A star trail photograph records the apparent motion of the stars as the Earth rotates, turning points of light into arcs across the sky. Point the camera near the celestial pole, Polaris in the northern hemisphere, and the stars sweep in concentric circles; aim elsewhere and they streak in gentle curves. The technique belongs to astrophotography and rewards patience more than expensive gear.
You need a sturdy tripod, a wide and reasonably fast lens, a fully charged battery or two, and an intervalometer. A dark site away from city light pollution makes the difference between faint trails and a dramatic sky, so check a light pollution map and drive out if you can.
Stacking beats one long exposure
There are two approaches. The old way was a single exposure lasting an hour or more, but on a digital sensor that builds heat, noise, and the risk of one passing car or plane ruining the whole frame. The modern method is to shoot a long sequence of shorter frames back to back and stack them in software. An intervalometer fires the shutter continuously, and free programs such as StarStaX blend the frames into a single image of unbroken trails.
Settings for the individual frames
Shoot each frame at around 30 seconds, ISO 400 to 800, and the widest aperture that stays sharp, often f/2.8 to f/4. Set the intervalometer to fire repeatedly with a gap of one second or less so the trails do not show breaks. Two to three hours of frames produces long, sweeping arcs. Switch the lens to manual focus and set it precisely to infinity on a bright star using magnified live view, then leave it alone.
Include a strong foreground such as a tree, a building, or a mountain so the trails have something to anchor them, and compose with the pole star placed off-center for circular trails. Shoot in RAW for the cleanest stack, turn off long exposure noise reduction (it would double your interval and leave gaps), and consider shooting a few extra frames with the lens capped at the end to use as dark frames for noise removal.
Common mistakes to avoid
- Leaving gaps between frames, which break the trails into dashes. Keep the interval tight.
- Letting the battery die mid-sequence. Carry spares or use a battery grip, and watch for dew forming on the lens.
- Focusing with autofocus in the dark. Set infinity manually and confirm on a star.
- Shooting under a bright moon or near city lights, which washes out faint stars.
Frequently asked questions
How long does it take to get good star trails?
Plan for at least one to two hours of continuous shooting. Longer sequences give longer, more dramatic trails, and circular trails around the pole need a good span of time to form full arcs.
Single long exposure or stacked frames?
Stack short frames. It keeps noise low, lets you discard any frame ruined by a plane or car, and avoids overexposing the sky. A single hour-long exposure is rarely worth the risk on a digital camera.
Which direction should I point the camera?
Toward Polaris in the northern hemisphere for circular trails around the pole, or toward the celestial pole in the south. Pointing east or west gives long diagonal streaks instead of circles.
Stacking the frames into trails
Once you have a few hundred frames, free software does the blending. StarStaX and Sequator load the sequence and merge it using a lighten blend, which keeps the brightest pixel at each point so the moving stars accumulate into continuous trails. A gap-filling or comet mode smooths any small breaks between frames and can taper the trails like comets. Photoshop can do the same by loading the frames as layers set to the lighten blend mode. Blend in a single clean frame for the foreground so it stays sharp rather than smeared, and use any lens-capped dark frames you shot to subtract sensor noise.