To make a night sky trail image, shoot many short exposures in a row on a tripod, then combine them with stacking software. Each frame records a small piece of the sky’s apparent movement, and stacking joins them into long trails with less noise than a single very long exposure. The numbers below are general guidelines.
Why Stack Instead of One Long Exposure
A single exposure of an hour or more builds up sensor heat and noise, and can be ruined by a passing light. A stack of shorter frames keeps each frame clean, and if one is spoiled you can leave it out.
- Lower noise per frame.
- Easy to remove frames with plane trails or stray light.
- You can also build a time lapse from the same frames.
Step by Step Shooting Process
- Choose a dark location away from city lights, on a clear night with little moonlight.
- Mount the camera on a sturdy tripod and frame the sky with an interesting foreground. Pointing toward the celestial pole gives circular trails; other directions give straighter ones.
- Use a wide lens with a wide aperture, such as f/2.8 or wider, and focus manually on a bright point, then switch autofocus off.
- Set manual mode with an exposure of roughly 20 to 30 seconds, ISO around 1600 to 3200, and RAW format.
- Turn off long exposure noise reduction, because it doubles the time between frames and leaves gaps in the trails.
- Use an intervalometer to take continuous frames for one to three hours. Longer sessions make longer trails.
- Keep the battery fresh and the lens from fogging with a dew shield or small heater band.
Stack the Frames
Import the images into stacking software designed for trails, or into an image editor that can combine layers using a lighten or maximum blend mode. The lighten mode keeps the brightest pixel at each position, so the points of light build into lines.
| Step | What to do |
|---|---|
| Prepare | Apply the same basic edit to every frame |
| Check | Remove frames with planes or flashes |
| Blend | Combine using lighten mode |
| Finish | Adjust contrast, color, and noise |
For a cleaner foreground, use one well-exposed frame, or a blend of a few, for the ground, and the stacked result for the sky.
FAQ
Why are there gaps in my trails?
Gaps usually come from a delay between frames. Turn off long exposure noise reduction and set the interval to the shortest possible time.
How many frames do I need?
A common range is a few dozen to a few hundred. More frames mean longer trails, as long as your battery and card can handle it.
The Short Version
Shoot many 20 to 30 second frames on a tripod in dark skies, avoid gaps between frames, then blend them with a lighten mode. Remove spoiled frames and treat the foreground separately.
