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WFC3/IR Geometric Distortion - Time Evolution of Linear Terms w.r.t. Gaia DR3

Anne O'Connor, Varun Bajaj

TL;DR

This study investigates the long-term stability of WFC3/IR geometric distortion by comparing the IDCTAB-derived WCS to Gaia DR3-aligned WCS across 2009–2024 using the MAST pipeline. The analysis finds no significant linear evolution in the rotation, skew, or scale terms, while the shift term shows time-dependent offsets dominated by external pointing errors, with notable discontinuities around 2018–2021 that align with known jitter periods. The results indicate that the IDCTAB reference file remains largely valid over the 16-year lifetime, though small residual offsets persist and are largely corrected by standard image alignment with TweakReg or the MAST pipeline. Practically, observers should continue to verify image alignment for high-precision astrometry, as the shift term is influenced by telescope pointing and external factors beyond the distortion model.

Abstract

We examine the relative offsets of the linear terms in the geometric distortion between WFC3/IR and the Gaia DR3 catalog using the Mikulski Archive for Space Telescopes (MAST) pipeline WFC3/IR to Gaia DR3 alignment solutions to assess temporal stability over the lifetime of the WFC3 instrument (2009-2024). We find a period of increased uncertainty and offsets in the rotation term between 2018 and 2021, as seen in a previous analysis of WFC3/UVIS linear geometric distortion (O'Connor et al., 2024), corresponding with a period of increased jitter. We find a similar pattern of increased uncertainty between 2018 and 2021 in the skew offsets to Gaia DR3, as well. We find no significant linear temporal evolution in the rotation, skew, or scale offsets between the WFC3/IR IDCTAB distortion solution and Gaia DR3 over the 16-year lifetime of the WFC3 instrument; however, we do see temporal evolution in the shift offsets (the difference -in pixels- between the IDCTAB and Gaia WCS positions), which are dominated by telescope pointing inaccuracy external to the WFC3/IR geometric distortion solution. For observers requiring high-precision astrometry, we continue to recommend that observers verify or improve image alignment using the tweakreg routine.

WFC3/IR Geometric Distortion - Time Evolution of Linear Terms w.r.t. Gaia DR3

TL;DR

This study investigates the long-term stability of WFC3/IR geometric distortion by comparing the IDCTAB-derived WCS to Gaia DR3-aligned WCS across 2009–2024 using the MAST pipeline. The analysis finds no significant linear evolution in the rotation, skew, or scale terms, while the shift term shows time-dependent offsets dominated by external pointing errors, with notable discontinuities around 2018–2021 that align with known jitter periods. The results indicate that the IDCTAB reference file remains largely valid over the 16-year lifetime, though small residual offsets persist and are largely corrected by standard image alignment with TweakReg or the MAST pipeline. Practically, observers should continue to verify image alignment for high-precision astrometry, as the shift term is influenced by telescope pointing and external factors beyond the distortion model.

Abstract

We examine the relative offsets of the linear terms in the geometric distortion between WFC3/IR and the Gaia DR3 catalog using the Mikulski Archive for Space Telescopes (MAST) pipeline WFC3/IR to Gaia DR3 alignment solutions to assess temporal stability over the lifetime of the WFC3 instrument (2009-2024). We find a period of increased uncertainty and offsets in the rotation term between 2018 and 2021, as seen in a previous analysis of WFC3/UVIS linear geometric distortion (O'Connor et al., 2024), corresponding with a period of increased jitter. We find a similar pattern of increased uncertainty between 2018 and 2021 in the skew offsets to Gaia DR3, as well. We find no significant linear temporal evolution in the rotation, skew, or scale offsets between the WFC3/IR IDCTAB distortion solution and Gaia DR3 over the 16-year lifetime of the WFC3 instrument; however, we do see temporal evolution in the shift offsets (the difference -in pixels- between the IDCTAB and Gaia WCS positions), which are dominated by telescope pointing inaccuracy external to the WFC3/IR geometric distortion solution. For observers requiring high-precision astrometry, we continue to recommend that observers verify or improve image alignment using the tweakreg routine.

Paper Structure

This paper contains 16 sections, 3 equations, 12 figures.

Table of Contents

  1. Introduction
  2. WFC3/IR Geometric Distortion
  3. Data and Methodology
  4. WFC3/IR Data Sets For Testing
  5. Methodology
  6. MAST Pipeline: Image Alignment
  7. Extracting Transform Components from the WCS
  8. Clipping the Data
  9. Results
  10. Relative Shift Offsets
  11. Relative Rotation Offsets
  12. Relative Scale Offsets
  13. Relative Skew Offsets
  14. Discussion
  15. Conclusions
  16. ...and 1 more sections

Figures (12)

  • Figure 1: Shift term offsets, WFC3/IR to Gaia DR3, by filter.
  • Figure 2: Shift term offsets, WFC3/IR to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
  • Figure 3: Rotation term offsets: WFC3/IR to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
  • Figure 4: Rotation term offsets by filter: WFC3/IR to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
  • Figure 5: Scale ratio: WFC3/IR to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
  • ...and 7 more figures