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Gehrels, T. 1971, Asteroids and Comets. Int. Union Geodesy Geophys. Rept. Trans. Amer.
If one wants to study the physical characteristics of individual asteroids, he must first find them and be prepared to track them accurately during observation. To make the necessary predictions, the positions of asteroids are measured in reference frames defined by stars of known coordinates as the input information for calculation of orbits and ephemerides. Continuation of positional observations over lengthening arcs provides the basis for improvements of orbits and increased accuracy of ephemerides. Direct photographs, from which positions relative to background stars are measured, may be made with relatively short-focus instruments of wide field. Astrographs with multicomponent lenses of various designs or catadioptric systems of the Schmidt or Maksutov type are in common use. Typical instruments have fields of several degrees diameter and reach to 16 to 17 mag on plates of scale 1 to 3 arcmin/mm. The overwhelming majority of astrometric observations of minor planets are obtained with such instruments. On a wide-field plate, images of a number of minor planets are usually found in all ecliptic fields; with powerful instruments the number may be very large. A limited number of faint objects of compelling interest may be observed with powerful long-focus instruments such as the 154 cm f/13.5 NASA reflector of the Catalina Station of the Lunar and Planetary Laboratory. Observations of objects as faint as 20 to 21 mag can be obtained with this instrument, but the limited field (30' at scale 10 arcsec/mm in the focal plane) restricts its practical use to special objects for which a fairly reliable prediction of position (within 5' to 10') can be made. Particularly for faint objects observed at long focal length, it is nearly essential that the motion of the asteroid be compensated during the exposure. On plates taken by this “Metcalf method,” star images appear as trails whose length corresponds to the amount of the differential motion during the exposure. The asteroid of perfectly matched motion appears as a small round dot. If asteroids are relatively bright, or the optical system very fast, exposures may be guided carefully on a star, in the same way as for conventional astronomical photography. The moving object will almost invariably appear elongated to some degree on plates taken by this “Wolf method.”