Astrometric performance of the Gemini multi-conjugate adaptive optics system in crowded fields

TL;DR

This paper evaluates the astrometric capabilities of the Gemini Multi-conjugate adaptive optics System (GeMS) in crowded fields, highlighting its high precision in single-epoch data and identifying limitations in multi-epoch observations due to systematic errors.

Contribution

It provides a detailed assessment of GeMS/GSAOI's astrometric performance, including error sources and limitations in multi-epoch measurements.

Findings

Achieves <0.2 mas error in single-epoch, un-dithered data with sufficient stars.

Multi-epoch data shows an additional systematic ~0.4 mas error.

Time-variable distortion from gravity flexure is a major error source.

Abstract

The Gemini Multi-conjugate adaptive optics System (GeMS) is a facility instrument for the Gemini-South telescope. It delivers uniform, near-diffraction-limited image quality at near-infrared wavelengths over a 2 arcminute field of view. Together with the Gemini South Adaptive Optics Imager (GSAOI), a near-infrared wide field camera, GeMS/GSAOI's combination of high spatial resolution and a large field of view will make it a premier facility for precision astrometry. Potential astrometric science cases cover a broad range of topics including exo-planets, star formation, stellar evolution, star clusters, nearby galaxies, black holes and neutron stars, and the Galactic center. In this paper, we assess the astrometric performance and limitations of GeMS/GSAOI. In particular, we analyze deep, mono-epoch images, multi-epoch data and distortion calibration. We find that for single-epoch, un-dithered data, an astrometric error below 0.2 mas can be achieved for exposure times exceeding one minute, provided enough stars are available to remove high-order distortions. We show however that such performance is not reproducible for multi-epoch observations, and an additional systematic error of ~0.4 mas is evidenced. This systematic multi-epoch error is the dominant error term in the GeMS/GSAOI astrometric error budget, and it is thought to be due to time-variable distortion induced by gravity flexure.