Serial Charge Transfer Efficiency in ACS/WFC

TL;DR

This work quantifies serial CTE in ACS/WFC by exploiting hot-pixel trails in calibration dark frames to derive trap densities and trail profiles per serial quadrant. It builds a forward-model serial CTE correction using a triple-exponential trail and a time-dependent trap-density scaling, integrating these parameters into CALACS via an updated PCTETAB file and serial-before-parallel correction order. The authors implement and validate the serial correction on post-SM4 full-frame data, demonstrating photometric recoveries of about $0.005$–$0.02$ mag and astrometric shifts of roughly $0.01$–$0.035$ pixels, with time-dependent adjustments to maintain performance. The approach improves data quality and is slated to provide science data corrected for both serial and parallel CTE in the MAST archive, while highlighting residual biases for bright stars and pointing to future refinements, including longer trail lengths and refined time constants. $t_0=49000$, $t_1=60500$ (MJD) are adopted for post-SM4 corrections to account for evolving serial CTE.

Abstract

We present a dedicated study of CCD serial ($x$-direction) charge transfer efficiency (CTE) in ACS/WFC. Following past studies of parallel ($y$-direction) CTE, we use the serial CTE trails behind hot pixels in calibration dark frames to characterize charge trapping and release in the serial registers of the WFC detectors. Serial CTE trails are sharper and longer than parallel CTE trails. Many fewer charge traps come into play during serial pixel transfers than parallel transfers, which explains why parallel CTE is much worse than serial CTE. We find that serial CTE can cause losses of $\sim$0.005-0.02~mag in stellar photometry and shift stellar centroids by $\sim$0.01-0.035 pixels. The pixel-based algorithm in CALACS that corrects for parallel CTE losses in WFC data has been modified to include a correction for serial CTE losses. The PCTETAB reference file has also been updated to include serial CTE parameters. The pixel-based correction for serial CTE currently runs only on full-frame WFC images obtained after SM4 (May 2009). Shortly following the publication of this report, science data corrected for both parallel and serial CTE will be available in the MAST archive.

Figures (20)

• Figure 1: Schematic of the ACS/WFC CCDs including active pixel arrays (white regions), prescans and overscans (gray regions), and parallel and serial transfer directions (red arrows). Reproduced from the ACS Instrument Handbook stark2024b.
• Figure 2: Section of quadrant D in a 2022 stack of 48 long dark frames (1000.5s each) corrected for post-flash. Hot pixels and CTE trails are dark gray and black in this grayscale image. Parallel and serial CTE trails extend towards the top and left of the image, respectively. Parallel trails are visible above the background for several pixels, often 20 or more. Serial trails are typically evident as a single bright pixel immediately to the left of hot pixels. A notable exception is the serial trail of the highly saturated hot pixel located at approximately (2730, 1590), which is faintly visible for almost 40 pixels.
• Figure 3: Schematic of a pixel grid including a hot pixel (black pixel labeled HP) and serial and parallel CTE trails. The serial and parallel readout directions are to the left and down, respectively, as shown by the arrows. The serial and parallel trails (grayscale pixels) extend to the right and top of the panel, respectively. Pixels selected for trail analysis are outlined with a dashed purple line, including the four "upstream" pixels (light blue), the hot pixel, and 100 pixels "downstream", i.e., the serial trail. 100 background pixels (light red) are selected from the rows above and below the serial trail.
• Figure 4: Example of kernel density estimation (KDE) peak-finding for the first three pixels in the serial CTE trail of 30k e$^-$ hot pixels. The hot pixels are from quadrant B in the 2022 long dark stack. Histograms show the sigma-clipped distribution of pixel values of first three pixels in blue (+1), green (+2), and orange (+3). The grey histogram shows the background regions, and is the same in both panels. The curves and vertical lines show the KDE result and peak of the KDE for each distribution.
• Figure 5: Serial CTE trail for 30k $e^-$ hot pixels located in quadrant B of long dark stacks from 2022. The left panel shows the full trail, excluding the hot pixel. The right panel zooms in on the second pixel and beyond to better show the scatter in the trail. The dashed black line marks zero signal. A running median of the last 70 pixels (solid black line) shows that the serial trail still contains $\sim$0.5-1.0 e$^-$/pixel 100 pixels beyond the hot pixel.