Star Trails

What it does

Finds the longest exposure you can use on a fixed tripod before stars start to look like short streaks instead of points. It offers two methods on one screen: the 500 Rule and the NPF Rule.

What you need

Nothing beyond your lens and camera details. No location, network, or sensor. It works offline, and your entries are remembered the next time you open it.

How to use it

500 Rule (quick estimate):

1. Enter Focal Length (mm).
2. Choose your Crop Factor from the list, from Micro Four Thirds (2.0×) to Medium Format: Fuji GFX (0.79×).

NPF Rule (more precise):

1. Choose NPF Mode: Simplified (no declination) or Complete (with declination).
2. Enter Aperture (f-number), Pixel Pitch (microns), and Focal Length (mm).
3. In Complete mode, also enter Declination (°) (0 to 90) and Tolerance Constant (K).

Reading the results

Each rule has its own Max Exposure Time card, in seconds, with the numbers used shown underneath. Use the copy icon to copy the result. Use the reset icon to clear every field on the screen (it asks first). The Crop Factor and NPF Mode choices are kept.

Which to use:

• 500 Rule: a fast rule of thumb when you just need a starting point. It accounts for sensor size through the crop factor, but ignores aperture and pixel size.
• NPF Rule, Simplified: better when you know your aperture and pixel pitch. Use it for a sharper result than the 500 Rule.
• NPF Rule, Complete: adds the declination of the sky area you are pointing at and a tolerance setting for how much trailing you accept. Stars near the celestial pole move less, so they allow longer exposures.

Tips

• Treat the result as a ceiling. If stars still look soft, shorten the exposure.