contrail

In the top slide, a line of exhaust left by an airplane trails across the sky; in the bottom image, we see the wake left by a boat on the ocean.

Because of some inevitable random signal present in many images (such as cosmic ray artifacts or satellite and airplane trails), the sum and average methods are not my first choice because they include these artifacts in the stacked result, though they produce the highest signal.

If any of these images suffer from bad guiding or satellite and airplane trails, the imager often has little recourse other than to throw away the bad frames, possibly ruining the ability to create a color image.