CTE in Space Astrometry

An imperfect CTE in CCD detectors is one of the most important instrumental issues affecting both photometry and astrometry, especially in space-based observations. We discuss the CTE effect in the images taken with the Hubble Space Telescope's, Advanced Camera for Surveys in Wide Field Channel (ACS/WFC). ACS is the only imaging instrument capable of delivering sub-mas astrometry from a single observation, and it is important to take into account any instrumental systematic in positions such as the CTE effect.