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Each year comma we will check the absolute gains of the ACS-WFC readout amplifiers. The data are pairs of full-frame flats in the F435W filter taken at five different exposure times. From experience comma the exposure times we will use correspond to signal levels of approximately 2.5k comma 5k comma 10k comma 20k comma and 40k DN. We will measure the signal and variance in 25 regions across a 5x5 grid on a WFC amplifier and fit a linear function to the variance as a function of signal (the photon transfer curve) comma the slope of which is proportional to the absolute gain for that amplifier.