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Go / no go for deep sky

Star trail exposure calculator

How long can you expose before stars turn into short lines? Enter your lens and camera for the 500 rule and the more accurate NPF rule.

// Your camera and lens

Enter the focal length marked on the lens, not the equivalent. If you know your camera’s exact pixel size, type it to override the estimate from the megapixels.

// Longest sharp exposure, no tracker
NPF rule
12 s
Trail about 3.4 px
500 rule
21 s
Trail about 6.1 px
Equivalent focal length
24 mm
Pixel size used
6.00 µm
Star drift (this pointing)
15.0″ per second

Use the shorter number if in doubt. The NPF rule accounts for your pixel size and aperture, so it is the more reliable of the two on modern high-resolution cameras. Trails of a few pixels are normal at these limits and look fine at normal viewing size; for pin-sharp stars when pixel-peeping, use about half the time. Stars drift slower toward the pole, so the allowed time grows.

Questions

What is the 500 rule?
A rule of thumb: divide 500 by your full-frame-equivalent focal length to get the longest exposure, in seconds, before stars visibly trail. With a 24 mm full-frame lens it gives about 21 seconds. It dates from the film era and ignores pixel size, so on high-resolution sensors it tends to allow trails you can see when zoomed in.
What is the NPF rule?
A more careful formula, (35 × aperture + 30 × pixel size in micrometers) ÷ focal length, giving seconds. It accounts for how sharp your sensor and lens can be, so a small, densely packed sensor gets a shorter time than a big one. Many astrophotographers treat it as the better guide.
Why does it matter where I point?
Stars near the celestial equator cross the sky fastest, about 15 arcseconds every second. Closer to the pole they move more slowly, so you can expose longer. The calculator divides the time by the cosine of the declination you choose.
What if I use a star tracker?
Then these limits no longer apply, because the mount cancels the Earth's rotation. Your exposure is limited instead by polar alignment, focal length and the tracker itself. Use the polar alignment tool to set up, then turn on sidereal tracking.

Using a tracker? Polar align with the Polaris clock, or check tonight’s verdict.