A heliograph (from Ancient Greek ἥλιος (hḗlios) 'sun' and γράφειν (gráphein) 'to write') is a solar telegraph system that signals by flashes of sunlight (generally using Morse code from the 1840s) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter. The heliograph was a simple but effective instrument for instantaneous optical communication over long distances during the late 19th and early 20th centuries. Its main uses were military, surveying and forest protection work. Heliographs were standard issue in the British and Royal Australian armies until the 1960s, and were used by the Pakistani army as late as 1975. While these solar telegraphs are now obsolete, the daylight signalling mirrors required worldwide in oceangoing lifeboats are often called "heliographs" in Commonwealth nations.
Description
There were many heliograph types. Most heliographs were variants of the British Army Mance Mark V version (Fig.1). It used a flat round mirror with a small unsilvered spot in the centre. The sender aligned the heliograph to the target by looking at the reflected target in the mirror and moving their head until the target was hidden by the unsilvered spot. Keeping their head still, they then adjusted the aiming rod so its cross wires bisected the target. They then turned up the sighting vane, which covered the cross wires with a diagram of a cross, and aligned the mirror with the tangent and elevation screws, so the small shadow that was the reflection of the unsilvered spot hole was on the cross target. This indicated that the sunbeam was pointing at the target. The flashes were produced by a keying mechanism that tilted the mirror up a few degrees at the push of a lever at the back of the instrument. If the Sun was in front of the sender, its rays were reflected directly from this mirror to the receiving station. If the Sun was behind the sender, the sighting rod was replaced by a second mirror, to capture the sunlight from the main mirror and reflect it to the receiving station. The U.S. Army's Signal Corps heliograph used a flat square mirror that did not tilt. This type produced flashes by a shutter mounted on a second tripod (Fig 4). The heliograph had certain advantages. It allowed long-distance communication without a fixed infrastructure, though it could also be linked to make a fixed network extending for hundreds of miles, as in the fort-to-fort network used for the Geronimo military campaign. It was very portable, did not require any power source, and was relatively secure since it was invisible to those not near the axis of operation, and the beam was very narrow, spreading only 50 ft (15 m) per 1 mi (1.6 km) of range. However, anyone in the beam with the correct knowledge could intercept signals without being detected. In the Second Boer War (1899–1902) in South Africa, where both sides used heliographs, tubes were sometimes used to decrease the dispersion of the beam. In some other circumstances, though, a narrow beam made it difficult to stay aligned with a moving target, as when communicating from shore to a moving ship, so the British issued a dispersing lens to broaden the heliograph beam from its natural diameter of 0.5 degrees to 15 degrees. The range of a heliograph depends on the opacity of the air and the effective collecting area of the mirrors. Heliograph mirrors ranged from 1.5 to 12 in (38 to 305 mm) or more. Stations at higher altitudes benefit from thinner, clearer air, and are required in any event for great ranges, to clear the curvature of the Earth. A good approximation for ranges of 20 to 50 mi (32 to 80 km) is that the flash of a circular mirror is visible to the naked eye at a distance of 10 mi (16 km) for each inch of mirror diameter, and farther apart seen with a telescope. The world record distance was established by a detachment of U.S. Army signal sergeants by the inter-operation of stations in North America on Mount Ellen (Utah), and Mount Uncompahgre (Colorado), 183 mi (295 km) apart on 17 September 1894, with Army Signal Corps heliographs carrying mirrors only 8 inches (20 cm) on a side.
History
… excerpt ends here. Continue reading the full article.
