A Comprehensive Astrometric Calibration of HST's WFPC2. I. Distortion Mapping

TL;DR

This paper presents a detailed calibration of the geometric distortions in HST's WFPC2 camera, improving astrometric accuracy for long-term proper-motion studies by analyzing extensive archival data and refining distortion correction methods.

Contribution

It provides a comprehensive analysis of WFPC2 distortions beyond previous models, offering updated correction maps and coefficients for enhanced astrometric precision.

Findings

No high-frequency systematics beyond 34th-row correction detected.

Low-frequency systematics of 30-50 mas identified at 50-pixel resolution.

Updated distortion coefficients and correction maps provided for each filter.

Abstract

Wide field planetary camera 2 (WFPC2) exposures are already some 20 years older than Gaia epoch observations, or future JWST observations. As such, they offer an unprecedented time baseline for high-precision proper-motion studies, provided the full astrometric potential of these exposures is reached. We have started such a project with the work presented here being its first step. We explore geometric distortions beyond the well-known ones published in the early 2000s. This task is accomplished by using the entire database of WFPC2 exposures in filters F555W, F606W and F814W and three standard astrometric catalogs: Gaia EDR3, 47 Tuc and $\omega$Cen. The latter two were constructed using HST observations made with cameras other than WFPC2. We explore a suite of centering algorithms, and various distortion maps in order to understand and quantify their performance. We find no high-frequency systematics beyond the 34th-row correction, down to a resolution of 10 pixels. Low-frequency systematics starting at a resolution of 50-pixels are present at a level of 30-50 millipix (1.4-2.3 mas) for the PC and 20-30 millipix (2-3 mas) for the WF chips. We characterize these low-frequency systematics by providing correction maps and updated cubic-distortion coefficients for each filter.