How long the sensor collects light for one frame. For deep sky, take many exposures of 30 s to a few minutes and stack them; for planets, exposures of a few milliseconds freeze the seeing. Without tracking, exposure is limited by the NPF rule.
Related terms
- Stacking
Adding or averaging many images of the same object, aligned on the stars, to reduce noise. Stacking 16 frames improves the signal-to-noise ratio by a factor of 4. Software such as Siril or DeepSkyStacker does the job.
- NPF rule
A formula by Frédéric Michaud (Société astronomique de France) for the longest exposure without star trails on a fixed tripod: t (s) = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel size in µm and f the focal length in mm. At f/2.8 with 4 µm pixels and a 24 mm lens it gives about 9 s. It is more reliable than the old 500 rule with today’s sensors.
- Gain (ISO)
The amplification of the sensor’s signal, set as ISO on a camera. Raising the gain does not capture more light: it makes the image brighter but reduces dynamic range. A medium to high gain lowers read noise, which helps with short exposures and planetary work.
- Signal-to-noise ratio
The strength of the useful signal (the object’s light) compared with the random fluctuations that make the image grainy. It is the measure of an astrophoto’s quality. It grows as the square root of total exposure time: four times as many frames give an image half as noisy.
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