Principal Component Analysis for ACS/WFC Superbias Temporal Variation

TL;DR

This study analyzes the long-term stability of ACS/WFC superbias calibration frames over 15 years, using PCA, and assesses whether less frequent updates could maintain calibration quality.

Contribution

It demonstrates that superbias frames have remained stable over 15 years and evaluates the potential for reducing update frequency without compromising calibration accuracy.

Findings

Superbias structure has remained stable post-SM4.

No significant global trends or shifts detected in superbias frames.

More frequent updates may be needed due to hot columns and increasing dark noise.

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.