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Principal Component Analysis for ACS/WFC Superbias Temporal Variation

Alyssa M. Guzman, Norman A. Grogin

Abstract

We examined the long-term behavior of the superbias calibration frames for the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) aboard the Hubble Space Telescope (HST). Superbias frames are used to remove detector-level bias structure from science images and are currently generated after an anneal and delivered monthly. The primary goal of this study was to determine whether the frequency of superbias generation could be reduced without compromising calibration quality, potentially aligning with the Wide Field Camera 3 UVIS (WFC3/UVIS) approach of generating only one superbias per year. We analyzed superbias frames produced from 2007 through 2024 to investigate whether these calibration products have changed significantly over time, and whether the frequency of superbias generation and delivery could be safely reduced without loss of calibration accuracy. In addition to visual inspections and pixel-level comparisons, we employed Principal Component Analysis (PCA) to evaluate whether any long-term, global structure exists beneath the apparent noise in these frames. Our findings show that the superbias structure has remained fairly stable post-Servicing Mission 4 (SM4), a 15-year period, and no significant or unexpected global trends or systematic shifts were detected. However, due to unstable hot columns and increasing readout dark observed in ACS/WFC data, it is likely that these calibrations still benefit from more frequent superbias updates than the annual cadence adopted for WFC3/UVIS.

Principal Component Analysis for ACS/WFC Superbias Temporal Variation

Abstract

We examined the long-term behavior of the superbias calibration frames for the Advanced Camera for Surveys Wide Field Channel (ACS/WFC) aboard the Hubble Space Telescope (HST). Superbias frames are used to remove detector-level bias structure from science images and are currently generated after an anneal and delivered monthly. The primary goal of this study was to determine whether the frequency of superbias generation could be reduced without compromising calibration quality, potentially aligning with the Wide Field Camera 3 UVIS (WFC3/UVIS) approach of generating only one superbias per year. We analyzed superbias frames produced from 2007 through 2024 to investigate whether these calibration products have changed significantly over time, and whether the frequency of superbias generation and delivery could be safely reduced without loss of calibration accuracy. In addition to visual inspections and pixel-level comparisons, we employed Principal Component Analysis (PCA) to evaluate whether any long-term, global structure exists beneath the apparent noise in these frames. Our findings show that the superbias structure has remained fairly stable post-Servicing Mission 4 (SM4), a 15-year period, and no significant or unexpected global trends or systematic shifts were detected. However, due to unstable hot columns and increasing readout dark observed in ACS/WFC data, it is likely that these calibrations still benefit from more frequent superbias updates than the annual cadence adopted for WFC3/UVIS.
Paper Structure (8 sections, 2 equations, 13 figures)

This paper contains 8 sections, 2 equations, 13 figures.

Table of Contents

  1. Introduction
  2. Data & Analysis
  3. Linear Regression Analysis
  4. Principal Component Analysis
  5. Results
  6. Linear Regression Analysis
  7. Principal Component Analysis
  8. Conclusion

Figures (13)

  • Figure 1: The layout of ACS/WFC full-frame images, including parallel and serial readout directions, physical pre-scans, and virtual overscans acs_ihb.
  • Figure 2: The layout of the WFC3/UVIS full frame detector wfc3_ihb.
  • Figure 3: First (2007) and last (2024) original standardized superbias frames used in this analysis, with WFC1 shown at the top and WFC2 at the bottom.
  • Figure 4: First (2007) and last (2024) standardized superbias frames with gradients applied along the $x$-axis (top) and $y$-axis (bottom).
  • Figure 5: PCA variance plots for the original superbias frames. Left: Variance ratio explained by each component, showing a steep drop after the first few components for both chips, followed by a flattening trend (“elbow”). Right: Same variance data on a logarithmic scale, revealing the long tail of minor variance contributions beyond the top 20 components. There are 127 total PCs for WFC1 and 126 components for WFC2.
  • ...and 8 more figures