What is my camera’s pixel scale on this telescope?
The pixel scale (also called image scale or plate scale) is how much sky one pixel covers, in arcseconds. It follows from your camera’s pixel size and the telescope’s focal length: scale = 206.265 × pixel size (µm) ÷ focal length (mm). This astrophotography pixel scale calculator gives it, compares it with your seeing to tell you whether you are over- or under-sampling, and shows the field of view your sensor frames.
It handles reducers, Barlows and binning, and works for any camera and telescope combination. Everything runs in your browser.
How to use the Astrophotography Pixel Scale & Sampling Calculator
- Enter the pixel size in micrometres from your camera’s specification.
- Enter the focal length and any reducer or Barlow factor; 1 means none.
- Add the sensor size in pixels to get the field of view, and the seeing FWHM for the sampling verdict.
- Set the binning if you bin on the camera or in software.
- Read the scale, sampling and field, or look at the seeing table.
Astrophotography Pixel Scale & Sampling Calculator features
Image scale
Arcseconds per pixel, with binning applied.
Sampling check
Pixels per seeing FWHM and a verdict from undersampled to oversampled.
Field of view
Width, height and diagonal in arcminutes and degrees.
Best focal length
The focal length that puts 2.5 pixels across your seeing.
Formulas and a worked example
Scale (″/px) = 206.265 × pixel size (µm) ÷ focal length (mm). Field (′) = scale × pixels ÷ 60. Sampling = seeing FWHM ÷ scale.
Example: a 3.76 µm camera on a 600 mm telescope gives 206.265 × 3.76 ÷ 600 = 1.29″/px. With 2.5″ seeing that is 1.93 pixels per FWHM, slightly undersampled. A 6248 × 4176 sensor frames 134.6′ × 90.0′, about 2.2° × 1.5°.
Over- and under-sampling
Sampling theory says to record a star profile properly you need at least two pixels across its full width at half maximum, and 2–3 is the usual target. Fewer and stars become blocky; more and you spread the light over extra pixels, lowering the signal in each for no gain in detail. The fix for oversampling is binning, a shorter focal length or a reducer; for undersampling, a longer focal length or a Barlow, or drizzle stacking. The seeing is the blur of the air on the night, so good guiding and a calm sky matter as much as the numbers. For the finest detail the optics can show, see the telescope resolution calculator.
Limits
- Use the true effective focal length; field flatteners and reducers change it.
- Seeing varies hour to hour; the table shows the range.
- The field of view is for a flat sensor and small angles, accurate for astrophotography frames.
- Pixel size is the physical size of one photosite, not the sensor width; check the maker’s datasheet.
Astrophotography Pixel Scale & Sampling Calculator FAQ
How do you calculate pixel scale for a telescope?
Multiply the pixel size in micrometres by 206.265 and divide by the focal length in millimetres. The answer is in arcseconds per pixel.
What is a good image scale for astrophotography?
Roughly one third to one half of your seeing FWHM, so 0.8–1.2″/px for 2.5″ seeing. Wide fields at 2–4″/px are fine for large nebulae.
What does binning do to image scale?
Binning 2×2 doubles the arcseconds per pixel but the field of view stays the same, and it increases signal per pixel.
What is the Nyquist rule in astrophotography?
Sample at least twice across the smallest feature you want to keep, in practice 2–3 pixels per FWHM.
Does a longer focal length always give better images?
No. It narrows the field and darkens each pixel, so you need longer exposures, tighter guiding and steadier air. Match the scale to your seeing, then choose the focal length that frames your target.
How do I find my seeing FWHM?
Measure it from stars in a recent image (most stacking software reports it) or use a local forecast; 2–3″ is typical.