... 4.7.1 Gain Calibration . ... DN for both the WFC and HRC (prior to Cycle 14 the WFC default was 1 e ? ... This technique relies on analysis of pairs of identical exposure flat fields taken at a range of intensity levels. ... As documented in ACS ISR 2004-01 , on-orbit data was used to redetermine the mean absolute gain values relative to the standard adopted for the default gains on each camera, e.g., WFC GAIN = 1 and HRC GAIN = 2 values were retained and other gains adjusted relative to these. ...