A recording night sky photometer which surveys the sky in 3 min 53 sec and possesses high spectral purity has been constructed and is in use at Sacramento Peak, New Mexico. The photometer is used to record the night sky emission lines at 5577, 5890, and 6300 A. It employs three interference filters 6 inches in diameter, of about 15 A band width, of about 70% transmission, and centered upon the above emission lines. It also employs a single element quartz birefringent filter used to modulate the emission lines. This arrangement of filters was first employed by Blamont and Kastler [ Ann. géophys. 7, 73 ( 1951)] and has been effectively used by Koomen et al. [ Koomen, Lock, Packer, Scolnik, and Tousey, J. Opt. Soc. Am. 44, 827 ( 1954)]. The ac signal in the photomultiplier is then proportional to the emission intensity. This combination of filters provides high spectral purity and corresponding freedom from the effects of continuum in the vicinity of the emission lines.
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