Heliography
(Communicating with Mirrors)
by James Riddle - KD7AOI
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Heliography (Communicating with Mirrors) by James Riddle - KD7AOI
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Definitions Heliograph: n. [HELIO- + GRAPH] (Greek - sun writer) 2. a mirrored communications device for sending messages in Morse Code by flashing reflected sun's rays to a distant station. Also heliogram, heliographics, heliography, heliographer, heliographing and heliographed. (I often abbreviate heliograph as "Hg", not to be confused with Mercury though its flash is every bit as quick.) But, I really love this 1896 description: �An instrument called the heliograph, or sun telegraph, constructed with small mirrors made to turn upon both a horizontal and vertical axis, mounted upon a tripod, so arranged as to make the flashes appear and disappear in rapid succession, is to a limited extent in use in the army; and by it messages may be transmitted much faster than with flags or torches, and it can be used at longer ranges. It is manipulated by a key, similar to the electric telegraph instrument. Mirror signaling was first used by the North American Indians.� Source: �As Told by Heliograph�, The Land of Sunshine, a Magazine of California and the Southwest, Los Angeles, October, 1896, pg 213. As shown on the Home Page, there are three basic types of heliographs: 1) the "British Mance" pattern that during "keying" the instrument tilts the mirror upwards, thereby projecting the solar beam onto the distant station, 2) the "American Service Model" whereby the mirror remains directed on the distant station while its beam is interrupted by keying a screen (shutter) set up on a separate tripod, and 3), the "Portuguese Pattern" which combines features of the first two into a single unit. As a "rule of thumb" the range of a heliograph is ten miles per inch of the mirror�s diameter. Therefore, a 5" mirror, whether round or square, would have a range of about 50 miles, the 1.8" mirror of about 18 miles. The diameter of the reflected flash of a flat mirror is about 49 feet per mile, independent of the mirror�s size or shape, however, the larger the mirror the brighter the image. Therefore, the beam width at ten miles would be about 500', at fifty miles it would be about 2,500', or about 1,250' on either side of (or above and below) the targeted station if perfectly centered. It is essential to maintain regular adjustments of the sun mirror to compensate for the apparent movement of the sun. The projected image is round regardless of the mirror's shape, it of course being that of the reflected sun.
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