6.6 Dealing With CTE Losses in WFC3 UVIS Images

There are several ways of mitigating the effects of 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; this section provides 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 2024; 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).

In order to predict the number of background electrons in a planned image through a particular filter with a given exposure time, users should use the WFC3/UVIS imaging exposure time calculator (ETC). The ETC provides an estimate of the predicted total background based on an input observing mode and will provide a warning if the background falls below the current recommendation. In images where the predicted natural background is less than the recommended level, observers should use post-flash to make up the difference. The ETC does not include an estimate of the actual astronomical scene in its calculations, and knowledge of what background to expect from your source can help you add just the right amount of post-flash. In general, though, it is better to flash too much rather than not enough.

NOTE: The post-flash (PF) levels in APT and the ETC represent the average across the FOV. The PF illumination pattern varies by ~ +/-20% across the FOV, and is brightest in B/D quadrants, falling off towards A&C corners. As a consequence, observers desiring to ensure a specific electron/pixel level across the entire FOV will want to increase the requested levels by ~20%.

Backgrounds above the recommended level provide increased protection against CTE losses, but adding background also adds noise, and the net signal to noise ratio for marginal sources ends up being essentially flat between backgrounds of 20 and 50 electrons. Splitting up long exposures into multiple dithered exposures is commonly done to improve PSF sampling and remove detector artifacts and cosmic rays. However, from a CTE-minimization perspective, it can be better to take fewer, longer exposures to reduce the amount of postflash required (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 CTE correction in calwf3 for general scenes of point and extended sources (Section 6.4) or the empirical formula correction for point sources (see Section 6.5).