ACS Instrument Handbook for Cycle 25
help@stsci.edu
Table of Contents Previous Next Index PDF


Advanced Camera for Surveys Instrument Handbook for Cycle 25 > Chapter 2: Considerations and Changes After SM4 > 2.5 CTE Considerations

2.5
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 option (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 readout, 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.
For option (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 by 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 LED post-flash (ACS ISR 2014-01) to increase the background. For the latest details and recommendations on ACS/WFC post-flash capabilities, please refer to the webpage:
http://www.stsci.edu/hst/acs/performance/cte/cte_postflash.html

Advanced Camera for Surveys Instrument Handbook for Cycle 25 > Chapter 2: Considerations and Changes After SM4 > 2.5 CTE Considerations

Table of Contents Previous Next Index PDF