2.5 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 using either a pixel-based correction algorithm to restore the values of the pixels or correcting photometry to account for the losses, and/or (2) adjusting the observing strategy to increase the CTE during the CCD readout process.

For option (1), the CALACS pipeline now employs a second generation pixel-based CTE correction algorithm described in ACS ISR 2018-04. This routine works well when CTE losses are not severe. Extensive testing has shown that for typical ACS backgrounds, the 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 2012-05 to correct the measured photometry for 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. A calculator is available to apply this CTE correction formula to measured photometry.

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). 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 2012-04. 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 20 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. For the latest details and recommendations on ACS/WFC post-flash capabilities, please refer to ACS ISRs 2018-02 and 2014-01.