An analysis is made of the factors affecting the photographic exposure time for celestial objects. The minimum number of quanta that must be received from a star, end the minimum number per unit solid angle that must be received from an extended source, to reach the photographic threshold, are determined, for the case when the camera is carried on an astronomical mount. The treatment is then extended to the case of a satellite, or other rapidly moving object near the earth, when the camera position is fixed with respect to the earth.
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