Star trails with a phone: when the blur is the subject

Star trails look like the photograph a phone cannot take. They come out of very long exposures, twenty minutes, an hour, and no mobile camera app gets anywhere near those times: the ceiling usually sits at thirty seconds. There is physics under the commercial limit too, because a phone sensor held open for an hour would warm up until it handed back a grey rectangle full of hot pixels. What matters is that almost nobody shoots those marathon exposures any more, not even the person with a DSLR bolted to a tripod beside yours. The current technique is something else, and a phone walks straight into it.

An hour of sky is worth fifteen degrees

The geometry is as simple as it looks: Earth turns 360 degrees in twenty-four hours, so fifteen degrees every hour. An hour of shooting draws a fifteen-degree arc. In a phone’s main wide camera, covering roughly 74 degrees horizontally, that arc takes up a fifth of the frame width. Half an hour gives you half of it. Ten minutes produce two and a half degrees of trail, five lunar diameters laid end to end: perfectly visible, and they read as a botched hand-held shot.

That figure applies to stars on the celestial equator, the ones riding the widest circle. Climb towards the pole and the circles tighten, so the trails shorten in proportion to the cosine of declination. Which is why framing decides the photograph before any setting does. Point north and you get concentric circles. Point east or west and the stars rise and set along near-straight diagonals that enter one side of the frame and leave the other. Point south and wide parallel arcs open up, concave towards the top.

The centre of those circles falls near Polaris without landing on it. Sky & Telescope puts the star some forty arcminutes from the celestial pole today, a shade under two thirds of a degree, and the precession of Earth’s axis will keep closing that gap until March 2100, when it reaches twenty-seven arcminutes. After that it drifts away again, slowly. In practice Polaris leaves a trail of its own: a small circle a degree and a third across, close to three lunar diameters, easy to spot at a hundred per cent magnification.

Maximum instead of mean

Here the method parts company with what your app does on every other night of the year. In live stacking frames get aligned on the stars and then averaged, and the averaging does the work: noise changes at random from frame to frame and cancels out, signal sits in the same place every time and survives. Trails throw away half of that procedure. No alignment, since the movement of the stars is precisely the subject. No averaging either, because averaging two hundred different positions gives you a very faint haze where a track should be.

The right operation is the maximum, pixel by pixel: across all the frames you keep the brightest value. Software calls it lighten blending, the same thing you find in a Photoshop layer or inside StarStaX. A star sitting in one pixel on frame one and in another one fifteen degrees away on frame two hundred ends up filling all two hundred of them, in order. The trail is a string of successive positions, stitched together by the software.

The maximum has a flaw that mirrors the one averaging has, and it pays to know about it before queueing up three hundred files. By always keeping the highest value, every pixel of background sky ends up wearing the noise peak of the whole series: the more frames you add, the higher and grainier the background gets, where normal stacking would have done the opposite. Hot pixels make it worse still, because they sit in the same spot and stay bright: they survive any blend, and the finished image acquires a sky full of fixed little stars that do not exist. A batch of darks shot with the lens covered at the end of the session, subtracted before blending, clears them out in a single pass.

The gap between one frame and the next

Anyone shooting trails with a DSLR fights dashes. Between the end of one exposure and the start of the next the camera writes the file, and during that second or two the sky keeps moving: the track is left with a hole, and the string turns into a dotted line. Blend modes exist for the sole purpose of filling those holes.

On a phone the problem dissolves on its own, and two multiplications show why. The sky drifts at a touch over fifteen arcseconds per second, and one pixel of the wide camera covers about seventy of them: we have run those numbers already. A two-second interval moves a star thirty arcseconds, less than half a pixel. The hole is real, it is calculable, and you will never see it. Making it visible takes an interval longer than five seconds, and at that point your problem is the app rather than the blend.

What to set, and for how long

Keep the ISO as low as it goes while the stars still register. It sounds wrong to anyone arriving from deep sky work, where everything gets pushed to drag out faint signal, and here it flips: the stars that make up the trails are bright by definition, while the noise is the thing you are about to multiply with a maximum operator. Single exposures run as long as the sky allows without burning the background, fifteen or twenty seconds from a dark site, as little as five under streetlights. Interval as short as the app permits. RAW, always.

Total running time decides whether you have a photograph or a test shot. Under half an hour the trails stay timid little dashes. An hour is the minimum for an image that stands on its own, and two hours around Polaris close the arcs enough to read as circles. That is also two hours of a phone shooting without a break: bring a power bank, check your free storage while you are still at home, and switch on airplane mode, because one notification halfway through costs you four frames.

Focus and white balance both need locking by hand, and this is where most nights get ruined. If the automatics step in at frame one hundred and twenty, the colour of the sky jumps, and lighten blending averages nothing away: that jump arrives intact in the final image, as a hard-edged band.

Then there is an advantage of the sequence that no single exposure will ever have. A plane crosses the field and ruins one frame out of three hundred, and that frame gets deleted from the folder before blending: the trail loses twenty seconds out of an hour, and nobody will ever notice. In a single exposure the same plane takes the whole night with it. The same goes for a Starlink train, for the head torch of whoever turns up late at the car park, for a cloud passing through.

The foreground is half the job

A sky full of concentric circles, on its own, stays an exercise in applied geometry. What makes a trail photograph readable is the motionless thing in front of it: a lone tree, a bell tower, the profile of a ridge, the silhouette of a ruin. You need something that declares stillness, so the rotating sky finally has something to rotate against.

This is where the Moon changes jobs. For the Milky Way it is a sentence. For trails it becomes an ally, lighting the foreground with a cold even light that no torch can imitate, and taking away only the faintest stars, the ones that would have left no track anyway. Suburban skies hold up too: trails are made of bright stars and they survive a background that would kill any nebula. It is one of the few outings where the weather and the lunar calendar almost always let you go.

Every other photograph of the sky shows you something distant and still. This one shows you something close that moves, namely the ground under the tripod.