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.
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.
- Exposure
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.
- Dark frame
An exposure taken with the shutter closed or the tube capped, with the same duration, gain and temperature as the lights. It records thermal signal and hot pixels, which software then subtracts. Usually 20 to 50 are taken.
← The whole glossary