Star Trail Calculator

Why Stars Trail

Earth rotates 360° in approximately 24 hours (15°/hour or 0.0042°/s). During a long exposure, this rotation causes stars to appear as streaks rather than points. The amount of trailing depends on your focal length, sensor resolution, and the star's position in the sky.

The 500 Rule

The simplest method for estimating maximum exposure time to avoid star trails:

• t = 500 / (f × CF)
• t = maximum exposure time in seconds
• f = focal length in mm
• CF = crop factor (1.0 for full frame, 1.5 for APS-C, 2.0 for Micro 4/3)

Variants include the 400 Rule (more conservative, sharper stars) and 600 Rule (more relaxed, suitable for lower-resolution sensors or smaller prints).

The NPF Rule

A more precise formula developed by Frédéric Michaud for the Société Astronomique de France. It accounts for aperture, pixel size, and star declination:

• t = (35 × N + 30 × p) / (f × cos(δ))
• N = aperture f-number
• p = pixel pitch in µm (sensor width / image width × 1000)
• f = focal length in mm
• δ = declination of the target region (−90° to +90°)

Stars near the celestial equator (δ = 0°) move fastest and trail first. Stars near the poles (δ = ±90°) move slowest, allowing longer exposures.

500 Rule vs NPF Rule

• 500 Rule: Quick, easy to remember, good for general use. Tends to overestimate safe exposure time on high-resolution modern sensors.
• NPF Rule: More accurate for modern high-megapixel cameras. Accounts for pixel density, aperture diffraction, and sky position. Recommended when pixel-level sharpness matters.

Creating Star Trails

To intentionally create star trail images, you need total exposure times measured in minutes or hours. Two approaches:

• Single long exposure: Set bulb mode and expose for 15–60+ minutes. Risk of noise, battery drain, and light pollution overexposure.
• Image stacking: Take many shorter exposures (e.g., 30 s each with an intervalometer) and combine them in software. This is the preferred modern method as it reduces noise and allows removing frames affected by aircraft or clouds.

Practical Tips

• Sharp stars: Use the widest aperture available (f/1.4 – f/2.8) and the highest ISO you can tolerate (1600 – 6400) to keep exposure time under the calculated limit.
• Pointing north/south: Stars near Polaris (north) or Sigma Octantis (south) trail less. Point toward the celestial equator for the longest trails.
• Focal length matters: Wider lenses (14 – 24 mm) allow much longer exposures before trailing becomes visible.
• Stacking interval: When stacking, minimize gaps between exposures (1–2 s) to avoid broken trails.

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