WFC3 Data Handbook V. 4.0

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6.1	CTE Losses And Background

ACS/WFC has spent more than twice as long in space as WFC3/UVIS (12 years versus 5 years as of Cycle 21), yet in some ways the CTE losses in WFC3/UVIS have become more severe than those in ACS/WFC. Indeed, CTE losses became apparent for WFC3/UVIS even before it had been on orbit for a year. At first it was unclear why this was the case, but we have been able to trace it to the basic fact that CTE losses are worse when the background is extremely low. It has long been known that lower detector backgrounds lead to more CTE losses, but the pathological nature of the losses at low background had not been fully appreciated.

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, so even after 5 years its typical dark current is about 1 e-/pixel over a 1000s exposure, while that of ACS is about 15 times this after having spent only 2.4 times longer in space. 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 WFC science exposures almost never have backgrounds less than 25 electrons. (For a summary of WFC3/UVIS backgrounds, WFC3 ISR 2012-12 and for ACS, ACS ISR 1204).

It is also very likely that the comparably high radiation degradation in the UVIS detectors was related to an extreme minimum in solar activity during the first years of WFC3 on-orbit operations. Solar minima are related to increased levels of detector-degrading radiation within the orbit of HST.