Free Astro Pixel Scale Calculator

Image scale = 206.265 × pixel size (µm) ÷ focal length (mm). The result, in arcseconds per pixel, tells you whether your seeing, tracking, and optics are matched to your sensor.

Your setup

Camera

Focal length (mm)

Pixel size (µm)

Typical seeing (arcsec FWHM)

Result

Pixel scale

—

arcsec / pixel

Seeing sampling

—

pixels per seeing disk

Verdict

—

for sharp round stars

🔭 The 1–2″/px rule: most amateur setups land between 1 and 2 arcseconds per pixel — a good match for typical seeing (2″) and common mounts. Below ~0.5″/px you're oversampling (huge files, no new detail, tracking demands explode); above ~4″/px you're undersampling (blocky stars, wasted resolution).

Why pixel scale matters

Pixel scale determines how much sky each pixel covers. It's the single best number for judging whether a camera + lens/telescope pairing makes sense before you buy:

Undersampled (> 3–4″/px): stars land on single pixels — square, blocky stars that don't improve with more exposure.
Sweet spot (1–2″/px): several pixels across each star image; good for most mounts and skies.
Oversampled (< 0.7″/px): the optics or seeing can't deliver that detail — you collect noise and huge files for nothing, and tracking errors become visible.

Rule of thumb: pixel scale ≈ seeing ÷ 3 gives near-critical sampling. With 2″ seeing that's ~0.67″/px — most hobbyists choose slightly coarser to keep guiding easy.

Pixel scale FAQ

What's the formula for pixel scale?

Pixel scale (″/px) = 206.265 × pixel size (µm) ÷ focal length (mm). Example: a camera with 3.8 µm pixels behind a 400 mm lens gives 206.265 × 3.8 ÷ 400 ≈ 1.96″/px.

Is smaller pixel scale always better?

No. Below the seeing limit you're oversampling — more pixels per star than the atmosphere delivers. The result is larger files, longer exposures for the same signal, and stricter guiding for zero real resolution gain.

How do I find my camera's pixel size?

Pixel size = sensor height (mm) × 1000 ÷ vertical resolution. ShutterMath stores this for every camera in the list automatically — just pick your camera.