Star Trail Calculator – Exposure, Frames and Trail Length

Plan star trails: exposure, frames and how long the streaks will be

This star trail calculator answers the three planning questions behind a star trail photo: how long can one frame be before stars smear, how many frames do you need to stack for a trail of a given length, and how long will the streaks be in pixels. Enter your sensor, lens, aperture and the settings you plan to use.

It also gives the two classic rules of thumb for a sharp night sky, the 500 rule and the NPF rule, so you can pick a frame length for stacking that keeps each individual frame crisp. Everything runs in your browser.

How to use the star trail calculator

1. Choose your sensor size, then type the image width in pixels from your camera’s specifications.
2. Enter the focal length and aperture. Add the declination of the stars you are pointing at if you know it; leave 0° for the longest trails.
3. Set the exposure per frame, the gap your camera needs between frames and the number of frames, or the trail time you want.
4. Read the trail arc, trail length in pixels, frames needed and the 500 rule and NPF exposure limits. Use the button to copy the safer of the two into the exposure box.

Star Trail Calculator features

How fast the sky turns

The Earth makes one turn against the stars in a sidereal day of 23 h 56 min 4 s, so the sky rotates 360° ÷ 23.9345 h = 15.04° an hour, usually rounded to 15°. A star at declination δ draws an arc whose angular length is 15.04° per hour × cos δ: longest at the celestial equator (δ = 0°) and shrinking towards zero at the celestial pole, where stars barely move. Pointing at the pole star therefore gives concentric circles.

To turn that angle into pixels the calculator divides the image width by the lens’s horizontal angle of view. This is accurate for the centre of a normal lens and approximate for wide angles, where projection and distortion stretch the edges. Frames stack by blending the brightest pixels, so a gap of a second or two between frames shows as a thin break in each streak; the gap-in-pixels figure tells you whether it will be visible.

The 500 rule and the NPF rule

The 500 rule says the longest exposure in seconds before stars show as trails is 500 ÷ (focal length × crop factor). It dates from the film era and gives soft stars on modern high-resolution sensors. The NPF rule, devised by Frédéric Michaud, is stricter and also depends on aperture and pixel pitch: (35 × N + 30 × p) ÷ f, with p the pixel pitch in micrometres. Both are rules of thumb for sharp stars, not laws; the declination note above does not change either published rule, but stars near the pole allow longer exposures.

For star trails you do not need pinpoint stars, so frames of 30 s or more are common. The rules matter when you want both sharp stars and trail-free single frames, for example for a Milky Way shot in the same session.