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WFC3/UVIS: External CTE Monitoring 2009-2024

Benjamin Kuhn

TL;DR

The paper analyzes long-term CTE flux losses in the WFC3/UVIS detector from 2009 to 2024, using star-cluster calibrations and varying backgrounds achieved via post-flash to map losses as a function of flux, background, and readout distance. It employs a bi-variate empirical flux-loss correction (Noeske2012 framework) to non-CTE-corrected FLT data and contrasts results with pixel-based FLC corrections, delivering updated coefficients (Tables 6–9) for use by observers. The study finds that CTE losses intensify with time and are mitigated by higher backgrounds, but FLC corrections can over-correct at backgrounds above ~40 e$^-$ pix$^{-1}$, highlighting the need for careful application. The results establish practical guidance (e.g., 20–25 e$^-$ pix$^{-1}$ background) and provide a comprehensive data set and coefficients to support ongoing CTE monitoring and correction refinements.

Abstract

This report examines Charge Transfer Efficiency (CTE) flux losses in the Wide Field Camera 3 UVIS detector aboard the Hubble Space Telescope. Spanning approximately 14 years of observations from October 2009 to February 2024, the study analyzes CTE flux loss trends across various total background levels and source fluxes. In addition to analyzing the present state of CTE flux losses, we provide updated coefficients for the empirical model for point source photometry corrections in both non-CTE-corrected (FLT) and CTE-corrected (FLC) data. Between 2009 and 2023, the rate of CTE flux loss for a 500-2000 e$^-$ source, farthest from the readout, in FLT data with a 1-3 e$^-$/pix background, is measured to be $\sim$0.05 $Δ$mag/2051 pix/year. The recommended minimum total background level to mitigate CTE losses remains at 20-25 e$^-$/pix. At that level, we find that 500-2000 e$^-$ sources, farthest from the readout, in 2024 FLT data can suffer $\sim$23$\%$ flux loss/2051 pix. The FLC data provide some relief, but we measure a $\sim$12$\%$ flux loss/2051 pix in 2024. There continues to be a slight over-correction in some FLC results that contain backgrounds above 40 e$^-$/pix. In 2024, 8000-20000 e$^-$ sources farthest from the readout in a 40, 60, or 90 e$^-$/pix background are over-corrected by $\sim$1, 2, and 3$\%$, respectively.

WFC3/UVIS: External CTE Monitoring 2009-2024

TL;DR

The paper analyzes long-term CTE flux losses in the WFC3/UVIS detector from 2009 to 2024, using star-cluster calibrations and varying backgrounds achieved via post-flash to map losses as a function of flux, background, and readout distance. It employs a bi-variate empirical flux-loss correction (Noeske2012 framework) to non-CTE-corrected FLT data and contrasts results with pixel-based FLC corrections, delivering updated coefficients (Tables 6–9) for use by observers. The study finds that CTE losses intensify with time and are mitigated by higher backgrounds, but FLC corrections can over-correct at backgrounds above ~40 e pix, highlighting the need for careful application. The results establish practical guidance (e.g., 20–25 e pix background) and provide a comprehensive data set and coefficients to support ongoing CTE monitoring and correction refinements.

Abstract

This report examines Charge Transfer Efficiency (CTE) flux losses in the Wide Field Camera 3 UVIS detector aboard the Hubble Space Telescope. Spanning approximately 14 years of observations from October 2009 to February 2024, the study analyzes CTE flux loss trends across various total background levels and source fluxes. In addition to analyzing the present state of CTE flux losses, we provide updated coefficients for the empirical model for point source photometry corrections in both non-CTE-corrected (FLT) and CTE-corrected (FLC) data. Between 2009 and 2023, the rate of CTE flux loss for a 500-2000 e source, farthest from the readout, in FLT data with a 1-3 e/pix background, is measured to be 0.05 mag/2051 pix/year. The recommended minimum total background level to mitigate CTE losses remains at 20-25 e/pix. At that level, we find that 500-2000 e sources, farthest from the readout, in 2024 FLT data can suffer 23 flux loss/2051 pix. The FLC data provide some relief, but we measure a 12 flux loss/2051 pix in 2024. There continues to be a slight over-correction in some FLC results that contain backgrounds above 40 e/pix. In 2024, 8000-20000 e sources farthest from the readout in a 40, 60, or 90 e/pix background are over-corrected by 1, 2, and 3, respectively.
Paper Structure (7 sections, 3 equations, 3 figures, 9 tables)

This paper contains 7 sections, 3 equations, 3 figures, 9 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Results
  4. CTE Flux Losses as a Function of Flux Bin
  5. CTE Flux Losses as a Function of Date
  6. Tabulated CTE Flux Losses Results
  7. Conclusion

Figures (3)

  • Figure 1: Residual CTE flux losses as a function of $log_{10}(flux)$ separated by approximate total background level in e$^-$/pix. Results for non-CTE-corrected FLT files and CTE-corrected FLC files are on the left and right, respectively (notated in red lettering at the top right of each panel). The upper x-axis on each panel shows the approximate central pixel flux in electrons. All the data come from February 16, 2024 observations of Omega Centauri.
  • Figure 2: Residual CTE flux losses as a function of log$_{10}$ flux separated into two different background levels. The top row shows the evolution of losses in a $\sim$ 1-3 e$^-$/pix background between 2009-2024. The bottom row shows results in a $\sim$ 20-25 e$^-$/pix background between 2016-2024. Results for non-CTE-corrected FLT files and CTE-corrected FLC files are in the left and right columns, respectively. The color coding corresponds to the observation date. Note that the y-axis range for the top and bottom rows differ. The top row spans -0.1 through 1.2, and the bottom ranges from -0.04 to 0.4.
  • Figure 3: Residual CTE flux losses as a function of observation date separated into three main flux bins. As the legends show, the flux bins are 500-2000, 2000-8000, and 8000-32000 e$^-$. The top row shows the evolution of losses in a $\sim$ 1-3 e$^-$/pix background between 2009-2024. The bottom row shows results in a $\sim$ 20-25 e$^-$/pix background between 2016-2024. Results for non-CTE-corrected FLT files and CTE-corrected FLC files are in the left and right columns, respectively.