Star trails from a phone or camera, without uploading anything
Point a phone or camera at the night sky from one spot and take a series of photos: thirty seconds apart for half an hour, or all night. Choose them here and the stars' paths across the sky are drawn as continuous arcs, the way a single hours-long exposure would record them, but without the glow and noise a long exposure collects. The gaps between your photos are filled from the measured turning of the sky, so the arcs are unbroken. The same photos also give one stacked sky: the stars aligned on each other, the noise averaged down, and trees, hills or buildings kept where they were.
It works by plate-solving one of the photos (finding exactly where it points and how the lens draws the sky, the same engine as Plate Solver), fitting each photo's rotation about the celestial pole on hundreds of stars, and separating sky from ground automatically. Everything runs on your own device.
How to shoot
- Prop the phone up and don't touch it: a tripod, a wall, a rolled-up jacket. It must not move between photos.
- Use an interval app (or your camera's built-in intervalometer) to take a photo every 10–60 seconds, each as long an exposure as the phone allows. Night mode is fine. Turn the flash and HDR off.
- Take as many as you can; ten or more is where it gets good. Two photos already give a result. From four, passing planes, satellites and people are rejected (that needs four samples of each pixel); from a handful, the sky and the ground are told apart more surely and the noise drops with the square root of the count. For trails it's the time span that counts: thirty minutes gives short arcs; two hours gives the classic circles around the pole.
- Keep the originals: the capture time in each photo's EXIF data is what orders them and fills the gaps. Screenshots and edited copies lose it.
What you get
- Star trails: each pixel keeps its brightest moment above the static sky background, so hot pixels and lit ground don't streak. The gaps between photos are filled along the measured rotation.
- Stacked sky: the photos aligned on the stars and averaged with outliers rejected (planes, satellites, meteors), with the foreground stacked in place.
- Natural or Vivid: two finishes. Vivid adds Milky Way contrast, star reduction and saturation.
- Labels: the constellations and named stars from the plate solve, switchable on and off.
- Export a JPEG straight to your share sheet on a phone, or download it.
What photos work
- JPEG, HEIC (iPhone) or PNG from the same phone or camera, all the same size, taken from one spot.
- Wide and normal lenses: phone main cameras, 14–50 mm on a full-frame camera, fisheyes.
- Some open sky with sharp stars in at least one photo, so it can be plate-solved.
- On a phone, the work is done at about 3000 px across to fit in memory; on a computer at up to 4096 px. A hundred phone photos take a few minutes.
Private, free, works offline
Your photos are read and processed in your browser and never uploaded. After the first visit the page works offline. Anonymous usage counts are collected; never photos or locations (privacy).
Questions
Why does it need WebGPU?
Aligning and combining a hundred photos is a lot of arithmetic; the graphics chip does it in seconds where the processor alone would take many minutes. Chrome and Edge (Android, Windows, Mac), and Safari 26 and later, have WebGPU.
The camera moved a little. Is that a problem?
The sky is aligned on the stars, so the trails and the stacked sky still work. If the photos don't line up on a fixed tripod, the ground is taken from one photo instead of being averaged, and the page says so.
What if a plane or a car's headlights crossed a photo?
In the stacked sky, outliers are rejected, so a plane's streak disappears. In the star trails, anything that moved across the sky is kept, since that's what trails are; a plane leaves its dashed line.
Can I use RAW files?
Not here: browsers don't decode RAW. The desktop version of the same method (skystack) takes RAW frames and dark frames and writes a 16-bit TIFF.
Why didn't it work?
Usually no photo could be plate-solved: clouds, very few stars, a street-lit sky, or photos that differ in size. Try a set with more open sky, and make sure the photos are the originals.