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Post-SM4 Internal CTE for WFC


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EPER CTE values were measured (manual region selections) with algorithms that are similar to ACS-R SMOV Optimization (ACS TIR 2009-01). FPR CTE values were measured with algorithms that are similar to Internal Monitoring of ACS CTE (ACS ISR 2005-03).

From post-SM4 data, it seems that the coefficients in ACS ISR 2005-03 now underestimate the EPER parallel CTE loss. M. Mutchler pointed out that CTE has been degrading faster since sometime between default gain and temperature change (in early 2006), which was after the ISR was published (2009; private communication).

There is no post-SM4 data for HRC.

CTE Equation & Fitted Coefficients

This is the equation defined on Page 12 of ACS ISR 2005-03:

CTE(s,d) = 1.0 - (n + c*(d-52335))*(sp)

Here are the explanations of the variables and coefficients:

Name Type Explanation
CTE Measured Charge transfer efficiency.
s Measured Signal level in electrons.
d Known MJD of observation. Zero point is set around SM3b launch (52335).
n Fitted coefficient Multiplier of the power law at launch.
c Fitted coefficient Rate of increase of the multiplier.
p Fitted coefficient Power of the power law.

CTE Tail Profiles for EPER

The upper and lower panels are EPER CTE tails for serial and parallel CTE, respectively. Each line is bias-subtracted signal (electrons) taken from an image with roughly the same signal level from epoch as mentioned in legend. Effects of horizontal streaking from the new electronics in CEB-R are visible in Amps A and D plots; they sometimes make post-SM4 EPER CTE measurements more challenging.

EPER 1620e Amp A
EPER 1620e Amp B
EPER 1620e Amp C
EPER 1620e Amp D

CTE Tail Profiles for FPR

Only serial CTE is available for FPR. Each line is bias-subtracted signal (normalized to average signal) taken from an image with roughly the same signal level from epoch as mentioned in legend. FPR is measured at quadrant boundary far from physical overscan; hence, bias gradient change in ACS-R Dual-Slope Integration (DSI) should not affect the results.

FPR 1620e Amp A
FPR 1620e Amp B
FPR 1620e Amp C
FPR 1620e Amp D