Astrophotography Calculator

500 Rule: 500 ÷ (focal length × crop factor) ≈ max seconds. Simple but outdated for high‑resolution sensors; often allows trailing.

NPF Rule: Max exposure (s) = k × (35×Aperture + 30×Pixel pitch) ÷ (Focal length × cos(declination)). Uses aperture, pixel pitch, declination, and your pixel tolerance k for a camera‑aware shutter speed. Prefer NPF for sharp stars on modern bodies.

Declination is the sky’s version of latitude. Just as places on Earth have a latitude (0° at the equator, ±90° at the poles), every star or deep-sky object has a declination that says how far north or south it is on the celestial sphere.

Why it matters for exposure: the Earth spins, so stars appear to move across your frame. Stars near declination 0° (the celestial equator) cross the sky fastest, so they blur into trails sooner, you need a shorter max exposure. Stars near the celestial poles (±90°) move slowest, so you can use longer shutter speeds before trailing shows.

You can look up your target’s declination in a star chart, planetarium app (e.g. Stellarium), or on astronomy sites. The Milky Way core is often near 0°, so when shooting the core you’re in the “short exposure” zone. Entering declination here lets the calculator tailor the result to where you’re actually pointing.

Pixel tolerance (the k value in the NPF formula) is how much star movement you’re willing to allow before you consider it “trailing.” Think of it as a strictness dial: at 1.0 the calculator gives you the shortest exposure that keeps stars as tight, round points, the strictest standard. At 2.0 or higher you allow a bit more movement, so you get a longer recommended exposure (and can use lower ISO or capture more light), but when you zoom in on the image you may see stars as slightly oval or soft instead of perfect dots.

Beginners often start at 1.0 for the sharpest stars; if your images are too dark or noisy, try 1.5 or 2.0 and see if the slight softness is acceptable for your use (e.g. web sharing vs large prints). The Compare line under the main result shows the exposure at 1.0 vs 2.0 so you can see the tradeoff at a glance.

Pixel pitch is the physical size of each light‑sensitive “pixel” on your camera sensor, usually given in micrometers (µm). A 6 µm pixel is literally 6 millionths of a meter across. Smaller pixels pack more resolution into the same sensor area but are more sensitive to tiny movements, so for astro, a camera with smaller pixels (e.g. 3–4 µm) will show star trailing sooner than one with larger pixels (e.g. 6–8 µm) at the same focal length and exposure.

The NPF rule uses pixel pitch because it directly affects how far a star’s image moves across a single pixel during the exposure. You can find your camera’s pixel pitch in the manufacturer’s spec sheet or on sites like DxOMark. If you know your sensor width in mm and your horizontal resolution in pixels, you can compute it: Pixel pitch (µm) = (sensor width in mm × 1000) ÷ horizontal resolution. Example: 36 mm width and 6000 px → (36 × 1000) / 6000 = 6 µm. In this calculator, choose Custom and enter sensor width plus horizontal resolution to have pitch computed automatically.