FCW or Full Well Capacity or Well Depth

updated: 2021-05-19

Example: ZWO ASI 294

FCW describes how many electrons a pixel is able to collect before it becomes saturated. A pixel with 5.5μm is able to accumulate up to 20,000 electrons, one with a size of 7,4μm up to 40.000 electrons.

The higher the full well capacity is the better is the maximum signal to noise ratio. Consumer cameras with pixel sizes of 1.7μm only require about 1,000 electrons before getting saturated.

Also other strong noise effects like photonic noise, digitizing noise, dark noise come so strong, depending on the number of steps of bits (8, 10 or even 12 steps) during the digitizing, that they can the image quality drastically.

The dynamic of the sensor can also be affected by very small pixel structures. The detection capability of the sensor is limited by dark noise induced by thermal effects. The sensor requires considerably more light generated electrons than those generated by dark noise to return meaningful results.

The pixel dynamic is the ration between the full well capacity and  dark noise.


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