Revised Astrometric Calibration of the Gemini Planet Imager

TL;DR

This paper revises the astrometric calibration of the Gemini Planet Imager by identifying and correcting issues in the data reduction pipeline, resulting in more accurate measurements of celestial objects.

Contribution

The authors provide a corrected astrometric calibration for GPI, improving the accuracy of position angle and separation measurements for astronomical observations.

Findings

Updated plate scale and north offset angle for GPI

Reduced systematic errors in astrometric measurements

Enhanced precision in GPI's astrometric data

Abstract

We present a revision to the astrometric calibration of the Gemini Planet Imager (GPI), an instrument designed to achieve the high contrast at small angular separations necessary to image substellar and planetary-mass companions around nearby, young stars. We identified several issues with the GPI Data Reduction Pipeline (DRP) that significantly affected the determination of angle of north in reduced GPI images. As well as introducing a small error in position angle measurements for targets observed at small zenith distances, this error led to a significant error in the previous astrometric calibration that has affected all subsequent astrometric measurements. We present a detailed description of these issues, and how they were corrected. We reduced GPI observations of calibration binaries taken periodically since the instrument was commissioned in 2014 using an updated version of the DRP. These measurements were compared to observations obtained with the NIRC2 instrument on Keck II, an instrument with an excellent astrometric calibration, allowing us to derive an updated plate scale and north offset angle for GPI. This revised astrometric calibration should be used to calibrate all measurements obtained with GPI for the purposes of precision astrometry.