Star Trail Calculator
Calculate exposure time for star trail photography
Star Trail Settings
Recommended Settings
Shooting Star Trails — Field Tips
Trails need long or stacked exposures — not single-frame limits
The 500 rule (used by the NPF calculator on another page) exists to prevent star movement as points. For trails, you want that movement — the longer the exposure, the longer the arc. A single uninterrupted exposure over 20-30 minutes accumulates color noise aggressively; stacking 30-second frames with 1-2 second gaps gives the same arc length with dramatically lower noise, and you can discard frames where a plane crosses the field.
Polar alignment determines trail shape
Pointing toward Polaris (Northern Hemisphere) or Sigma Octantis (Southern) places the celestial pole near center frame and produces tight, circular concentric arcs. Point 90 degrees away from the pole and trails become straight horizontal streaks — useful for a different aesthetic but not the classic circular look. The calculator gives correct exposure duration, but framing relative to the pole is the compositional decision that shapes the image.
Light pollution destroys the gradient
Sky glow from a city 30-40 km away brightens the horizon enough that a 2-hour stack blows out the lower sky even at f/4 and ISO 400. In moderate light pollution, start with ISO 800 and 30-second frames so individual frames look dark — the stack brightens them. Check your histogram on the first few frames: if the peak sits at 30-40% from the right, you have roughly 3-4 hours before the sky clips.
Frequently Asked Questions
How long do I need to expose for star trails of a specific arc length?
Stars move 15 degrees per hour due to Earth’s rotation. To calculate exposure time, divide your desired arc in degrees by 15. For a 45-degree arc you need 3 hours of total exposure. The calculator converts this to either a single long exposure time or the number of 30-second frames needed for a stacked sequence.
Should I use a single long exposure or stack multiple shorter frames for star trails?
Stacking multiple 30-second frames is almost always better. A single 3-hour exposure accumulates noise, risks satellite and airplane streaks that are impossible to remove, and drains the camera battery. With a stacked sequence, you can remove interference frames, apply noise reduction per frame, and combine cleanly. Most star trail software (StarStax, Sequator) automates the blending.
Where should I point my camera to get circular star trails?
Point near Polaris in the Northern Hemisphere or Sigma Octantis in the Southern Hemisphere. These polar stars barely move, becoming the pivot point of perfect concentric circles. Pointing at the celestial equator produces straight horizontal trails, which are visually different but require shorter exposures to show clear motion.
How does focal length affect the appearance of star trails?
Wider lenses show more sky and produce shorter apparent trail lengths per degree of arc in the frame, making circular patterns more visible. Telephoto lenses stretch the apparent movement, making even short trails look dramatic, but capture less of the circular pattern. A 14 to 24mm lens on full frame is a popular choice because it fits Polaris and the surrounding circle comfortably.
What is the most common mistake that ruins star trail shoots?
Leaving gaps between frames is the biggest mistake when stacking. Even a 1-2 second interval between shots creates small dark breaks in each trail, making the final composite look fragmented rather than smooth. Set the interval to the minimum your intervalometer allows, typically 0.5 to 1 second, to ensure the trails are continuous in the blended result.