CCD Pixel Scale Calculator for Astrophotography
Calculate pixel scale in arcsec/pixel from camera pixel size and telescope focal length.
Includes oversampling check and field of view per frame.
Pixel scale tells you how much sky each pixel of your astrocamera covers. It is the single most important number in astrophotography setup, and it is the one beginners almost universally get wrong by buying a camera that is mismatched to their telescope.
The formula:
pixel_scale_arcsec_per_pixel = (pixel_size_microns × 206.265) / focal_length_mm
A ZWO ASI2600 with 3.76 micron pixels behind a 540 mm refractor: pixel scale = (3.76 × 206.265) / 540 = 1.44 arcsec per pixel. Same camera behind a 2000 mm SCT: 0.39 arcsec per pixel.
The Nyquist rule sets the sweet spot. To resolve a feature, you want about two pixels per resolved arcsecond. So for a typical backyard with 2-3 arcsec seeing, your pixel scale should be 1.0 to 1.5 arcsec per pixel. Below 0.5 arcsec per pixel you are oversampling — the seeing smears every star across many pixels and you gain nothing in resolution while losing signal-to-noise per pixel. Above 3 arcsec per pixel you are undersampling — stars become tiny squares and the image looks pixelated.
This is why a 2000 mm SCT (great for visual planetary work) is the wrong scope for galaxies with most modern small-pixel cameras — you end up at 0.4 arcsec per pixel, badly oversampled, and your exposures need to be 4× longer to get the same per-pixel SNR. The same camera at 540 mm hits the sweet spot for deep-sky work.
Field of view per frame is the other half of camera-scope matching:
FOV_arcmin = (sensor_size_mm × 3438) / focal_length_mm
A 26 mm wide sensor at 540 mm focal length: FOV = (26 × 3438) / 540 = 165 arcmin = 2.76°. That comfortably frames the Andromeda galaxy. Same sensor at 2000 mm gives only 0.74° — too tight even for M31 in one frame.
Practical tradeoffs to remember. CMOS sensors with smaller pixels (under 3 microns, like the IMX571 in some recent ASIs) push the right focal length down to short refractors and astrographs. Older CCDs with 9 micron pixels (like the original ST-10) want long focal lengths to hit the right scale. Bin your camera 2x2 for galaxies on a long focal length and you effectively double the pixel size, fixing the oversampling at the cost of half the resolution.
One detail people miss. The pixel scale calculation assumes diffraction is not the limit. For a perfect optical system, the Airy disk at f/5 is roughly 6.7 microns across at visible wavelengths. If your pixel scale is much finer than the Airy disk, the optics are limiting the image, not the camera. Work the other direction: figure your seeing-limited resolution first, then pick a pixel scale that matches.