Photographic Astrometry

a branch of astrometry devoted to methods of solving astronomical problems by means of photographs of the stellar sky. Photographic astrometry is concerned with the measurement of the celestial coordinates of the stars, planets, artificial celestial bodies, and other objects, the determination of the proper motions of celestial objects, the measurement of the trigonometric parallaxes of stars, and the study of the motions of the components of binary stars.

The methods of photographic astrometry are based on the determination of the empirical relation between the rectangular coordinates of some group of stars—the reference stars—measured with a coordinate-measuring machine on an astronegative and the equatorial celestial coordinates of the same stars obtained from a star catalog. This relation makes it possible to determine the equatorial coordinates of any other celestial object, such as a star or planet, using the rectangular coordinates measured on a photograph. The proper motions of the reference stars are taken into account when calculating the coordinates of celestial objects, and corrections are introduced for distortions due to light refraction in the atmosphere, the annual and diurnal aberration of light, the aberrations of optical systems, and other factors. Extensive use is made of Turner’s method in the calculations.

The origin of photographic astrometry dates to 1857, when G. Bond made multiple photographs of the binary star Mizar and measured the positional angle of the components on the photographs. In the 1890’s the methods of photographic astrometry became widely used. A new branch of photographic astrometry emerged with the first photographic observations of artificial earth satellites in the early 1960’s. One of the main difficulties in calculating the coordinates of the transits of satellites across the stellar sky from a photograph is the necessity of calculating the exact times when the images of a fast-moving satellite are formed on the photograph (to an accuracy of up to 0.1–1 msec). In observations of faint satellites that do not leave a noticeable trace on a photographic emulsion, the photographic plate (or film) is moved in the focal plane of the camera objective with the moving image of the satellite. This makes it possible to increase the effective exposure of the satellite. The necessity of allowing for such movements of the photographic plate relative to the reference stars is another feature of calculating coordinates from satellite photographs.

The astrograph is the main instrument used in photographic astrometry. Wide-angle astrographs with high-power lenses, in particular satellite-tracking cameras, are used to observe artificial earth satellites, meteors, and a number of other celestial objects.