The world record distance was established by a detachment of U.S. A good approximation for ranges of 20–50 miles is that the flash of a circular mirror is visible to the naked eye for 10 miles for each inch of mirror diameter, and farther with a telescope. 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.
Heliograph mirrors ranged from 1.5 inches to 12 inches or more. The range of a heliograph depends on the opacity of the air and the effective collecting area of the mirrors. 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. In the Boer War, where both sides used heliographs, tubes were sometimes used to decrease the dispersion of the beam. However, anyone in the beam with the correct knowledge could intercept signals without being detected. 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 feet per mile of range. 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 campaign. This type produced flashes by a shutter mounted on a second tripod (Fig 4). Signal Corps heliograph mirror did not tilt. 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. If the sun was in front of the sender, its rays were reflected directly from this mirror to the receiving station. 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. This indicated that the sunbeam was pointing at 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. Keeping their head still, they then adjusted the aiming rod so its cross wires bisected the target. 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. It used a mirror with a small unsilvered spot in the centre. Most heliographs were variants of the British Army Mance Mark V version (Fig.1). Fuess in Berlin (on display at the Museum of Communication in Frankfurt)