Brief description

A heliograph is a wireless telegraph that signals by flashes of sunlight (generally using Morse code) reflected by a mirror. The flashes are produced by momentarily pivoting the mirror, or by interrupting the beam with a shutter.

Use / Function

• Primary use: Long-distance optical communication.
• Secondary uses: Surveying (as a target), military signaling, and emergency rescue signaling.
• Scale: Local to regional (line-of-sight distances up to 50 km or more).

Operating principle

Reflected Sunlight:

1. Light Source: Uses the Sun as a powerful, free source of light.
2. Reflection: A high-quality plane mirror reflects sunlight toward a distant receiver.
3. Alignment: A sighting device (vane or second mirror) ensures the reflected beam is precisely aimed at the target station.
4. Signaling:
   • Shutter Method: A screen or shutter is placed in front of the mirror to block or release the beam.
   • Mirror Tilt Method: The mirror itself is tilted slightly to move the beam on and off the target.
5. Coding: The sequence of light flashes (long and short) translates into a code, such as Morse code.

How to create it

1. The Mirror Assembly

• Use a flat, high-quality glass mirror.
• Mount it on a pivot that allows for both horizontal (azimuth) and vertical (elevation) adjustment.

2. The Signaling Mechanism

• Shutter: Build a wooden frame with louvers (like a blind) that can be opened and closed rapidly using a key or lever.
• Tilting Mirror: Attach a spring-loaded key to the mirror mount. Pressing the key tilts the mirror to aim the light at the receiver; releasing it returns the light to a “rest” position (away from the receiver).

3. Sighting and Alignment

• Place a sighting rod or vane a few feet in front of the mirror.
• Align the mirror so that the sun’s reflection hits the tip of the sighting rod while the target station is visible behind it.
• For situations where the sun is behind the operator, a second mirror (duplex mirror) is used to reflect light onto the main signaling mirror.

Materials needed

• Plane Mirror: A glass mirror with a silvered back. (Mirror)
• Wood: For the tripod, base, and shutter assembly. (Wood)
• Metal (Brass or Iron): For pivots, hinges, and the signaling key. (Metal)
• Glass: If needed for protective enclosures or sighting devices.

Variants and improvements

• Heliograph vs. Signal Lamp: The heliograph is superior during the day as it uses the intense power of the sun; signal lamps (using kerosene or electricity) are used at night.
• Portable Tripods: Early military heliographs were designed to be mounted on sturdy, foldable wooden tripods for field use.
• Automatic Tracking: Advanced versions could include a “heliostat” mechanism to slowly rotate the mirror to compensate for the sun’s movement across the sky.

Limits and risks

• Weather Dependence: Requires clear, sunny skies. It is useless at night, during heavy rain, or in thick fog.
• Line of Sight: Requires a clear, unobstructed path between the sender and receiver.
• Interception: Anyone with a line of sight to the sender or receiver can see the flashes. Messages must be encrypted if secrecy is required.
• Eye Safety: Looking directly at the reflected beam at close range can be blinding.

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