## How Many Images Should I Stack for Noise Reduction (in Starry Landscape Stacker or Sequator)?

With programs like Starry Landscape Stacker or Sequator image stacking for noise reduction has become very easy to do.

The one question I get is how many images should i stack?

This question can actually be answered analytically with math. If you recall your stats class, the variance of the sum of n variables is the sum of their covariances. Therefore…

Ok just kidding. I won’t go into the math. But if you want to dig further look at the Wikipedia article on variance and assume that the photons have a Poisson distribution.

The way the math works out is that if you have a stack of N photos and average them, the resulting noise is 1/sqrt(N) of the original noise.

In other words, if you stack 4 photos, the noise will be reduced by 50%. Stacking 16 photos reduces noise by 75%. Here’s a table summarizing the noise reduction for a few values:

Looking at the results, there’s clearly diminishing returns. In practice, I usually take anywhere from 10-20 images when stacking. I may not use them all, but I have them available as an option.