Telescopes · Imaging
Astrophotography image scale calculator.
Enter camera pixel size and focal length to see arcseconds per pixel and how much sky the sensor covers.
Inputs
Image scale
Enter pixel size and focal length.
How image scale is worked out
Image scale is how much sky one pixel covers: IS = 206.265 × pixel size (µm) / focal length (mm), in arcseconds per pixel. Binning multiplies the pixel size. Sensor field of view is the image scale times the number of unbinned pixels across, divided by 3,600 to give degrees.
Worked example. A 3.76 µm pixel on an 800 mm telescope: 206.265 × 3.76 / 800 = 0.97 arcseconds per pixel, the same example Astrobackyard gives. With 6,248 × 4,176 pixels the field is 1.68° × 1.12°.
Astrobackyard describes about 1 to 2 arcseconds per pixel as a practical sweet spot, notes undersampling at about 2.5 or more and oversampling at about 0.5 or less in average seeing, and calls these rough guidance rather than strict rules.
Frequently asked questions
How do I calculate image scale in astrophotography?
Multiply the camera pixel size in microns by 206.265 and divide by the telescope focal length in millimetres. The result is arcseconds per pixel.
What image scale is best?
There is no single answer. Astrobackyard suggests a practical sweet spot of 1 to 2 arcseconds per pixel for backyard imaging, as a rough guide.
What is undersampling and oversampling?
Undersampling is a large image scale (about 2.5 arcsec/pixel or more) that loses fine detail; oversampling is a very small one (about 0.5 or less in average seeing) that spreads light over too many pixels.
Does binning change image scale?
Yes. 2x2 binning doubles the effective pixel size and so doubles the arcseconds per pixel.
How do I find my sensor field of view?
Multiply the image scale by the number of pixels across and down, then convert arcseconds to degrees. Enter your pixel counts above.
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Scope
Geometric estimate that ignores lens distortion and focal length errors. Education only.