The angle of sky covered by one pixel, in arcseconds per pixel: 206.265 × pixel size (µm) ÷ focal length (mm). 3.76 µm pixels behind a 500 mm focal length give 1.55″/px. For deep sky, aim for about a third to a half of the seeing (often 1 to 2″/px).
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Related terms
- Arcseconds per pixel
The unit of sampling. Too large (undersampling) and stars turn square and detail is lost; too small (oversampling) and light is spread over too many pixels, raising noise. For planets, aim for a focal ratio of about five times the pixel size in µm.
- Sensor
The electronic chip in a camera or astronomy camera, made of millions of pixels that turn light into a signal. Its key figures are the pixel size (in micrometres), the sensor dimensions and its noise. Astronomy cameras are often cooled to reduce thermal noise.
- Seeing
The restlessness of the atmosphere that makes images dance and boil; it is also what makes stars twinkle. It is measured in arcseconds (the size of a star in a photo) or on scales from 1 to 10 (Pickering) or I to V (Antoniadi). It limits high magnification far more often than the instrument does.
- Focal length
The distance between the main lens (or mirror) and the point where it brings a star to focus, in millimetres. A long focal length gives a larger image and a narrower field. A 1,200 mm telescope magnifies 60 times with a 20 mm eyepiece.
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