FLAT FIELD DETERMINATIONS USING AN

The traditional method of measuring ACS flat fields (FF) involves a complicated analysis of multiple observations of a region of the 47 Tuc globular cluster at overlapping field positions. The analysis of the dithered 47 Tuc images suffers from source crowding and possible systematics related to the CTE correction and the high density of sources. New programs 13167 and 13602 avoid these problems by observing a single bright star at several locations around the field of view (FOV) in F435W and F814W. A discrepancy of ~3% with a 10 level of significance exists between the two FF measurement techniques and is currently unexplained.