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Plan

  • Tonight
  • Night Outlook
    • Weather
  • Targets
    • Saved targets
  • Plan a night
  • Night plans
    • Logbook

Tools

  • Sky atlas
  • Field of View
  • Exposure Lab
  • Calculators
  • Horizon Studio
  • Sequence Studio
    • Nights
    • Projects

Setup

  • Settings
  • About & Support
  • What’s new

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Sky & timeNew

Tripod trailing calculator (NPF rule)

How long before stars trail on a tripod?

All 16 calculators
Polar alignmentTripod trailingSky darknessPlanetary imaging

Lens

mm
f/
°

Camera

µm
mm
mm

Telescope, camera and sky are shared by every calculator in this visit and kept in the link.

The 500 rule trails here — use NPFDrawn to scale

Longest sharp exposure (NPF)

8.8s

Expose up to 8.8 s for pin-point stars. The 500 rule would allow 13.6 s and smear stars over 6.3 pixels.

500 rule says
13.6 s
Trail at the 500 rule
6.3 px
Crop factor
1.53×

With your numbers

t = (35·N + 30·p) / f ÷ cos δ (simplified NPF, F. Michaud)

= (35·2.8 + 30·3.76) / 24 ÷ cos 0° = 8.8 s

500 / (f · 1.53) = 13.6 s

Simplified NPF rule (F. Michaud); stars near the pole move slower (÷ cos δ). Use it as a starting point and check your first frame.