A new differential variable-star photometry system has been designed, constructed, and tested at the Rothney Astrophysical Observatory of the University of Calgary. The system uses a single telescope, photometer, and detector and consists of gated pulse-counting electronics synchronized to a chopped secondary mirror. The mirror is driven to four positions, permitting rapid alternate detection of a variable star, comparison star, and two sky-background regions. Within certain constraints the system is insensitive to the effects of transparency, sky-brightness, and detector-sensitivity variations occurring on time scales longer than the system's duty cycles. The system has proved effective under as much as three magnitudes of cloud and attendant sky-brightness variation.

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