How the Astrophotography Pixel Scale Calculator works
Pixel scale connects your camera pixels with angular sky detail. It is one of the central checks for matching a camera to a telescope and to the seeing at your site. The core relationship used here is Pixel scale ≈ 206.265 × pixel size(µm) ÷ focal length(mm). Sensor field is set by sensor dimensions and focal length. If you are building a complete observing or imaging setup, use the Focal Ratio Calculator alongside this page rather than treating one number in isolation.
Worked example and interpretation
A 3.76 µm pixel on a 750 mm focal-length telescope gives about 1.03 arcseconds per pixel before binning.
For broader physical context, the Astronomy.tools CCD suitability reference is a useful external reference.
When this calculator is useful
Use this calculator to compare cameras, focal reducers and Barlows, estimate sensor field of view, and judge whether a setup is heavily undersampled or oversampled for the seeing you normally experience.
When the result involves angular size, field, observing conditions or a current target, continue with Field of View Calculator. For a different but related perspective, Night Sky Tonight can connect the formula to an actual observing, telescope or Solar System context.
Diagrams
Assumptions, accuracy and practical limits
Seeing changes, guiding error, focus, optical blur and deconvolution all affect final resolution. Nyquist-style sampling guidance is a planning tool, not a rigid pass/fail rule.
A second external cross-check is available from NASA Universe. Not for operational or published work.
Reference table: image scale
| Pixel size | 400 mm | 750 mm | 1500 mm |
|---|---|---|---|
| 2.4 µm | 1.24″/px | 0.66″/px | 0.33″/px |
| 3.76 µm | 1.94″/px | 1.03″/px | 0.52″/px |
| 5.86 µm | 3.02″/px | 1.61″/px | 0.81″/px |
A sampling of roughly one third to one half of your typical seeing FWHM per pixel is a common target, so 1.0–1.5″/px suits nights with 2–3″ seeing.
Related SpaceTracker.live tools
Continue with Focal Ratio Calculator, Field of View Calculator and Night Sky Tonight.
Common mistakes and scale lookup
- Forgetting binning. Binning 2×2 doubles the pixel size, and so doubles the arcseconds per pixel.
- Ignoring seeing. Sampling much finer than a third of the seeing size produces a soft image without extra detail.
- Using the wrong units. The pixel size should be in micrometres and the focal length in millimetres.
| Pixel size | 400 mm | 800 mm | 1,200 mm |
|---|---|---|---|
| 2.4 µm | 1.24″ | 0.62″ | 0.41″ |
| 3.76 µm | 1.94″ | 0.97″ | 0.65″ |
| 4.63 µm | 2.39″ | 1.19″ | 0.80″ |
| 5.86 µm | 3.02″ | 1.51″ | 1.01″ |
More questions answered
What pixel scale is best?
Roughly one-third to one-half of your typical seeing size, so about 0.7–1.0″ per pixel for 2″ seeing.
What happens if my pixel scale is too coarse?
Stars become blocky, which is called undersampling. Dithering and drizzle stacking can recover some detail.
Worked scenarios: matching a camera to a telescope
These examples use the calculator's formula, pixel scale = 206.265 × pixel size (µm) ÷ focal length (mm).
- 3.76 µm pixels at 800 mm: 0.97 arcseconds per pixel. With typical 2″ seeing this is a good match, because it samples the seeing at about two pixels per blur.
- Sensor field: a 6248 × 4176 pixel sensor at that scale covers about 1.68° × 1.12°, enough for the Pleiades (about 2°) only with a mosaic, but plenty for the Orion Nebula.
- Too fine: 2.4 µm pixels at 1,200 mm give 0.41″ per pixel. Unless your seeing is much better than 2″, you are oversampling. Binning 2 × 2 gives 0.83″ per pixel and a stronger signal per pixel.
- Planet check: Jupiter is about 45″ across at its best, which is about 46 pixels at 0.97″ per pixel, enough to record belts and the Great Red Spot.
Pair this with the Focal Ratio Calculator to see how the same optics change exposure time.
Frequently asked questions
What pixel scale should I aim for?
Between one third and one half of your seeing FWHM. In 3″ seeing that is about 1–1.5″/px.
What does binning do to pixel scale?
2×2 binning doubles the effective pixel size and therefore the arcseconds per pixel, trading resolution for sensitivity.
Is a small pixel scale always better?
No. If it is much smaller than the seeing allows you are oversampling and only lowering signal per pixel.