Although Britain has not established a network of optical stations for tracking artificial Earth satellites, very many optical observations have been made both by an excellent group of amateurs who make visual observations to an accuracy of 1' to 2' of arc and 0.1 s of time and by a number of kinetheodolite stations. Precision measurements of satellite positions are also made using two cameras which have been designed and built in Britain. These cameras are comparable with the Baker-Nunn cameras used by the Smithsonian Astrophysical Observatory of America in their world-wide network of optical tracking stations, and like the Baker-Nunn cameras they have been specially designed for observing satellites. The British cameras, however, unlike the Baker-Nunn cameras, do not track the satellite but record its trail from a bearing and elevation fixed in relation to the Earth. Consequently, they are not as sensitive as the Baker-Nunn cameras but errors which might arise from the tracking system have been eliminated. The sensitivity of a fixed camera depends not only upon the sensitivity of the photographic emulsion but also upon the aperture and focal length of the optical system and its optical quality. The limiting magnitude of a system is given by E/d kId/ where E is the sensitivity of the photographic emulsion, d , the diameter of the image, D , the diameter of the camera aperture, K the transmission coefficient of the system and t the exposure time. For a fixed camera the exposure time, t , can be expressed as djojF, where co is the angular velocity of the satellite in radians per second and F is the focal length of the optical system. For a tracking camera, t is given by d/V-V0. where V— V0 is the difference between the velocity of the satellite and the tracking velocity of the camera. The sensitivity of a tracking camera with a given aperture, therefore, increases as its tracking velocity approaches that of the satellite and, with an accurately predicted velocity of the satellite, a considerable increase in the camera sensitivity can be obtained. However, the sensitivity of a fixed camera of the same aperture can be increased only by reducing the focal length. As measuring accuracy is determined by the scale of the image on the photographic plate, the focal length must not be too short, otherwise accuracy will suffer.
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