The Impact of Degraded Charge Transfer Efficiency on Extended Sources in ACS/WFC

Abstract

Using repeat imaging of a galaxy cluster taken over a seventeen-year baseline, we examine the impact that degraded Charge Transfer Efficiency (CTE) has on photometric measurements of extended sources using the ACS/WFC on HST. We examine how measured brightnesses depend on time since ACS installation, source location on the WFC detectors, source brightness, and local background level in individual exposures. We find that global brightness measurements using large apertures are generally reliable within $\sim$0.05 magnitudes across the WFC detectors if exposure backgrounds are above $20e^-/{pixel}$ and sources are brighter than $\sim300e^-$ in a single exposure. However, brightness measurements on smaller scales can suffer deficiencies in excess of 0.1 mags (sometimes, significantly more) in recent data unless sources are very close to the CCD serial registers ($\lesssim 512$ pixels), or brighter than $\sim3000\,e^-$ in a single exposure. We also show how degraded CTE can result in artificial asymmetries in galaxy light distributions, which are largely mitigated if backgrounds are $>20e^-/{pixel}$ and targets are not far ($>1536$ pixels) from the serial registers. As expected, brightness measurements in later epoch data are best when using CTE-corrected images (FLC/DRC), but our results imply that the pixel-based CTE correction algorithm employed by the ACS reduction pipeline does not necessarily place charge back into its original location within extended sources. Based on this study, users are advised to keep backgrounds above the already recommended $30e^-/\mathrm{pixel}$, ensure targets will have at least $\sim 300e^-$ in a single exposure, and place targets close to the serial registers if analysis of their spatially resolved properties is needed.

Figures (14)

• Figure 1: The galaxy cluster CL0024$+$16 used for this study, imaged in F435W as part of GO 10325.
• Figure 2: The same zoomed in region of sky seen in images from the three epochs used in our analysis. Both DRZ (left) and DRC (right) images are shown. No post-flashed images are shown. The scaling is identical between all images. The white vertical band seen in the 2013 and 2021 images is the WFC chip gap which was not covered by the 4-point dither pattern.
• Figure 3: (top) Total Kron Magnitude distribution of detected sources from the 2004 data set with total Kron $SNR>10$. (bottom) Total background level (including sky, dark current, and post-flash) measured in individual exposures of the 2021 imaging data at the positions of detected sources, broken up by the image post-flash duration. Due to the non-uniformity of the flash, post-flashed data span a range of backgrounds. The backgrounds in the 2013 data span a comparable range.
• Figure 4: $\Delta m_{total}$ versus $\Delta y$ for DRZ (top) and DRC (bottom) images taken in 2013 (left) and 2021 (right) using total Kron magnitudes. Points are color-coded by the local background of each source the original exposures. Large points show binned median values as a function of $\Delta y$ and background. Global brightness measurements are generally reliable to within $\lesssim 0.05$ mags except when backgrounds fall below $20e^-/\mathrm{pixel}$ in more recent data.
• Figure 5: $\Delta m_{central}$ versus $\Delta y$ for DRZ (top) and DRC (bottom) images using apertures covering only the centers of galaxies. Unlike for global brightness measurements, there is a clear decline in recovered central brightness with increasing time, increasing $\Delta y$, and decreasing background.