The charge transfer efficiency (CTE) of the ACS CCDs (WFC and HRC) declines as damage from the space radiation environment accumulates. Here we provide information about our efforts to monitor the CTE, and methods of mitigating and calibrating it. The ACS Team currently provides two tools for correcting signal losses incurred by the imperfect CTE of the ACS/WFC CCD detectors: ACSCTE and the Photometric CTE Calculator.
ACSCTE
ACSCTE is a standalone python tool included in the python package ACSTOOLS. It is simply a wrapper around the C-code implementation of the pixel-based CTE correction algorithm developed by Jay Anderson and Luigi Bedin described here. The most recent updates to the algorithm are described ACS ISR 2018-04. This is the same algorthim used in the ACS calibration pipeline, CALACS, to correct for CTE losses in ACS/WFC frames.
As of 2024, ACSCTE includes an additional correction for serial (x-direction) CTE. All post-SM4 (May 2009) full-frame WFC images have been reprocessed to incorporate this correction. See ACS ISR 2024-07 for complete details.
A more detailed summary of the pixel-based CTE correction algorithm and its performance can be found at the Pixel-Based CTE Corrections Webpage.
Photometric CTE Calculator
This calculator corrects ACS/WFC photometry for parallel (y-direction) CTE losses using an empirical model derived from observations of 47 Tucanae described ACS ISR 2022-06. The tool is calibrated for photometry extracted from observations obtained after SM4 in May 2009 only. For information on the expected accuracy, as well as, the observations and model used in the webtool please refer to the Photometric CTE Corrections webpage. To access the webtool, click the link below
ACS/WFC Photometric CTE Webtool (Currently under maintenance. Please use the acsphotcte module within acstools.)
CTE Monitoring and Reports
CTE Monitoring and Reports
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Internal monitoring of ACS CTE
Internal monitoring of the decay of parallel CTE of the ACS/WFC CCDs is accomplished with the extended pixel edge response (EPER) test. EPER images have extra-large overscans in which to accumulate trapped charge during readout of the lamp-illuminated active pixels. Comparing the amount of trapped charge in the overscans to the signal level of the illuminated pixels provides an estimate of CTE per pixel. EPER images of various signal levels have been obtained since ACS was installed on HST, which provides a long baseline over which to track CTE losses. Recent analysis of parallel CTE for ACS/WFC is described here (ACS ISR 2018-09). Future work includes an analysis of serial CTE in WFC from EPER data.
WFC Parallel EPER
EPER images have extra-large overscans in which to accumulate trapped charge during readout of the lamp-illuminated active pixels. Comparing the amount of trapped charge in the overscans to the signal level of the illuminated pixels provides an estimate of CTE per pixel. EPER images of various signal levels have been obtained since ACS was installed on HST, which provides a long baseline over which to track CTE losses.
Modeling the CTE
Parallel CTE from EPER data has a power-law dependence on signal level and a linear dependence on time. The best-fitting power-law index for the dependence on signal level is 0.5 ± 0.01.
The rates of decrease of CTE as a function of time are given in the table below. They are measured for specific, standard signal levels obtained regularly since SM4.
Signal level (e-) Rate of Decrease of CTE per year \(180\) \(−4.7×10^{−5}\)
\(430\) \(−2.8×10^{−5}\) \(1600\) \(−1.5×10^{−5}\) \(3400\) \(−9.4×10^{−6}\) \(7100\) \(−6.5×10^{−6}\) \(42000\) \(−2.7×10^{−6}\) The above rates of CTE decrease are determined from direct linear fits to the CTE trends, rather than a combined power-law (signal level) and linear (time) model as described in ACS ISRs 2018-09 and 2005-03. The combined model struggled to accurately fit the CTE decrease with time, likely due to CTE re-trailing in the EPER overscans.
Parallel CTE as a function of signal level (Q) in electrons. The points represent CTE measurements in a 16-column-wide bins in the EPER overscan. The points are color-coded by anneal date. Earlier data are located towards the top of the plot (higher CTE) and later data towards the middle/bottom. No difference between the WFC1 and WFC2 chips is seen. The color-matched curves are the best-fit power law models evaluated at the anneal date of the datasets. 
Parallel CTE as a function of time. The points represent CTE measurements in a 16-column-wide bins in the EPER overscan, color-coded by signal level. The black stars represent median CTE values for each anneal. No difference between the WFC1 and WFC2 chips is seen. The gray lines are the best-fit linear models for each signal level. Separate fits were performed for pre- and post-SM4 observation dates. The gray shaded band is the time period during which ACS/WFC was offline. External monitoring of the ACS CTE for point sources
Observations of a single field in globular cluster 47 Tucanae are obtained for the external CTE monitoring program for ACS/WFC. Pairs of images are separated by two large slews about half the size of the WFC field of view (102"). The slews are performed in both the X and in the Y directions to vary the number of transfers for each star. Monitoring of both the serial and parallel CTE is possible by comparing the measured magnitudes of each star at its position in each frame.
The empirical photometric correction for stellar data discussed above is determined from this monitoring program, and the analysis is described more fully in ACS ISR 2012-05and ACS ISR 2022-06.
Studies of ACS CTE for extended sources
Observations of galaxy cluster CL0024+16 taken in 2004, 2013, and 2021 have been used to assess the impact of degraded CTE on extended sources (on size scales of a few 10s to a few 100s of pixels). Global magnitudes measured with large apertures were found to be generally unaffected by degraded CTE to within 0.05 magnitudes as long as backgrounds are above 20e- and target brightnesses are above ~300e- per exposure. Measurements on smaller scales (e.g., local regions within galaxies) can suffer significantly underestimated brightnesses in excess of 0.1 mags, even in images corrected with the pixel-based CTE correction algorithm. For these reasons, users are strongly encouraged to keep backgrounds above 30e- and place targets close to the CCD serial registers if they intend to conduct spatially resolved analyses of extended sources. At this time, no correction model has been developed for extended sources. Further details can be found in ACS ISR 2025-02.
The figure below shows the magnitude loss for measurements of central brightness (within half a Kron radius) for extended sources as a function of global brightness per exposure and background level. Dark and light shaded points indicate objects close to (<512 pixels) and far from (>1536 pixels) the CCD serial registers. Losses in data from both 2013 (left) and 2021 (right) are shown and demonstrate the progression of CTE degradation. Similarly, losses in DRZ (top) and DRC (bottom) images are shown, demonstrating that although the pixel-based CTE correction algorithm reduces the impact of degraded CTE, it does not perfectly correct the data in all cases.
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ISRs
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(ACS ISR 2025-02): The Impact of Degraded Charge Transfer Efficiency on Extended Sources in ACS/WFC
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(ACS ISR 2024-07): Serial Charge Transfer Efficiency in ACS/WFC
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(ACS ISR 2024-02) The Impact of CTE on Point Source Detection in Simulated ACS/WFC Imaging Data
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(ACS ISR 2024-01) Evolution of Sink Pixels in ACS/WFC and Connection to Charge Transfer Efficiency
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(ACS ISR 2022-06) ACS/WFC CTE photometric correction: improved model for bright point sources
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(ACS ISR 2018-04): Improving the Pixel-Based CTE-correction Model for ACS/WFC
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(ACS ISR 2012-05): A new accurate CTE photometric correction formula for ACS/WFC
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(ACS ISR 2010-03): Empirical Pixel-Based Correction for Imperfect CTE
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(ACS ISR 2010-01): Pixel-based correction for CTI in the HST ACS
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(ACS ISR 2009-01): Updated CTE photometric correction for WFC and HRC
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(ACS ISR 2004-06): Time Dependence of ACS WFC CTE Corrections for Photometry and Future Predictions
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(ACS ISR 2004-04): Elevated temperature measurements of ACS CTE
Whitepapers
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- CTI over the History of the ACS WFC poster for AAS Meeting, January 2011
- Post-SM4 ACS CTE poster for AAS Meeting, January 2010
- ACS CTE poster for AAS Meeting, January 2003
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- Justification for onboard FPR/EPER CTE calibration, ACS TIR 1999-03 (available upon request)
- ACS CTE OPRs
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