Measuring High-Precision Astrometry with the Infrared Array Camera on the Spitzer Space Telescope

TL;DR

This paper enhances the astrometric precision of the Spitzer Space Telescope's IRAC instrument by optimizing position estimation routines and deriving new distortion corrections, enabling more accurate measurements of faint mid-IR sources.

Contribution

It introduces improved distortion corrections and demonstrates that PRF fitting yields minimal errors, significantly advancing high-precision astrometry with IRAC.

Findings

Achieved ~20 mas astrometric accuracy at SNR 100.

Derived new 7th and 8th order distortion corrections.

Validated improvements using archival star-forming region data.

Abstract

The Infrared Array Camera (IRAC) on the Spitzer Space Telescope currently offers the greatest potential for high-precision astrometry of faint mid-IR sources across arcminute-scale fields, which would be especially valuable for measuring parallaxes of cold brown dwarfs in the solar neighborhood and proper motions of obscured members of nearby star-forming regions. To more fully realize IRAC's astrometric capabilities, we have sought to minimize the largest sources of uncertainty in astrometry with its 3.6 and 4.5 $\mu$m bands. By comparing different routines that estimate stellar positions, we have found that Point Response Function (PRF) fitting with the Spitzer Science Center's Astronomical Point Source Extractor produces both the smallest systematic errors from varying intra-pixel sensitivity and the greatest precision in measurements of positions. In addition, self-calibration has been used to derive new 7$^{\rm th}$ and 8$^{\rm th}$ order distortion corrections for the 3.6 and 4.5 $\mu$m arrays of IRAC, respectively. These corrections are suitable for data throughout the mission of Spitzer when a time-dependent scale factor is applied to the corrections. To illustrate the astrometric accuracy that can be achieved by combining PRF fitting with our new distortion corrections, we have applied them to archival data for a nearby star-forming region, arriving at total astrometric errors of $\sim$20 and 70 mas at signal to noise ratios of 100 and 10, respectively.