Combination

Frame combination improves signal to noise ratio and image sharpness, because some random errors occuring in only a few frames and differently in each one, such as wind or movements due to tracking errors, are diminished.
Before the stack itself, we must select the frames to stack and align or register them, so we will have three stages:

• Selection - it is important to select only the best frames before stacking, as moved frames deform star shape and produce sharpness and fine detail loss in the final image. Some programs evaluate frame quality to facilitate this selection.
• Aligning - depending on the software used different frame aligning methods can be used.
• Stacking - the result of the stack can be computed by average, sum, median or sigma clip.

Below you can see an individual frame and the result of the combination of 70 pframes like this one.

Increase in contrast and low noise is evident.
Se nota el aumento de contraste y la baja del nivel de ruido.