The ACS camera has a very high-speed shutter; even the shortest exposure times are not significantly affected by the finite traversal time of the shutter blades. On-orbit testing reported in ACS ISR 2003-03
has verified that shutter shading corrections are not necessary to support 1% photometry for either the HRC or WFC.
A total of 4 exposure times are known to be in error by up to 4.1%; the nominal 0.1 seconds HRC exposure is really 0.1041 seconds (updates for reference files have been made to use the correct values in pipeline processing). No significant differences exist between exposure times controlled by the two shutters (A and B), with the possible exception of non-repeatability up to ~1% on the WFC for exposures in the 0.7 to 2.0 second range. The HRC provides excellent shutter time repeatability.
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