Bias Subtraction and Correction of ACS/WFC Frames

Calibrated ACS/WFC science frames processed through the CALACS pipeline exhibit a residual offset in their absolute levels at the edge separating the Amp A-B and Amp C-D quadrants. This effect can be attributed primarily to uncertainties in the bias level subtraction. We present an analysis of the overscan levels and of the amplitude and behavior of the residual offsets for a large sample of bias frames. The scientific impact of this residual is discussed.