WFC3/UVIS EPER CTE 2009-2025

TL;DR

This study analyzes the decline of Charge Transfer Efficiency in the WFC3/UVIS detector from 2009 to 2025, revealing a non-linear decline better modeled by quadratic or cubic functions and identifying an approximately 8-9 year periodic oscillation in residuals.

Contribution

It provides a detailed analysis of CTE decline over time using EPER data, demonstrating that a quadratic or cubic model fits the data better than a linear one and identifying periodic residual oscillations.

Findings

CTE declines at a rate of 0.0001 per year at the lowest signal level.

Quadratic and cubic models fit the CTE decline data better than linear models.

Residuals show an approximately 8-9 year periodic oscillation.

Abstract

In this report, we examine the behavior of Charge Transfer Efficiency (CTE) on the WFC3/UVIS detector over time as computed by the Extended Pixel Edge Response (EPER) technique, using internal calibration data acquired from 2009 through 2025. We find that the CTE has continued to decline as expected, with a steeper loss rate for lower signal levels. The lowest signal level (160e-) has continued to decline at a rate of 0.0001 per year, with a total overall decline of 0.0015. Analyses from 2016 and 2020 found that the rate of decline was not well fit by a linear function. This report verifies the rate of decline is instead better fit by a quadratic function (which results in the smallest min. and max. residuals, on average) or a cubic function (which has the best "goodness of fit" $\chi^2$ and $R^2$ values). We continue to see periodic oscillations of the residuals around all three fit lines (linear, quadratic, and cubic) on which we perform a Lomb-Scargle periodogram analysis of the residuals. We find a periodicity of about 8 years for the residuals around the linear fit lines and about 9 years for the quadratic and cubic fit lines.