Sensor Noise: How to Measure the Invisible
You are counting sand grains falling in an hourglass, but each time the wind adds or removes a few. In the most sensitive cameras, the task is similar: you need to figure out how many "sand grains" (electrons) the sensor itself added, and how many came from light. This inherent sensor noise is called readout noise, and it hinders seeing weak signals — for example, the light of a distant star.