6.2 CTE Losses And Background

ACS/WFC, installed in HST in March 2002, has been in space about 7 years longer than WFC3, yet in some ways the CTE losses in WFC3/UVIS have become more severe than those in ACS/WFC. CTE losses became apparent for WFC3/UVIS even before it had been on orbit for a year. In fact, CTE losses for even moderately bright targets were 2-3 time greater than expected CTE losses based on ACS performance in its first years on-orbit ( ACS ISR 2009-01). However, the faster degradation seen by UVIS 2009/2010 was also occurring on ACS as well ( Massey, 2010, MNRAS 409L, 109). As discussed in the CTE White paper, one contributing factor was likely the solar minimum:

  1. the strength and extent of the South Atlantic Anomaly, where most of the CTE traps are thought to be formed, is known to be anti-correlated with solar activity (e.g. Casadio et al. 2010) 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 the low image background. WFC3/UVIS images tend to have much lower backgrounds than ACS images for several reasons. First, WFC3/UVIS was designed to be sensitive to low S/N objects, with low readnoise (~3 e-/pixel) and low dark current (~8 e-/pixel/hr in 2017 and increasing by ~1 e-/hr/year). The dark current of ACS is about 50-55 e-/hour. 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 background low. Finally, images obtained with the WFC3/UVIS complement of UV and narrow-band filters often have extremely low sky backgrounds. For all these reasons, even deep UVIS science exposures frequently have backgrounds of less than 5 electrons, whereas similar ACS/WFC science exposures almost never have backgrounds less than 25 electrons. (For a summary of WFC3/UVIS backgrounds see, WFC3 ISR 2012-12 and for ACS, ACS ISR 2012-04.)