The charge transfer efficiency (CTE) of ACS/WFC has progressively degraded over the lifetime of ACS. The ACS ETC does not take this into account when estimating exposure time or signal-to-noise ratio (SNR). The true amount of time needed to obtain a specific SNR will likely be longer than stated by the ETC, and the SNR reached in a specific time will be lower. This issue is true for both point sources and extended sources.

See Phase-II Program Development and Design Issues: CTE issues for detailed guidance on how to minimize and correct losses due to degraded CTE.

Chapter 9 of the ACS Instrument Handbook and this Jupyter notebook now contain examples of how to estimate exposure time to reach a fixed SNR given the impact of degraded CTE.