Optimal Differential Astrometry for Multiconjugate Adaptive Optics. I. Astrometric Distortion Mapping using On-sky GeMS Observations of NGC 6723

Abstract The Extremely Large Telescope and the Thirty Meter will use state of art multiconjugate adaptive optics (MCAO) systems to obtain full D 4 advantage that their apertures can provide. However, reach astrometric potential these facilities for on-sky science requires understanding any residual distortions from find ways measure eliminate them. In this work, we Gemini optic system (GeMS) observations core NGC 6723 better understand performance MCAO. We develop new methods distortion fields observing system, which probe at highest possible spatial resolution. also describe examining time-variable static components distortion. When applied GeMS South Adaptive Optics Imager (GSAOI) data, are able see effect field rotator subpixel level, empirically derive due optical design GeMS/GSAOI. argue resulting maps a valuable tool monitor future instrumentation. Our overall astrometry pipeline produces high-quality proper motions with an uncertainty floor ?45 ? as yr ?1 . motion dispersion profile radius ?10? out ?1? based on ?12,000 stars. produce optical–near infrared color–magnitude diagram, clearly shows extreme horizontal branch main-sequence knee cluster.