Teaching · For Teachers · Astronomy

Sharp stars, or
short trails: it is
in the seconds.

Type your lens, camera and where in the sky you are pointing. The calculator gives the longest exposure before the Earth's turning smears the stars, by two rules photographers use.

Find the exposureWhat you get

The star exposure calculator

The worked example is a made-up Milky Way photo with an 18 mm f/2.8 lens on an APS-C camera. Nothing you type leaves this browser.

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What you get

Teaching · For TeachersFree

What you finish with
The longest untracked exposure by the NPF and 500 rules, the equivalent focal length and a table for other lenses. It prints as a summary and downloads as a spreadsheet.
Who it is for
Astronomy and earth science students and teachers, photography students, astronomy clubs and families on a dark-sky trip.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose the sensor size and type the focal length and aperture.
  2. Type the pixel pitch from the camera's specifications.
  3. Type the declination of what you are shooting.
  4. Read the exposure time and set the camera.
  5. Print the table for the field.

Find the exposure

How the exposure is worked out

Two rules of thumb for a turning sky.

The worked example: Milky Way core, APS-C500 rule18.5 s500 ÷ (1.5 × 18)NPF top21535 × 2.8 + 30 × 3.9÷ 18 mm11.94 sat the equator÷ cos 29°13.7 sfor the Milky Way coreThe worked example: Milky Way core, APS-C500 rule18.5 s500 ÷ (1.5 × 18)NPF top21535 × 2.8 + 30 × 3.9÷ 18 mm11.94 sat the equator÷ cos 29°13.7 sfor the Milky Way core
The sky turns 15 degrees an hour. Longer lenses magnify that motion, smaller pixels record it sooner, and a wide aperture blurs stars enough to hide a little of it.

The rules

The 500 rule divides 500 by the full-frame equivalent focal length; it dates from film and lower-resolution screens. The NPF rule (35 × f-number + 30 × pixel pitch in micrometres) ÷ focal length, from the Société Astronomique de France, accounts for today's small pixels and for the slight blur a wide aperture adds. Dividing by the cosine of the declination lengthens exposures for stars that circle close to the pole.

Why stars trail

The Earth turns about 15 degrees an hour, so stars drift across the frame. A longer lens magnifies that drift; a smaller pixel records it as a streak sooner. Beyond these limits, stack several short exposures or use a tracking mount that turns with the sky.

Sources

  1. Société Astronomique de France. The NPF rule for the maximum exposure time without star trails (saf-astronomie.fr).
  2. NASA. Earth's rotation and the apparent motion of the stars (science.nasa.gov).
  3. National Park Service. Night skies: photography tips and dark-sky parks (nps.gov/subjects/nightskies).
  4. OpenStax. Astronomy 2e, section 2.1: the sky above, celestial sphere and declination (openstax.org).

The worked example is made up. These are rules of thumb; check the first frame at full size. Your settings stay in this browser. Support: team@caveatacademic.com.