Star Trail & 500 Rule Calculator

Stars move across the sky as the Earth turns, about 15 degrees per hour. Photographers either fight that motion, keeping stars as sharp points for Milky Way shots, or embrace it, stacking many exposures into long, curving star trails. This calculator covers both.

Sharp stars: the NPF rule

The NPF rule, developed by Frédéric Michaud of the Société Astronomique de France, estimates the longest exposure before stars start to look like short lines. It considers the aperture, the focal length, the size of the sensor’s pixels and the part of the sky you are pointing at. Enter your sensor and its resolution, and the calculator derives the pixel pitch for you.

Why pixel size matters

Small pixels record finer detail, so they also reveal motion sooner. A 24-megapixel full-frame camera has pixels of about 6 micrometres; a 45-megapixel one about 4.4. For a 24 mm lens at f/2.8, NPF gives about 5.4 seconds on the first camera; the old 500 rule would suggest almost 21 seconds, which shows clear trailing on modern sensors.

The 500 and 300 rules

For comparison, the calculator also shows the 500 rule, 500 divided by the focal length times the crop factor, and the stricter 300 rule often suggested for high-resolution sensors. They are quick estimates, fine for small images, but the NPF result is the more reliable choice for large prints.

Declination

Stars near the celestial pole move in small circles and barely trail; stars near the celestial equator, such as Orion’s belt, move fastest. Declination expresses that: 0 degrees is the equator, 90 the pole. Enter the declination of your target to get a longer allowed time when pointing toward the pole.

Planning star trails

For trails, photographers shoot many consecutive exposures and stack them, because a single exposure of hours would be noisy and risky. Enter the total shooting time, the length of each exposure and the gap between them. The calculator returns the number of frames, the gaps that will appear in the trails, the storage needed at about 30 MB per RAW file, and how long the frames last when played as a 25 fps time-lapse.

Keeping the trails continuous

Gaps between frames create dotted trails. Set the camera’s gap to the minimum, often one second, and use an intervalometer. Exposures of 20 to 30 seconds at ISO 400 to 1600 work for most dark-sky locations.

Practical checklist

Charge spare batteries, since cold drains them quickly. Turn off image stabilization on a tripod, and long-exposure noise reduction during stacks, because it doubles each exposure time and creates gaps. Use a lens heater or hand warmer against dew. Focus manually on a bright star using live view magnification.

Finding the celestial pole

In the northern hemisphere, Polaris marks the pole within about a degree; find it by following the two end stars of the Big Dipper’s bowl. In the southern hemisphere there is no bright pole star; the Southern Cross and the two Pointers lead the way to the empty pole.

Related tools

Find the darkest time of night with the Sun Angle Calculator, which shows twilight times. Plan a time-lapse from the same frames with the Timelapse Calculator. Convert focal lengths across sensors with the Crop Factor Calculator.