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

TL;DR

This study addresses the challenge of achieving high-precision astrometry with WFC3/UVIS by quantifying how the linear terms of the geometric distortion solution evolve when referenced to Gaia DR3 over a $13$-year baseline. Using $7{,}491$ well-fit full-frame UVIS exposures across five filters, the authors compute Gaia-aligned WCSs and compare them to the initial IDCTAB-based WCSs to extract temporal changes in shift, rotation, scale, and skew, reporting small evolutions ($<0.2$ px over 13 years) plus notable filter-dependent scale offsets up to $0.3$ px. Rotation offsets remain small but show increased scatter after 2017, coinciding with higher pointing jitter; the dominant source of uncertainty is telescope pointing rather than distortion itself. The results support continuing to realign UVIS data with tweakreg for high-precision work, and they prompt future efforts to implement time-dependent distortion corrections in the WFC3/UVIS calibration files and to explore global scale corrections with Gaia. Overall, the paper provides actionable guidance for observers to maintain sub-pixel astrometric accuracy across long baselines by combining Gaia-based alignment with standard DrizzlePac tools.

Abstract

We align more than 7,400 WFC3/UVIS exposures to the Gaia DR3 catalog to examine the time evolution of the linear terms (shift, rotation, scale and skew) of the geometric distortion solution between 2009 and 2022. We find small linear temporal changes in the scale and skew terms (less than 0.2 pixels in 13 years) which are generally dominated by intrinsic scatter (up to $\pm$ 0.3 pixels). Concurrently, a larger filter-dependent offset in the scale term is observed, with a maximum difference of 0.3 pixels between F275W and F814W images at all epochs. A small rotation offset to Gaia of 0.003 $\pm$ 0.004 degrees is measured from 2009 to mid-2017, after which the offsets are as large as 0.01 degrees, with a large scatter. MAST pipeline processing includes an additional alignment step which corrects UVIS images for any residual linear terms with respect to Gaia DR3 when there are at least 10 matched sources. In addition to any pointing offsets, this step accounts for any evolution in the distortion linear terms described here. For observers requiring high-precision astrometry, we recommend using the tweakreg routine to realign images using a 4-parameter fit (x-shift, y-shift, rotation, and scale) or a 6-parameter fit (x-shift, y-shift, x-rotation, y-rotation, x-scale, and y-scale) depending on the number of matched sources. We provide links to DrizzlePac tutorials for improving both absolute and relative astrometry in WFC3 images.

Figures (10)

• Figure 1: RMSE of each image's fit to Gaia DR3 vs. the number of matches. A smaller RMSE indicates a more accurate fit. We cut out any images with an RMSE above 0.2.
• Figure 2: Shift term offsets, WFC3/UVIS to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
• Figure 3: Shift term offsets, WFC3/UVIS to Gaia DR3, by filter.
• Figure 4: Rotation term offsets: WFC3/UVIS to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
• Figure 5: Rotation term offsets by filter: WFC3/UVIS to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.