Introduction

Internal monitoring of the decay of parallel CTE of the ACS/WFC CCDs is accomplished with the extended pixel edge response (EPER) test. EPER images have extra-large overscans in which to accumulate trapped charge during readout of the lamp-illuminated active pixels. Comparing the amount of trapped charge in the overscans to the signal level of the illuminated pixels provides an estimate of CTE per pixel. EPER images of various signal levels have been obtained since ACS was installed on HST, which provides a long baseline over which to track CTE losses. Recent analysis of parallel CTE for ACS/WFC is described here (ACS ISR 2018-09). Future work includes an analysis of serial CTE in WFC from EPER data.

EPER images have extra-large overscans in which to accumulate trapped charge during readout of the lamp-illuminated active pixels. Comparing the amount of trapped charge in the overscans to the signal level of the illuminated pixels provides an estimate of CTE per pixel. EPER images of various signal levels have been obtained since ACS was installed on HST, which provides a long baseline over which to track CTE losses. The

Modeling the CTE

Parallel CTE from EPER data has a power law dependence on signal level (s) and linear dependence on time (d) described by,

$$CTE(s,d) = 1.0 - (n+c(d-52335))s^p$$

with the variables and coefficients defined below:

• CTE := Charge transfer efficiency measured from EPER image
• s := signal level in electrons.
• d := MJD date of the observations
• n := intercept of the linear time dependence
• c := slope of the linear time dependence
• p :=  exponent of the power law dependence

The best-fit coefficients quoted below are from the most recent analysis of EPER data (ACS ISR 2018-09).

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Epoch n c p
Pre-SM4 $$6.8×10^{−5} ± 4×10^{−6}$$ $$1.66×10^{−6} ± 4×10^{−8}$$ $$-0.528 \pm 0.003$$
Post-SM4 $$−1.3×10^{−4} ± 6×10^{−5}$$ $$2.00×10^{−6} ± 3×10^{−8}$$ $$-0.521 \pm 0.002$$

The rates of decrease of CTE as measured from EPER data for specific signal levels are given in the table below.

Signal level (e-) Rate of decrease of CTE per year
$$180$$

$$−4.9×10^{−5}$$

$$430$$ $$−3.1×10^{−5}$$
$$1600$$ $$−1.5×10^{−5}$$
$$3400$$ $$−1.1×10^{−5}$$
$$7100$$ $$−7.2×10^{−6}$$
$$42000$$ $$−2.9×10^{−6}$$

LAST UPDATED: 06/17/2019