2.6 CTE Considerations

Observers should consider the effect of charge transfer efficiency (CTE) losses on their data. These issues and recommendations are discussed in some detail for ACS/WFC and WFC3/UVIS in the CTE White Paper and on the ACS CTE Information webpage. CTE losses can be mitigated using two different approaches: (1) post-processing with either a pixel-based correction algorithm to restore the values of the pixels, or correcting point-source photometry to account for the losses (corrections for extended sources are not currently available), and/or (2) adjusting the observing strategy to maximize the CTE during the CCD readout process.

For option (1), the CALACS pipeline now employs a third generation pixel-based CTE correction algorithm described in ACS ISR 2018-04 and 2024-07. This routine works well when CTE losses are not severe. Extensive testing has shown that for typical ACS backgrounds (>30 e^–), the parallel (y-direction) CTE correction algorithm has a 75% reconstruction accuracy. For very low backgrounds, 90% of the charge can be lost as the CCD is read out, and these large losses cannot be reconstructed. An alternative technique for point sources is to apply the formula provided in ACS ISR 2022-06 to correct the measured photometry for parallel CTE losses as a function of CCD position, background level, source flux, and observation date. This formula is similarly unable to estimate accurate fluxes for objects that have been CTE-trailed beyond detectability. The ACS Photometric CTE Calculator is available to apply this CTE correction formula to measured photometry. This calculator is also available as a Python API, acephotcte, as part of acstools. Recent work presented in ACS ISR 2025-02 has demonstrated that extended sources may also be impacted by degraded CTE. No correction formula analogous to that for point sources is currently available.

For option (2), there are several strategies that can be employed. The simplest is to place the source near the WFC serial registers (located furthest from the gap between the WFC CCDs) to reduce the number of CCD parallel transfers. This can be accomplished by using the aperture WFC1-CTE (Table 7.7) and/or a suitable Y POS TARG. If this is not possible (e.g., if the source subtends >5 arcsec), then the observer should estimate the sky background using the information given in ACS ISR 2022-01. Here, empirical sky backgrounds are provided for all of the ACS filters as a function of exposure time and compared to Exposure Time Calculator (ETC) estimates. Observers should check that the sky background is above 30 e^– for a given exposure time. If the background is lower than this value, observers should consider increasing their exposure times or using a LED post-flash to increase the background, although the post-flash option is currently only recommended in very limited circumstances where mere basic detection is the main goal, and photometric comparisons across the field of view or photometric comparisons with data of different epochs or other telescopes and/or instruments is not the goal. For the latest details and recommendations on ACS/WFC post-flash capabilities, please refer to ACS ISRs 2024-02 and 2018-02.