A Fabry-Perot spectrometer specifically designed for measuring airglow intensities in twilight has been built using a feedback-controlled etalon from Queensgate Instruments, London. A single etalon passband (FWHM = 0.015 nm at 558 nm) is isolated by a narrowband (0.2-nm) interference filter. The passband of the etalon can be shifted in a random fashion by a digital command to a controlling circuit. Observing time is, therefore, used efficiently because only wavelengths of interest need be observed. Responsivity of the system is 1.3 counts sec−1 R−1 at 558 nm. The optics are contained in an outdoor climate-controlled enclosure connected by cables to a microcomputer at the indoor observing station. The computer controls all functions of the instrument and is capable of running long complex observing routines unattended using parameters specified by the observer. These parameters include meridian angles, filters (up to eight), integration times, and etalon steps. Some examples of twilight measurements are given.
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