![]() It is a thermal phenomenon resulting from electrons spontaneously generated within the silicon chip (valence electrons are thermally excited into the conduction band). Scientific CCDs have a readout noise of 2-3 electrons RMS. For a four second exposure, a total of four electrons/pixel are generated. It can be reduced at the expense of increased read out time. For instance, a given camera might have a dark current specification of 1.0 e /p/s. The curve has a minimum near T = 0.2 (or % T = 20%) or A = 0. Dark Shot Noise ( D): Dark current is a current that flows even when no photons are incident on the camera. Dark current noise is the statistical variation of dark current across the sensor. The dynamic range is expressed in decibel units according to the following equation: Dynamic Range 20 × Log (N sat /N noise ) where N (sat) is the linear full well capacity stated as the number of electrons and N (noise) is the total value of the read and dark noise, also expressed as the number of electrons. The equivalent background per pixel is then given as B84.1+5.22+. The relative uncertainty is largest when T is large (small A) or when T is small (large A). Therefore the total dark current (on which there will be shot noise) is only 13.5 electrons. The shape of the curve is very much like the Category 1 curve shown in red. ![]() The purple curve in Figure 8.2.1 is the sum of the contribution of Categories 1, 2 and 3. One of them, read noise, is actually a constant term for a given camera and gain/ISO setting. In this case k 3 = 0.0013 and like Category 2 the contribution of Category 3 uncertainty to the relative uncertainty in c is largest when T is large and A is small. We have to account for additional sources of electronic noise, such as read noise and dark current: SNR S/SQRT (S + DC + RN2) Dark current and read noise are different in nature.
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