Any unwanted signal in astrophotography (signal = light, radiation) is called noise. There are many types of noise depending on its source; shot / photon noise, read noise, thermal noise, bias noise and others.
One of the most important of these noise types is shot / photon noise. This type of noise is due to the particle nature of the light and the randomness of the photons hitting the sensor. Although shot / photon noise is the most common and inevitable type of noise, astrophotographers have two very important weapons to mitigate their effects, LONGER EXPOSURES and STACKING.
Every time we take a photo, our sensor is exposed to the “signal”, which is the light we want, and the “noise”, which is the unwanted signal, and the effects of both are observed in the photo. Noise is seen in the photograph as graininess. It has a sandpaper appearance. On the other hand, the signal appears as the details of the object photographed in areas exposed to more signal / light on the photograph.
One of the most important concepts in astrophotography is "Signal to Noise Ratio" or SNR. Noise is equal to the square root of the signal in this case;
SNR = Signal / √Signal
As can be seen from the above formula, as we extend the long exposures and stack these exposures, that is, increase the signal, the "Signal" will soon overcome "Noise" and our photo will be less noisy and more detailed.
To be able to stack, we have to stack the photos taken one after the other with the same framing and exposure settings. While the signal is in the same place in every frame, the noise is randomly distributed in the photograph. We can visualize this as raindrops falling on the sidewalk on random points. While stacking the photographs taken on top of each other, the pixel values of each point in the photograph are averaged and this average value is assigned to the pixels in the stacked photograph. This process is called the "average" or "average out" process. When this is done, a softer, noise reduced, and signal preserved final photo will be obtained.
As mentioned above, the main purpose of stacking is to increase the SNR, that is, the Signal Noise Ratio. In other words, if we compare a single photo with 9 stacked photos; In the photo where 9 photos are stacked, we will have 3 times less noise or 3 times more SNR, which is the square root of 9.
5 times less noise or 5 times more SNR etc. when 25 photos are stacked.
In addition to this advantage of the stacking method that provides noise reduction, there is of course also a disadvantage. The photo obtained after stacking will be a soft photo. Therefore, sharpening and some improvement processes should be applied after stacking.
We recommend that you attend our “Milky Way Astrophotography Stacking and Post-Processing” Course to be held on March 27, 2021 to learn about all types of noise, stacking methods, Sequator and Starry Landscape Stacker softwares and post-stacking astrophotography processing.
Below you can see some examples of a single frame shot and multiple frames stacked.
ISO 8000 | 10-sec exposure ISO 8000 | 28 frames x 10-sec
ISO 32000 | 30-sec exposure ISO 32000 | 30 frames x 30-sec