Astrophotography Pixel Scale & Field of View Calculator
Match a camera to a telescope: pixel scale in arcseconds per pixel from pixel size and focal length (with reducer or Barlow), field of view in degrees and arcminutes, sampling verdict against your seeing (under-, well- or over-sampled), a binning suggestion, and whether the Moon, M31, M42 or the Pleiades fit the frame.
How it works
pixel scale (″/px) = 206.265 × pixel size (µm) ÷ focal length (mm) · FOV (°) = sensor dimension (mm) ÷ focal length × 57.3 · sampling: aim for 2–3.5 pixels across the seeing FWHM · effective focal length = focal × reducer/Barlow factor
A telescope projects the sky onto the sensor at a scale set only by its focal length: one radian of sky becomes one focal length of image, so a pixel a few microns wide covers a small angle that comes out in arcseconds through the 206,265 constant. The atmosphere blurs every star into a disc a couple of arcseconds wide, and information theory says you need two to three pixels across that disc — fewer and the stars become blocky squares, many more and you dilute the light over pixels that record nothing the seeing has not already erased. Field of view is the same projection applied to the whole sensor, which decides whether Andromeda fits or needs a mosaic, and it is why reducers and Barlows, which change the effective focal length, trade field for scale.
Worked example
A 600 mm refractor with a 3.76 µm APS-C camera gives 206.265 × 3.76 ÷ 600 = 1.29″/pixel, a field of 2.24° × 1.49° (134′ × 89′). In 2.5″ seeing that is 1.9 pixels per star FWHM — on the undersampled edge, so drizzle or a longer focal length would help; a 0.8× reducer would widen the field to 2.8° × 1.9° but coarsen the scale to 1.6″/px. M31 at 178′ needs a mosaic; M42 fits easily.
Frequently asked questions
Is oversampling bad?
Not for resolution — you lose nothing — but each pixel gets less light, so exposures need to be longer for the same signal-to-noise. Binning recovers it. Planetary imaging deliberately oversamples (0.1–0.2″/px) with lucky imaging.
What about the Bayer matrix on colour cameras?
A one-shot-colour sensor samples each colour at half the pixel density, so effective sampling is coarser than the nominal scale suggests; aim for the finer end of the range (3+ px per FWHM) with OSC cameras.
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