ACS's survey mode is ten times faster

Simulated globular star cluster field as viewed by the current HST imaging camera (WFPC2) and by the ACS Wide Field Camera. In this illustration, both instruments are used in survey mode to image the star cluster to the same absolute flux limit (the same 'depth').

The outlines of the field of view for each instrument are shown in green. In the same total exposure time, the ACS instrument can image to the same depth an area ten times larger than WFPC2 can.