Astrophotography Exposure Calculator

Every untracked exposure is a race against Earth rotation. The planet turns once per sidereal day of 23 hours 56 minutes 4 seconds, so a star on the celestial equator drifts at 15.041 arcseconds per second of time, and that rate falls as the cosine of declination - about 0.75 at 41 degrees, 0.5 at 60 degrees, and near zero beside Polaris. How much of that drift lands on the sensor depends on image scale, which is 206.265 times pixel pitch in microns divided by effective focal length in millimeters. The 500 Rule and 300 Rule collapse all of this into a single division by effective focal length, which is focal length times crop factor: 1.0 for full frame, 1.5 or 1.6 for APS-C, 2.0 for Micro Four Thirds. The NPF approach instead takes pixel pitch and declination as explicit inputs, which is why it usually returns 30 to 50 percent shorter times. Concrete figures make the spread clear: a 24mm lens on full frame gets about 21 seconds under the 500 Rule and 12.5 under the 300 Rule, while a 200mm lens gets 2.5 and 1.5 seconds. Typical consumer pixel pitches run 3.7 to 6.5 microns. All three rules are heuristics, not physics - each encodes an unstated tolerance for how many pixels of trailing look acceptable. Atmospheric seeing of 1.5 to 3 arcseconds, focus error, mount vibration and wind commonly dominate the result, so treat any computed value as a ceiling to be verified by inspecting corner stars at 100 percent zoom.

Long-exposure astronomical photography began with the daguerreotype: John Adams Whipple and William Cranch Bond captured Vega through the Harvard 15-inch refractor in 1850, and Henry Draper recorded the Orion Nebula in 1880 using a 51-minute exposure on a dry plate. Those pioneers had already identified the core problem this calculator addresses, and their solution was mechanical - clock-driven equatorial mounts that rotate the instrument westward at the sidereal rate. E. E. Barnard extended the approach to wide-field work in the 1890s, producing the Milky Way survey plates that first showed dark nebulae as obscuring dust rather than empty voids. The divide-by-a-constant shortcuts are a much later, distinctly amateur invention. Guidelines of the rule of 600 and rule of 500 type circulated in 35mm film-era handbooks, where the grain of ISO 400 emulsion hid modest trailing and the tolerance was generous by necessity. Digital sensors of 20 to 60 megapixels, routinely inspected at full resolution on screen, exposed how lenient those numbers were, prompting wide adoption of the stricter 300 Rule. The NPF Rule - named for its inputs of aperture N, pixel pitch p and focal length f - is commonly credited to Frederic Michaud of the Societe Astronomique du Havre, and it spread through French astrophotography forums before being built into planning apps such as PhotoPills, which brought pixel-aware exposure limits to a mainstream audience.