Full well capacity, also known as saturation capacity, is a measure of the maximum number of electrons that a pixel in an image sensor can hold before it becomes saturated. When the pixel is saturated, it cannot record any additional information, and the resulting image will appear clipped and overexposed.

Full well capacity is an important characteristic of a camera or sensor, particularly in astrophotography where long exposure times and faint signals are often used. A larger full well capacity allows the sensor to capture a wider range of signal levels without saturation, which results in more accurate and detailed images. On the other hand, a smaller full well capacity limits the dynamic range of the camera, making it more difficult to capture both bright and faint objects in the same image.

Full well capacity is typically measured in electrons, and can vary depending on the sensor technology and pixel size. It is important to consider the full well capacity when selecting a camera or sensor for astrophotography or other applications where high dynamic range and low noise are desired.