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.
Related terms
- 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.
- Calibration
The step that corrects the light frames with darks, flats and bias frames before stacking. It removes hot pixels, vignetting and dust shadows. Done well, it makes processing the image much easier.
- Lucky imaging
The technique behind planetary and lunar photos: record thousands of very short frames, then software keeps only the sharpest, taken in the moments when the air is calm. Often 5 to 20% of the frames are stacked. It goes well beyond what the seeing lets you see at the eyepiece.
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