6.5 Empirical Corrections

Empirical corrections provide an alternate way to correct for CTE losses of observations of point sources or estimate future CTE loss levels. Noeske et al., ( WFC3 ISR 2012-09) studied star clusters NGC6791 and 47 Tuc and examined how the measured fluxes of stars varied when particular stars were placed close to and far from the readout register. They documented the losses experienced by point sources as a function of (1) the electron flux of the point source, (2) the detector Y position of the source, (3) the background level, and (4) the date of the observation.

These corrections are provided in the form of polynomials and can be used to estimate how many electrons were originally present in the aperture before the readout process. The conclusions in this ISR and in the more recent follow-up studies ( WFC3 ISR 2017-09, and WFC3 ISR 2016-17) mirror the conclusions from the pixel-based-model study: namely, that even a small amount of background (2-3 electrons) can have a significantly ameliorative effect on charge-transfer efficiency, and backgrounds of 10-30 e- can reduce CTE losses to the perturbation level (less than 20% losses for all stars).

The most recent empirical corrections are described in WFC3 ISR 2017-09, which also provides an analysis of the long term behavior of the UVIS CTE using observations of star clusters.