|ACS Data Handbook v. 9.0|
4.1.1 Read NoiseRead 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 influence of read noise is minimized by reducing the number of pixels in the photometry aperture, and by subdividing the total exposure into the minimum number of component readouts while still permitting an adequate cosmic ray and hot pixel removal, as well as pixel-phase sampling. 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. Figure 4.1 illustrates the history of the amplifier-based read noise (in electrons) since 2003.
2.0 1 5.32Valid after 20 January 2013; read noise was 4.04 e– prior to this date.
2 1 Since Cycle 14 (under Side 1 operations), the default gain has been GAIN = 2. Prior to this, GAIN = 1 was the default.
Table 4.3: CCD Gain and Read Noise Operated Under Side 2 of Original CEB
(July 2006 to January 2007)
4.1.2 A-to-D ConversionThe analog information (electrons) accumulated in the CCD is converted into data numbers (DNs) by the analog-to-digital converter (ADC). Both ACS CCD cameras employ 16-bit ADCs, which can produce a maximum of 216 −1 = 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 x 216. Any charge packet containing more than g x 216 electrons will reach the digital saturation limit (see Section 4.7.4). 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 quantizing noise. It can be shown (Janesick, J. R., 2001, Scientific Charge-Coupled Devices, SPIE Press) that quantizing 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 x g.The measured read noise (RN; reported in Tables 4.1, 4.2, and 4.3) is the quadrature sum of this quantizing noise and the intrinsic read noise (IRN) associated with a particular readout amplifier:RN = (IRN2 +(0.288675 x g)2)1/2