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

摘要

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