This concept is very important to wrap your head around, because thereвЂ™s a lot of talk about diminishing returns for the number of images that you stack. You see, the signal-to-noise ratio does not actually go up because of the number of exposures in a graph, but rather because the number of exposures is increasing the signal faster than it's increasing the noise.

The reason I want you to understand this is because I am about to tell you something that appears to contradict all the graphs youвЂ™ve ever seen about stacking. However, the world does not obey вЂњthe math.вЂќ Math serves physics, not the other way around. Math is a tool that can describe how the world works. This is due to a common misconception about math and the physical world: Stacking is often misunderstood, even by some very intelligent and gifted astrophotographers. Essentially, it's as if the camera had taken a single image with way more signal than you would normally obtain in a single, short exposure. Averaging images yields the same signal-to-noise ratio as summing up properly calibrated images, but it also has the advantage of keeping the numerical values that make up the image in the same range as that reported by the camera.

Typically, we do not sum (add together) images instead, we average them.

(Of course, it's slightly more complicated than this вЂ“ you do need to calibrate individual frames, which we'll discuss later). If you add all the exposures together, youвЂ™d have the equivalent of 1 hour of exposure time. Instead of a single 60-minute exposure, we might take twelve 5-minute exposures, or thirty 2-minute exposures. Stacking involves combining many short exposures. We can also get more signal by combining multiple exposures, a procedure called stacking. Not to mention, most cameras will saturate (become all white) if exposed too long, especially from a light-polluted location.Д«ut you don't need long exposures or fancy equipment to get more signal. Airplanes, satellites, and the weather all conspire to ruin a single hours-long exposure. Even with great care and finely tuned gear (as well as computer-controlled autoguiding), there are practical limits that prevent us from taking a single exposure that might last for several hours to get a high enough S/N ratio for a nice clean image. Simply taking longer exposures is tricky, due to equipment constraints, it's difficult for most mounts to keep stars on the same group of pixels for more than a few minutes. We know the noise level grows more slowly with time than signal does, so we can get more signal by exposing our images longer, or with larger, faster optical systems that deliver more light to our imaging sensors. So how do we deal with it? Why Stack ImagesД®xposing too long in a light polluted area can create a foggy image that will eventually turn solid white. Our deep-sky images contain shot noise calibration frames do, too. One of the most significant sources of noise is shot noise, a type of noise inherent in all digital images. Last month we laid the groundwork for further discussions of noise in our astrophotos. The secret to stacking images is increasing signal rather than just increasing the number of exposures.

Stacking software astrophotography