral operating characteristics of the instrument are shown in ammonia and atmospheric water vapor spectra in the region between 45 and 150 microns. Absorption lines separated by less than 1 cm−1 are well resolved throughout this spectral region.
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