a method of astronomical observation based on photographing celestial bodies with astro-graphs. Astrophotography was introduced as an astronomical technique in the middle of the 19th century. It replaced visual observations because of its advantages, including the ability of photographic emulsion to store light energy, which makes possible the observation of faint celestial bodies; the ability to obtain simultaneously in one photograph the images of many objects (for example, stars in the Milky Way) or of one object in all its details (for example, the corona of the sun); and objectivity and ease of data storage.
In a narrow sense, astrophotography refers to photographic astrometry, that is, the branch of astrometry in which photography is used to solve such problems as the determination of the positions of stars in the celestial sphere, the measurements of their movements and of the distances to them, the relative displacements of stars in binary and multiple systems or of satellites orbiting planets, and so forth. Most astrometrical problems are solved by measuring the angles between star positions at given time periods. Using the methods of astrophotography gives the measurement on a photograph of a specific region of the sky, the rectangular coordinates of the object to be studied, and a certain number of reference stars with the equatorial coordinates α and δ, which are known from catalogs. Measurements are made with the aid of special coordinate-measuring machines. Measurements by this method do not normally exceed a one-μm margin of error. The results of such measurements permit the determination of the α and δ coordinates for the bodies under study, which might be a large or small planet, a comet, a meteor, the moon, a star, and so forth.
Proper motions of stars are determined from photographs made at ten-year intervals. Distance calculations are based on measurements of angles between positions to a celestial body at different times of the year, that is, from different points of the earth’s orbit. In this way the distances to stars are determined with an accuracy to several thousandths of a second of arc, which corresponds to distances of 200–300 parsecs. Astrophotography makes possible the measurement of the relative position of binary star components if the distance between them is not less than 1”—otherwise, the star images on the photograph touch or overlap each other. Of exceptional interest are the invisible companions of stars, which cause noticeable periodic displacements of the stars themselves. The masses of such invisible companions have been found to be comparable to the masses of the planets in the solar system. Special instruments were designed in the 1950’s for photographing and determining the positions of artificial satellites of the earth moving rapidly through outer space, and special methods were also developed for determining their α and δ coordinates and the time periods of the observations.
A. N. DEICH