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The "readout noise "The Flat Field

The readout noise and readout noise is expressed in terms of electrons per pixel introduced into the final signal that occurred after the reading of the CCD. This is the first major source of "noise" with which we will inevitably coexist because generated by the same electronic components of the CCD camera.

Being a noise, can not have a precise value: mathematicians use to describe a particular function: the Gaussian curve or Gaussian :

Certainly at first sight, such a formula can be frightening, but " traduciamola "in a chart:

The chart above shows an example three Gaussians with the typical bell shape, all centered on the same average value (the Greek letter" mu ") zero, but of different sizes (the Greek letter sigma squared) that " variance. The variance is a measure of dispersion of data and there gives an immediate indication of the amount of "noise" in our data sample: the higher the variance, the wider the bell and noisier is the data in our possession. However, rather than using the variance, usually to indicate the dispersion of a set of data using its square root ("sigma") usually referred to as a standard deviation . All this we have seen in a simple practical application when we have addressed the topic of BIAS FRAME and in particular how to obtain the READ FRAME NOISE .

Returning to the formation of the readout noise, we can see that consists of two componenti:

1. la conversione di un segnale analogico in un numero non è mai perfettamente ripetibile: sia gli amplificatori integrati sul sensore che i convertitori analogico-digitali producono una distribuzione statistica di possibili risultati centrati su di un valore medio. Quindi anche nell'ipotetico caso di poter leggere lo stesso pixel due volte con la stessa identica carica, potrebbe produrre due valori leggermente differenti l'uno dall'altro.
2. l'elettronica stessa che compone la camera digitale può introdurre elettroni di disturbo nell'arco dell'intero processo di lettura e conversione portando inevitabilmente a fluttuazioni casuali del risultato finale di lettura.

La media di queste two components of uncertainty is what we call the readout noise. In the commercial
CCD readout noise can vary from 5 to over 20 e-/pixel. Obviously the lower this value the better the CCD camera in question. In particular, the CCD with a high readout noise are not suitable when you need to use the technique of the sum or average of multiple images to increase the signal to noise ratio: the final image will not have the same quality of a single long exposure of the same total time of integration, since each image will bring with them the contribution of the readout noise for each pixel that will contribute to the sum or average.