What star trails are
Stars appear to move across the sky because Earth rotates. In a long exposure, or in many shorter exposures combined, each star draws a streak. Pointing toward the celestial pole produces circular trails around a central point. Pointing elsewhere produces arcs or straighter lines.
Single exposure or stacking?
You can make trails two ways.
- One very long exposure (often many minutes to hours, using Bulb mode). It is simple, but sensor noise builds up, batteries drain, and a single mistake such as a passing light ruins the whole frame.
- Many shorter exposures stacked in software. This is the method most photographers prefer. Each frame stays low in noise, you can remove a bad frame, and you can reuse the same sequence for a time-lapse.
Gear
- A camera with manual mode and RAW.
- A wide lens, ideally fast. Our best lens for star photography article explains what to look for.
- A sturdy tripod.
- An intervalometer (built into some cameras, or an external remote) to fire frames automatically.
- Fully charged batteries, or an external power option for long sessions.
- Optionally, a lens heater or dew shield, since moisture can ruin a long session.
Choose your composition
To get circular trails, find the celestial pole. In the Northern Hemisphere, Polaris (found with the Big Dipper pointers, see the famous constellations) is close to it, so aiming near Polaris gives circles. In the Southern Hemisphere there is no bright pole star, so use a sky app to locate the south celestial pole. Include a foreground so the picture has a sense of place. Decide before you start, because you cannot change the composition during the sequence.
Camera settings
These are starting points.
- Mode: Manual, with manual focus set on a bright star and then left alone.
- Aperture: Wide open or slightly stopped down, for example f/2.8 to f/4.
- ISO: Often 800 to 3200. A lower ISO can reduce noise, because the trails build up across many frames.
- Shutter speed: Typically 20 to 30 seconds per frame, short enough to avoid visible trailing in each individual frame.
- Interval: Set the intervalometer so the gap between frames is as short as possible (around one second), so the trails join without breaks.
- Noise reduction: Off for each frame, in most cases.
- Total duration: Often 30 minutes to a couple of hours. Longer sequences give longer trails.
For general settings and focusing technique, see the night sky photography guide.
Shooting the sequence
- Set up on a stable surface and check your composition.
- Focus using live view on a bright star, then switch off autofocus.
- Take a few test shots to check exposure.
- Start the intervalometer, with enough frames to cover your total time.
- Leave the camera alone. Avoid bumping the tripod, and watch for dew.
Stacking the frames
StarStaX is a free tool made for this job. You load the frames, choose a blending mode such as “lighten” (also called maximum), and it combines them into a single image. Some modes add a fading, comet-like effect.
Photoshop can do the same. Load the images as layers and set each layer’s blend mode to Lighten, so the brightest pixels from each frame build up into trails. Many people use a script or the stack Maximum mode to automate it. Menu names vary between versions, so check yours.
Tips for stacking:
- Edit one frame first and apply the same changes to all, so the sequence stays consistent.
- If a plane crosses a frame, you can exclude that frame, though a small gap may appear in the trail.
- A separate foreground frame can be blended in if the foreground is noisy.
Common problems
- Gaps or dotted trails: The interval was too long. Use the shortest interval your camera allows.
- Foggy or fuzzy frames: Dew on the lens. Use a heater or shield.
- Noisy result: Lower the ISO, or shoot in cooler conditions.
- Light pollution glow: Pick a darker site (see camping astronomy) or reduce the gradient in editing.
- Uneven brightness: Keep exposure settings fixed and avoid auto modes.
What to try next
Once you have good results, consider using your frames for a time-lapse, or add a separate sharper sky frame to improve clarity.