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Advanced Camera for Surveys Instrument Handbook for Cycle 21 > Chapter 2: Changes After SM4 and Considerations for Cy21 > 2.5 CTE Considerations

Observers should consider the effect of 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 (MacKenty & Smith 2012). 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 the photometry to account for the losses, and/or (2) adjusting the observing strategy to increase the charge transfer efficiency during the CCD readout process.
For (1) the CALACS pipeline now employs a pixel-based CTE correction algorithm based on the work of Anderson & Bedin (2010). 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 a formula to correct the measured photometry for CTE losses, depending on CCD position, background, flux and observation date. This formula is also not able to estimate accurate fluxes for objects that have been severely trailed beyond recognition. Details are provided in ISR 2012-05.
For (2), there are several strategies that can be employed. The simplest is to place the source near the readout amplifier to reduce the number of transfers. This can be accomplished using the aperture WFC1-CTE (Table 7.6). If this is not possible (e.g. if the source extends > 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 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 short post-flash to increase the background. The ACS team is currently testing the post-flash capability of ACS and users should check the ACS web page for the latest details and recommendations.

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