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Calcimator

Astrophotography Exposure

Calculate the maximum untracked exposure time to avoid star trails using the NPF rule, 500 rule, and declination correction.

Inputs

mm
μm
°

Results

NPF Rule Exposure

7.7 sec

Declination-Adjusted

7.7 sec

500 Rule Exposure2.5 sec
300 Rule Exposure1.5 sec
Image Scale4.95 arcsec/px
Trail Length at NPF Time23.39 pixels
Sky Rotation0.0322°
Assumed Aperture (f/5)40 mm
How to Use This Calculator
  1. Enter your lens or telescope Focal Length (mm) — wider focal lengths allow longer untracked exposures.
  2. Look up your camera's Pixel Size (µm) in its specifications; typical DSLRs range from 3.7 to 6.5 µm.
  3. Enter the Target Declination (°) of your subject — stars near the celestial pole trail less, so longer exposures are possible.
  4. Use the NPF Rule Exposure (sec) as your maximum untracked shutter time to avoid visible star trails.
  5. Compare the 500 Rule and 300 Rule exposures — the NPF rule is more accurate for modern high-resolution sensors.
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Formula

t = (35×aperture + 30×pixel_size) / focal_length (NPF Rule)

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