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heliograph(hē`lēəgrăf) [Gr.,=sun-writer], signaling device using flashes of sunlight. It has two mirrors that are used to reflect sunlight on a distant point and a shutter through which the sunlight passes so that messages may be transmitted in telegraphic code by means of long and short flashes. It was used in ancient times and as recently as the 19th cent. by the U.S. army in the SW United States and by the British army in India.
(1) In meteorology, an instrument for automatically registering the duration of sunshine, that is, the time when the sun is above the horizon and not covered by clouds. There are many types of heliographs. The most widespread in the USSR is the Campbell-Stokes heliograph, in which a stationary sphere serves as a lens that gathers the rays of the sun onto a cardboard strip marked off by hour lines. The strip is burned through by the sun’s rays if irradiation exceeds 0.3 to 0.4 calories per sq cm per min. Because of the apparent daily motion of the sun, the burn is in the form of a line, the length of which serves to measure the duration of the sunshine. A continuously registering actinograph can also function as a heliograph.
REFERENCESternzat, M. S. Meteorologicheskie pribory i nabliudeniia. Leningrad, 1968. Page 209.
(2) In astronomy, a telescope adapted for photographing the sun. It is used to obtain photographs of all or part of the solar disk in a wide range of wave lengths. A heliograph can be used in combination with a coelostat. Because of the tremendous illumination generated by the sun, the aperture ratio of the heliograph objective can be minimal. In order to obtain pictures of the sun with large linear dimensions, as large a focal length is chosen for the heliograph as possible. At the same time, so as not to increase the size of the instrument, additional magnification systems are used. The heliograph is equipped with a fast-action shutter (usually of the curtain type), giving exposure times of from 0.02 to 0.001 seconds. One of the first heliographs was installed by the Russian astrophysicist M. M. Gusev at Wilno (Vilnius) in 1854.
(3) In military affairs in the 19th and early 20th centuries, a light-signaling device for transmitting messages (by Morse code) with a mirror used to reflect light rays. The heliograph is effective over distances of 18 to 40 km in daytime and 3 to 8 km at night.