1. The nature of the birefringent filter which permits separating continuum and line emission into dc and ac signals, respectively, is described in papers by Lyot(12, 13) and Ohman(14). Application of a birefringent filter for the observation of spectral lines in the air low has been described bymany investigators(6,8, 18-20), Dandekar, Pomalaza, and Beilew(20)discussed the theory of the birefringent filter as applied directly to airglow research.
2. Calibration of the ac channel requires the use of an external chopper for modulation. The modulation frequency of the external chopper was 33 Hz. The Referencesignal frequency from the photometer was maintained at 35 HBand was used for PAR amplifier reference. The ac channel, being externally chopped but referenced through the lock-in amplifier at a slightly different frequency, appears in the output as a slowly varying, sinusoidal signal of frequency 35 - 33 = 2 Hz. This requires that the time constant for the PAR and recorder be selected small enough to allow the recorder to follow this frequency.
3. To establish the lowest possible limit of detection for atmospheric lithium, all of the selectivity or discriminatory ability of the birefringent photometer must be realized. It is extremely important that each facet of the experimental techniques be investigated to insure that, inthe absence of a positive signal for guidance, the lower limit established is based on valid considerations. The proper phase setting of the lock-in amplifier was determined from a hollow cathode source with natural lithium as the cathode material. If it were definitely known that a particular amount of lithium is present in the atmosphere, then a valid verification of instrument capability could be established. During the course of tbij investigation, lithium of known quantity, under controlled conditions, was released from a rocket over Wallops Island, Virginia, between a height of 70 and 200 km during two twilight periods. This lithium provided a known lithium signal which could be used for instrumental evaluation under typical operating conditions. The launch dates for these rockets were the evening of July 16, 1966 and the early morning of July 17, 1966. The lithium emission fSom these releases was easily detectable even though the weather over and near Raleigh, North Carolina, was inclement. Signal strengths of several thousand Rayleighs were recorded when the photometer was oriented in the direction of the released vapor. Observations were not made during twilight on the days immediately following the lithium releases because of inclement weather. Observationsbefore and about a week after gave no indication of lithium in the atmosphere.
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