Satellite-Tracking Camera

an astronomical instrument for the photographic observation of artificial earth satellites. It is a wide-angle camera with a large-diameter lens, highspeed shutter, and a device for accurately registering the time at which the shutter opens and closes.

To ensure that a rapidly moving satellite can be photographed at several points in its orbit during a single pass through the region visible to the observation station, the camera is usually supported by a three- or four-axis mounting (seeTELESCOPE MOUNTING). The camera can be moved from one position to another by rotation about one of the axes. A three-axis mounting permits the apparent path of the satellite to be approximated by a great circle of the celestial sphere. In this case, the first and second axes constitute an altazimuth or equatorial mounting and are used to orient the third axis toward the pole of the approximating great circle. The camera rotates about the third axis. In the four-axis mounting, the fourth axis permits the optical axis of the camera to be directed nonperpendicularly to the third axis and thus makes possible a more accurate approximation of the apparent path of the satellite by a small circle of the celestial sphere.

Since most artificial satellites are faint objects and their image in the focal plane of a fixed camera moves rapidly, the luminous energy from a satellite is usually insufficient to produce on the photographic emulsion a blackened area whose position can be measured. For this reason, many cameras are provided with devices that compensate for the motion of the satellite image relative to the emulsion; as a result, the effective exposure time is increased. Such compensation can be carried either by slowly turning the entire camera to follow the satellite during the exposure or by moving the photographic plate or film at the same speed that the image of the satellite moves in the focal plane.

The satellite photographs obtained with the cameras depict individual positions of the satellite against the stellar background in the form of points or short lines. Through measurements of the photographs, the direction of the satellite can be determined to within 1” at moments in time that can be registered with an accuracy of approximately 1 millisecond.

One of the first satellite-tracking cameras was the Baker-Nunn camera. It was designed in the USA in 1957 and has been used by the Smithsonian Astrophysical Observatory for global research in satellite geodesy.

In the USSR, the AFU-75 automatic satellite tracking camera has been used in geodetic and geophysical research involving the observation of satellites. The AFU-75 has a four-axis mounting, a mechanism for moving the photographic film in order to observe faint satellites, and an equatorial platform. The equatorial platform is a mechanism that, during the exposure, rotates the camera in a simulation of its rotation about the polar axis. This motion is necessary if the images of the stars are to appear as points on the film. The camera is equipped with a lens whose diameter is 210 mm and whose focal length is 736 mm. AFU-75 satellite cameras have been installed in photographic-observation stations in the USSR and in many foreign countries, where they are used in scientific programs sponsored by the Academy of Sciences of the USSR.

The largest satellite-tracking camera in the USSR is the automatic VAU. It uses a three-axis mounting and has a catadioptric objective that was developed under the direction of D. D. Mak-sutov. The mirror is 1,070 mm in diameter and has a focal length of 700 mm. In accordance with a predetermined program, the camera is automatically shifted from one orientation to another. Faint objects are observed by rotating the camera about the third axis at speeds ranging from 0” to 6,000” per second.

Satellite-tracking cameras of various designs have been developed in the USA, France, Great Britain, the German Democratic Republic, the Federal Republic of Germany, and other countries.