// Explore astrophotography
Every sub is noisy. Stacking averages the noise away.
Add more exposures of the Trifid Nebula and watch faint detail come through. The nebula is the same in every sub, but the noise is different each time.
One pixel, sub by sub
Each dot is this spot in a single sub. The running average settles on the true brightness.
More subs, slower gains
Signal-to-noise grows with the square root of the subs, so doubling it takes four times as many.
Signal repeats, noise doesn’t. Light from the nebula lands on the same pixel in every sub. Noise pushes that pixel up or down at random, so averaging keeps the nebula and shrinks the noise.
Why the gains slow down. 4 subs are twice as clean as one, 16 are 4× as clean, and 64 are 8× as clean. Your first few hours change the image the most, and later hours mostly help the faintest detail.
What stacking can’t fix. Anything that repeats in every sub stays in the stack, including poor focus, trailed stars, and gradients. Brighter skies add noise, so they take many more subs to reach the same result.
This is a simulation. Shot noise, sky glow, and read noise are calculated for each pixel and added to my finished image of M20, then averaged as if each sub were taken separately. Real stacking also calibrates and aligns the subs and rejects outliers such as satellite trails.