Exposure Time for Stars Calculator Formula

Understand the math behind the exposure time for stars calculator. Each variable explained with a worked example.

Use the Exposure Time for Stars Calculator

Formulas Used

Max Exposure Time

max_exposure = (35 * aperture_ratio + 30 * pixel_size) / (focal_length * cos(declination * pi / 180))

Variables

VariableDescriptionDefault
focal_lengthFocal Length(mm)200
pixel_sizePixel Size(um)3.75
aperture_ratiof-Number (N)4
declinationDeclination(deg)0

How It Works

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.

    Worked Example

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

    focal_length = 200pixel_size = 3.75aperture_ratio = 4declination = 0
    1. 01t = (35*4 + 30*3.75) / (200 * cos(0))
    2. 02t = (140 + 112.5) / 200
    3. 03t = 252.5 / 200
    4. 04t = 1.26 seconds

    Ready to run the numbers?

    Open Exposure Time for Stars Calculator