6.2 CTE Losses And Background

ACS/WFC was installed on board HST in March 2002 and has been in space about 7 years longer than WFC3. While it took several years for CTE to become a concern for ACS observations, CTE losses became apparent for WFC3/UVIS even before it had been in orbit for a year. In fact, CTE losses for even moderately bright targets were 2-3 times greater than the expected CTE losses based on ACS's performance in its first years on-orbit (ACS ISR 2009-01).  It is worth noting that the faster degradation seen by UVIS 2009/2010 was also occurring on ACS as well in those years (see Massey 2010, MNRAS 409L, 109). As discussed in a CTE White paper in 2011, one contributing factor to WFC3/UVIS's rapid CTE losses was likely the solar minimum:

1. The strength and extent of the South Atlantic Anomaly, the region in HST's orbit where most of the radiation damage is thought to occur, is known to be negatively correlated with solar activity, and
2. The installation of WFC3 on HST coincided with solar minimum while the installation of ACS occurred during a period of higher solar activity.

An additional factor in the WFC3/UVIS CTE behavior is its typical image background. It has long been known that CTE losses are greater in images with low backgrounds. For several reasons, WFC3/UVIS images tend to have much lower backgrounds than ACS images. First, WFC3/UVIS was designed to be sensitive to low S/N objects, with low readnoise (~3 electrons/pixel) and low dark current (~12 electrons/pixel/hr in 2024 and increasing by ~0.5 electron/hr with every additional year on orbit). The dark current of ACS is about 50-55 electrons/hour, several times higher. Second, the WFC3/UVIS pixels are 40% smaller than the ACS/WFC pixels, and as such they intercept correspondingly fewer sky photons, again keeping the WFC3/UVIS background low. Finally, images obtained with WFC3/UVIS's many UV and narrow-band filters often have extremely low sky backgrounds.  For all these reasons, even long UVIS science exposures frequently have backgrounds of less than 5 electrons, whereas similar ACS/WFC science exposures rarely have backgrounds less than 25 electrons. (For a summary of WFC3/UVIS and ACS backgrounds, see WFC3 ISR 2012-12, and ACS ISR 2012-04, respectively.)

A recent report (WFC3 ISR 2021-09) has shown that, in addition to having lower backgrounds than ACS/WFC, the WFC3/UVIS detector is also more sensitive to CTE losses when the background is low. Figure 9 in that document shows that even though WFC3/UVIS has been in orbit for only 65% of the time ACS/WFC has, it already suffers more CTE losses than ACS/WFC when the background is below 20 electrons. The loss spectrum for ACS follows a simple power law, whereas that for WFC3/UVIS shows an upturn from the power law at low backgrounds (e.g. Figure 9 in WFC3 ISR 2021-09). This finding was unexpected, as WFC3 was designed to have a mini channel that would preserve charge from losses at the low-signal end.