Starlapse

Photograph the night sky properly, on a phone that was never meant to.

Starlapse locks the camera and keeps it locked. ISO, shutter, focus, white balance — set them once and they stay there for the whole session. No creeping exposure, no autofocus hunting in the dark and landing on nothing, no auto white balance turning the sky orange under a streetlight.

Then it does the thing the hardware cannot do alone. An iPhone sensor caps a single frame at about one second, so exposures longer than that are built by stacking hundreds of frames. That is not a workaround — it is better. Noise falls as the square root of the frame count, and the same night's light can be developed as pinpoint stars or as star trails afterwards, because nothing is thrown away.

WHAT IT DOES

• Manual camera that holds — ISO, frame exposure, focus, white balance, lens choice • Autofocus on starlight — steps the lens and measures star size, the way observatories focus • Self-timer, 3 or 10 seconds, so touching the phone does not shake the first frames • Stacking from 10 seconds to a full hour of collected light • Pinpoint stars — frames aligned on the stars themselves, so they stay points • Star trails — the sky's rotation drawn as arcs, ground kept sharp • Night time-lapse where every frame of the video is its own stack • Meteor detector — watches the sky and saves a short clip around anything that crosses it • Aiming overlay showing planets, bright stars, meteor radiants and the Moon

WHERE TO POINT

Most apps show you where a shower's radiant is. Starlapse tells you where to aim, which is not the same place: meteors streak away from the radiant, so the longest trails appear 40° off to the side. It works that out from your location and the time, keeps you clear of the Moon, and picks a height a tripod can actually hold.

FOCUS, WHICH IS THE HARD PART

Autofocus cannot see a night sky — there is no contrast to hunt on — and the infinity mark is not exactly infinity: it is a mechanical end stop, and the sharp point sits slightly inside it, at a place that differs between lenses and drifts as the phone cools. Starlapse steps the lens across its range, measures how large the stars come out at each position, and settles on the smallest. It takes about a minute and it is the difference between a night's work and a night of soft dots.

DEVELOP AFTERWARDS

The stack is kept as accumulated light, not as a finished picture. When the exposure ends you can move the curve — stretch, black point, exposure, saturation — and it re-develops from the original data. Same latitude a RAW file gives you, which matters when you are judging a night sky on a phone screen in the dark.

The curve is asinh, the one professional astronomical imaging uses. It lifts the faint Milky Way out of the noise while bright stars keep their cores and their colour instead of clipping to white discs.

HONEST LIMITS

• Aperture is fixed on iPhone. It is a property of the lens, not a setting. The app shows the f-number and what each lens gives up, and lets you choose — that is all any app can do here. • A single frame is about one second. Everything longer is stacked, and the interface always shows the arithmetic: "10 min of light = 600 × 1.00s". • Capture cannot run in the background. iOS stops the camera when an app leaves the screen. Starlapse keeps the screen on and dims it instead. • You need a tripod. An hour of stacking on a handheld phone is an hour of nothing.

PRIVACY

No account, no server, no analytics, and no network code at all. Location and motion are used on the device to work out where to aim, and never leave it. The source is public — github.com/fortunto2/starlapse — so that claim is checkable rather than merely asserted.