Internal CTE Monitoring for WFC

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.

WFC Parallel EPER

 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. 

Modeling the CTE

Parallel CTE from EPER data has a power-law dependence on signal level and a linear dependence on time. The best-fitting power-law index for the dependence on signal level is 0.5 ± 0.01. 

The rates of decrease of CTE as a function of time are given in the table below. They are measured for specific, standard signal levels obtained regularly since SM4.

The above rates of CTE decrease are determined from direct linear fits to the CTE trends, rather than a combined power-law (signal level) and linear (time) model as described in ACS ISRs 2018-09 and 2005-03. The combined model struggled to accurately fit the CTE decrease with time, likely due to CTE re-trailing in the EPER overscans.