Now that we know the meaning of gain and readout noise to a CCD camera, we see how we can determine the value if these quantities were not reported in the literature that accompanies our device.
There is a very simple and quick to make, though not very accurate, however we will give a reliable estimate of these two quantities.
Once installed on the CCD camera telescope, simply take two flat field, [F1] and [F2] and two BIAS FRAME [B1] and [B2]. ( Note: always use this convention to indicate a "frame" of the CCD camera, or that to enclose the identifier in square brackets: it is an array of numbers and not a single numerical value, this will be useful for easy reading of the mathematical formulas used here ).
The only important warning about the flat field: they shall not be a sky-flat in the highly unlikely the two flat field will have exactly the same illumination as will have been taken at different times and therefore with a luminosity del cielo differente. E' necessario riprendere due flat field con la tecnica del dome-flat ovvero utilizzando una sorgente luminosa stabile e costante.
Una volta ripresi e salvati su disco i due flat field e bias frame, utilizzando Astroart possiamo aprire le finestre della statistica per ogni frame ed annotarne il valore medio: indicheremo questi valori ripettivamente con avg[F1] , avg[F2] , avg[B1] e avg[B2] .
A questo punto sottraiamo un flat field con l'altro, cioè utilizzando il comando Matematica > Sottrai di Astroart eseguiamo l'operazione [F1]-[F2] . We'll get a completely black frame looking as if all the pixels have a null value: in fact they are the thresholds for display of the image that must be regulated and do it quickly and automatically, simply click your mouse on the bar status of the image. You should get a representation of the "noise map " in the flat field as in the image below. 
However it is not the graphical representation we are interested because its numerical value: return to open the annotiamoci statistics and the standard deviation: the denote by ds ([F1] - [F2]) . Repeating the same steps for the two bias frames will get the value ds ([B1] - [B2]) . Now we have all the information needed to apply the following formula to calculate the gain:
while applying the following to calculate the readout noise: 
always remember that the gain is expressed in electrons / ADU and the readout noise is expressed in electrons. 
