## Introduction

Since 2003, the ACS Team has taken observations of 47 Tuc to quantify the dependece of stellar photometry on the number of parallel and serial transfers. The observations employ a specialized dither pattern  that allows one to derive an analytical model that depends on the following parameters: stellar flux, background level, and the number of parallel transfers. Losses due to serial transfers are currently very small (< 2% at the edge of the chip and far from the amplifiers, where losses are the worst) and consistent with zero for the level of sky background usually achieved by GO observations (> a few electrons per pixel). In 2012, the original model was revisited and the assumption of a log-space linear relationship between the magnitude losses as a function of stellar flux (valid for the small CTE losses in pre-SM4 data) was shown to be an oversimplification of the CTE effects after ~10 years in space. This led to a new and improved model that also correctly accounts for the time-dependence of the derived coefficients (see ACS ISR 2012-05).