7.6 CCD Gain Selection

Please check for updates on the ACS website.

As quantified in Table 4.1, the WFC CCDs have selectable gain values of 0.5, 1.0, 1.4, and 2.0 electrons per digital number. Various factors should influence the gain selected in Phase II for your science program: level of support and calibrations provided, influence of associated readout noise on data quality, dynamic range on the bright end, and data compressibility for WFC in limited applications.

Only GAIN=2.0 is fully supported for the WFC. Calibration support will not be provided for the GAIN=0.5, 1.0, and 1.4 settings; users proposing their use should provide special justification and discussion of calibrations to be used. Please note that post-SM4 WFC images appear to have substantial (>10-3 fractional) amplifier crosstalk when the ADC saturates.

GAIN=2.0 completely samples the full well depth of nearly 85,000 e. Furthermore, charge is conserved even beyond filling the full well depth; for point sources at GAIN=2.0 it is possible to obtain valid aperture photometry several magnitudes beyond saturation by summing over all pixels affected by the bleeding due to saturation (see Section 4.3.1). GAIN=2.0 provides critical sampling of the readout noise, supporting robust background sky-level determination even at low values.