The Beacon That Spelled My Name to Polaris

My hobby is collecting hobbies, and hobby number eight is Morse Beacon Star-Trail Lightpainting — building a small LED Morse beacon and placing it in the foreground during long-exposure star-trail sessions so the blinking pattern writes a callsign into the photograph while the sky stacks behind it.

Three days ago I was holding the RF Waterfall Lithophane up to the kitchen window, watching the ridges of frozen radio glow in the afternoon light. An hour of invisible spectrum, carved into plastic. Retrospective art — the signal was long gone, but the shape remained.

What if the making were the artefact?

The idea arrived fully formed while I was putting the lithophane back on its shelf: skip the freezing, skip the carving, skip the printing. Let the light write itself in real time. Point a camera at the stars, put a blinking LED in the frame, and let the sensor accumulate both.

The stars would draw their arcs. The LED would blink my callsign. The photograph would hold a conversation between the rotation of the planet and the timing table of ITU-R M.1677.

Dit, Dah, Frame

The Morse timing standard has lived in my fingers since 1997. One unit for a dit, three for a dah, three between letters, seven between words. At 12 words per minute — the speed I use for CW beacons — one dit is exactly 100 milliseconds. This isn’t arbitrary: the standard is calibrated so the word PARIS, at 50 units total, takes precisely five seconds at 12 WPM. A metronome in the alphabet.

VE6SLP, my callsign, takes about six seconds to transmit once. In a 30-minute star-trail stack of 10-second exposures, that’s roughly 265 repetitions — 265 signatures blinking into the accumulating frame, each one catching a different slice of sky rotation.

The beacon itself is almost insulting in its simplicity. An ATtiny85 microcontroller, a green LED, a current-limiting resistor, a CR2032 coin cell. Twelve lines of code:

void loop() {
  morse("VE6SLP");
  delay(WORD_GAP);
}

The green LED matters more than I expected. Camera sensors peak at 555 nanometres, right in the green band — not by accident, but because silicon photodiodes were optimized to match human eye sensitivity. Green writes the thickest light trail per milliwatt. Red cuts through urban glow better but leaves thinner streaks. Blue disappears into the star colours.

I soldered it on the workbench in twenty minutes, programmed it in five, tested it against the dark ceiling of the garage. The blink pattern looked right. Short short short long — V. Short — E. Long short short short short — 6. And so on.

Stacking Against the Machine

The back deck faces north. Polaris sits about 53 degrees above the horizon at my latitude, which means the rotation arcs will be wide and dramatic — full circles if I run long enough. I mounted the beacon on a garden stake about four metres from the tripod, far enough that the LED resolves as a point source rather than a glowing blob.

Camera settings: ISO 800, f/2.8, 10-second exposures, intervalometer set to loop until I tell it to stop. The stacking software (Sequator) will blend the frames, pulling the star trails into continuous arcs while rejecting any single-frame anomalies — satellites, headlights, cosmic ray hits.

This is where it almost went wrong.

Stacking software uses sigma-clipping to reject transient pixels. A satellite crossing the frame appears in one exposure and vanishes; the algorithm flags it as an outlier and discards it. But my beacon blinks. It appears in every frame, yes, but at a different position in its cycle — sometimes mid-dit, sometimes dark. The algorithm could interpret that variability as noise.

I caught this before the first frame by switching from sigma-clip rejection to simple median stacking. Median preserves anything that appears in at least half the frames. The beacon blinks on roughly 40% of the time (the prosign structure of callsigns is heavy on dits), so it survives.

Sequator loaded the first 180 frames around 10:30 PM.

What the Sensor Saw

The preview rendered slowly, arcs extending from the star positions, the foreground stake a dark silhouette. And there — a broken green line, pulsing in discrete segments where the beacon fired, dark gaps where it rested.

The pattern was readable. Not by eye, not intuitively, but when I pulled up an expanded view and traced the segments: short, short, short, long — V. The letters were there, encoded in light, traced against the rotation of the planet.

There’s something disorienting about reading your own name written by photons you emitted hours ago, captured in the same frame as stars that emitted theirs thousands of years ago. The timescales don’t belong together. And yet the camera doesn’t care. It integrates everything that arrives.

The final 90-minute stack holds about 795 callsign repetitions. The green trace is thick and steady, a dense pulse train running along the bottom of the frame while the circumpolar stars wheel silently above. Cassiopeia’s W. The Big Dipper’s arc. And below them, stubborn, repeating, a grid of dits and dahs spelling VE6SLP into the archive.

I didn’t sign the photograph. The photograph signed itself.