Serial Charge Transfer Efficiency in ACS/WFC

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.

💡 Research Summary

This paper presents a dedicated investigation of the serial (x‑direction) charge transfer efficiency (CTE) of the ACS/WFC CCDs on the Hubble Space Telescope. While the parallel (y‑direction) CTE has been extensively studied and corrected in previous work, the serial CTE has received far less attention despite its gradual degradation over time and its subtle impact on high‑precision photometry and astrometry. The authors exploit hot pixels in the long‑exposure calibration dark frames as delta‑function charge sources. By selecting isolated hot pixels with signal levels around 30 000 ± 6 000 e⁻ that experience 1500–1900 serial transfers, they extract the hot pixel itself, four upstream pixels (toward the amplifier), and a 100‑pixel downstream trail. Background is estimated from rows above and below the trail. Combining data from an entire calendar year improves the signal‑to‑noise ratio sufficiently to detect the faint serial trails, which are typically only a single bright pixel but can extend up to ~40 pixels for the brightest hot pixels.

The authors construct an empirical CTE model analogous to the well‑established parallel model (Anderson & Bedin 2010; Anderson & Ryon 2018). The model parameterizes a trap‑density profile (how many traps affect charge packets of a given size) and a trail‑profile (the probability of charge release as a function of distance downstream). Because the dwell time per serial transfer (≈22 µs) is far shorter than that for parallel transfers (≈3212 µs), far fewer traps can capture charge during serial readout, leading to much smaller CTE losses. Nevertheless, some trap species have release times comparable to or longer than the serial dwell time, producing trails that can be longer than the parallel ones despite their lower amplitude.

To correct the data, the existing pixel‑based CTE correction step in CALACS (ACSCTE) is extended. The algorithm rotates each CCD quadrant by 90°, treating the serial direction as a parallel one, applies a single forward‑model iteration (sufficient given the small magnitude of the effect), then rotates the data back. Serial correction is performed before the parallel correction to avoid double‑counting deferred charge. Time dependence is incorporated via a linear scaling factor s = (t_obs − t₀)/(t₁ − t₀), with t₀ and t₁ chosen specifically for the serial case and stored in the updated PCTETAB reference file. The new reference file contains quadrant‑specific trap‑density and trail parameters, as well as the CTEDATE0/CTEDATE1 constants.

Quantitatively, the serial CTE introduces photometric losses of roughly 0.005–0.02 mag and centroid shifts of 0.01–0.035 pixel, an order of magnitude smaller than parallel CTE effects but still relevant for precision work such as variable‑star monitoring, weak‑lensing shape measurements, and astrometric studies of crowded fields. The correction is currently applied automatically to all full‑frame ACS/WFC images obtained after SM4 (May 2009). The authors note that the corrected data, now accounting for both parallel and serial CTE, will be made available in the MAST archive shortly after publication.

In summary, this study provides the first comprehensive, empirically calibrated model of serial CTE for ACS/WFC, demonstrates its implementation within the standard HST data‑processing pipeline, and quantifies its impact on scientific measurements. The work ensures that archival and future ACS/WFC observations can be analyzed with a consistent, high‑fidelity correction for charge‑transfer losses in both readout directions.