Image Combining using Integer & Real Pixels
A common procedure used to increase the signal to noise ratio ("S/N") of images is to combine them in software by mathematically merging the pixel values of the individual images. Image combining is sometimes termed "stacking". Increasing the signal to noise ratio improves the detection of weak features and shows fainter objects, and it improves the significance of any measured quantity like magnitude, surface brightness, edge sharpness, centroid position, etc. Image combining can also remove deviant pixels that do not differ from one image to another simply because of random noise. A deviant pixel might also be caused by a cosmic ray hit or radioactive decay, as CCD's are extremely sensitive to such radiation events.
Various methods are available for combining images to blend the goals of increasing the signal to noise ratio and removing non-repeating artifacts. In the limiting case of best performance, combining can improve the S/N by the square root of the number of images combined. For example, combining 16 images of the same field of view can increase the S/N as much as 4 times. Removing artifacts such as cosmic rays reduces the theoretical gain to somewhat less than √n.