This is a great suggestion! Thanks.

In fact, based on how Gnuastro's Arithmetic is designed, the weights can be numbers (like your example) or an actual image (giving a weight for each pixel).

This is very easy to implement in Arithmetic. I just don't have the time right now. I hope a volunteer can be found to implement it before I do ;-).

One crude approximation until then would be to simply call an image with more weight, more than once and increase the total number of inputs. For example in the command bellow, I am giving the first image a weight of 4, the second image a weight of 1 and the third image a weight of 2. I am using the `set-' operator to avoid reading each file into memory more than once.

astarithmetic im1.fits set-a im2.fits set-b im3.fits set-c a a a a b c c 7 mean --output=stacked.fits

But while this avoids reading the images into memory more than once, it consumes much more memory than the raw inputs and corresponding weights. So its just a very crude/rough fast implementation, for cases where the weights don't need much accuracy, until this feature is added.

Thanks again ;-).