Electrons which accumulate in the CCD wells are read out and converted to data numbers (DN) by the analog-to-digital converter (ADC). The ADC output is a 16 bit number, producing a maximum of 65,535 DN in one pixel.
The CCDs are capable of operating at gains of 1, 2, 4 or 8 electrons/DN. In principle, use of a lower gain value can increase the dynamic range of faint source observations by reducing the quantization noise; however, in practice this improvement is not significant. Table 4.2 shows the actual gain levels and readout noise in electrons for the four WFC amps, and the default C amp used for the HRC.
For the WFC, gain factors of 1 and 2 are fully supported, as are gain values of 2 and 4 for the HRC. The remaining two gain factors for each camera are available but unsupported so users of those modes must plan their own calibration. GAIN=2 is the default configuration for the WFC for observations in Cycle 14 and later cycles. The readout noise associated with GAIN=2 is on average only 0.28 e- higher per amplifier than that of GAIN=1. The noise increase brought about by the use of GAIN=2 is equivalent to that produced by adding a mere 1.7 e- of noise in quadrature to the noise of the GAIN=1 configuration; when the number of detected photons is larger than 3, the Poisson noise alone on the combination of source and sky would exceed this level. Thus, in terms of readout noise, the advantage of using GAIN=1 is minimal, whereas by adopting GAIN=2, the ability to do accurate photometry before saturation is extended by 0.32 magnitudes. In addition, more bright unsaturated sources could become available for cross-image registration, and for point sources, it may be possible to perform photometry several magnitudes beyond saturation for some cases. Further information about gain values can be found in ACS ISR 2002-03
and ACS ISR 2004-01
.
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