Kostenloser Exposure Time for Stars Rechner
Berechnen Sie maximum exposure time before star trailing mit the NPF rule. Kostenloser astrophotography Rechner.
Max Exposure Time
1.26 s
Max Exposure Time vs Focal Length
Formel
## Maximum Exposure Time for Point Stars The NPF rule estimates how long you can expose before Earth's rotation causes star trailing. ### NPF Formula **t = (35N + 30p) / (f * cos(dec))** - *N* = lens f-number - *p* = pixel size in micrometres - *f* = focal length in mm - *dec* = declination of the target Stars near the celestial equator trail fastest; those near the poles trail slowest.
Lösungsbeispiel
200 mm f/4 lens, 3.75 um pixels, shooting the celestial equator.
- 01t = (35*4 + 30*3.75) / (200 * cos(0))
- 02t = (140 + 112.5) / 200
- 03t = 252.5 / 200
- 04t = 1.26 seconds
Häufig Gestellte Fragen
How does the 500 rule compare?
The classic 500 rule (t = 500/f) is simpler but less accurate, especially with modern high-resolution sensors where trailing is visible at shorter exposures.
Does a tracking mount eliminate this limit?
Yes. An equatorial tracking mount follows the sky, allowing exposures of many minutes without trailing.
Why does declination matter?
Stars near the celestial poles move in smaller circles, so they trail less per unit time. The cosine factor accounts for this.