6.6 Dealing With CTE Losses in WFC3 UVIS Images

There are several ways of dealing with imperfect charge-transfer efficiency in WFC3/UVIS images. Sections 7 and 8 of WFC3 ISR 2021-09 provide a detailed discussion of the many issues and trade-offs.  Here, we give an overview.

The best way to deal with CTE losses is by minimizing them in the first place.  Neither the reconstruction algorithm nor the empirical correction is perfect, so minimizing the need for these corrections is the first line of defense in reducing CTE losses. The easiest way is to do so is to place the source close to the readout register, thereby minimizing the number of readout transfers during which CTE losses can occur. Of course, this is only possible for fields-of-interest that are relatively small.

Another way to minimize CTE losses is to ensure that the total image background (sky + dark + postflash if needed) has a minimum level of 20 electrons per pixel (as of 2021; see WFC3 CTE page for any updates). A total image background at the recommended value will enable at least half of the electrons of a marginal source to survive to the readout (see Figure 18 of WFC3 ISR 2021-09).  To predict the number of background electrons in a planned image through a particular filter with a given exposure time, users can consult the graphs found in WFC3 ISR 2012-12. The WFC3/UVIS imaging exposure time calculator also provides an estimate of the predicted total background and will provide a warning if it falls below the recommendation.  In images where the predicted natural background is less than the recommended level, observers should use post-flash to make up the difference.  

Backgrounds above the recommended level do provide more protection against CTE losses, however they also add more Poisson noise and as a consequence, the signal to noise ratio of the science targets will decrease. Splitting up long exposures into dithered sub-exposures is commonly done to improve PSF sampling and remove detector artifacts and cosmic rays. However it can be better to take fewer, longer sub-exposures to reduce the amount of postflash (and the resulting noise), thereby improving the signal to noise for faint sources.  

Once CTE losses have been minimized in a data set, it will still likely be important to correct the observations for whatever losses have occurred, or at the very least obtain an estimate of how the readout process may have degraded the original image. One can use either the pixel-based correction for general scenes of point and extended sources (Section 6.4) or the empirical formula correction (see Section 6.5) for point sources.