## 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).

## Recent Updates

Analysis of data from the Cycle 24 External CTE monitor program (CAL/ACS 14507, P.I. Chiaberge) show that the CTE trends are continuing according to the model presented in ACS ISR 2012-05. We derive new coefficients and provide an updated correction to be used for aperture photometry of ACS/WFC drizzled images taken after Servicing Mission 4. The updated correction allows an average photometric accuracy of better than 3% for stellar sources located at any distance from the amplifiers. In order to achieve the highest accuracy, the ACS Team has developed two tools for computing and applying the photometric CTE corrections.

LAST UPDATED: 04/15/2020

#### Please Contact the HST Help Desk with any Questions

https://hsthelp.stsci.edu