Frames Stacking

There are several ways to calculate the result of the stack:

• Average - the most commonly used.
  
• Sum - if the original frames are from a webcam (8 bits per channel) and the result will be saved in FITS or TIFF format (16 bits per channel or more), sum can increase the dynamic range.
  
• Median - recommended when the images are very noisy.
  
• Sigma clip - discards far from the average values, thus eliminating gross infrequent errors but as large movements, passage of satellites, etc.
  

A stacking technique called Drizzle can be used. It can increase the resolution obtaining details even finer than the pixels of the original frames.
This technique involves substantially greater processing times and in my experience I have not found significant differences in results.