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The focusing properties of star-tracker tubes can be summarized by a quantity, called rise distance, which measures the sharpness of transition between light and dark areas. Measurements showing the values achieved to date are presented. It is then discussed how shortcomings of the rise distance can be overcome by proper choice of the data handling process, i.e., the output circuitry. It is pointed out that the tubes can be operated either in the star tracker or in the dissector mode; one or the other may be preferable for a given task.