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 (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.