恒星曝光时间计算器 — 公式

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.

    计算示例

    200 mm f/4 lens, 3.75 um pixels, shooting the celestial equator.

    1. t = (35*4 + 30*3.75) / (200 * cos(0))
    2. t = (140 + 112.5) / 200
    3. t = 252.5 / 200
    4. t = 1.26 seconds