Readout noise is an irreducible contribution to the total error budget. It is linked to the readout process and there are no reduction steps that can minimize or remove it. The effect of the read noise can, however, be minimized by keeping the number of pixels, where signal is measured, small and by minimizing the number of CCD readouts (while still allowing for good cosmic ray and hot pixel removal). Read noise is independent of position on the CCD. Tables
4.1,
4.2, and
4.3 show gain and read noise values of the four WFC amplifiers for different time periods.
The analog information (electrons) accumulated in the CCD are converted into data numbers (DN) by the analog-to-digital converter (ADC). Both ACS CCD cameras employ 16-bit ADCs, which can produce a maximum of 2
16 =
65,535 DN. If the gain conversion factor
g is expressed in e
−/DN, the largest number of electrons representable by these ADCs is given by
g *
2
16. Any charge packet containing more than
g *
2
16 electrons will reach the digital saturation limit (see
Section 4.6.4). The analog-to-digital converter produces only discrete output levels. This means that a range of analog inputs can produce the same digital output. This round-off error is called quantization noise. It can be shown (Janesick J. R., 2001, Scientific Charge-Coupled Devices, SPIE Press.) that quantization noise QN is constant for a given gain setting when expressed in DN: QN(DN)
=
12
-1/2 = 0.288675. Quantizing noise can be converted into noise electrons as: QN(e
−)
=
0.288675
*
g. The total noise (TRN) associated with CCD readout noise (R) and quantizing noise is obtained by adding the two figures in quadrature (see
Table 4.1),
