2016ApJ...833..111D

Ground-based imagers at 8 m class telescopes assisted by multi-conjugate adaptive optics are primary facilities with which to obtain accurate photometry and proper motions in dense stellar fields. We observed the central region of the globular clusters Liller 1 and NGC 6624 with the Gemini Multi-conjugate Adaptive Optics System (GeMS) feeding the Gemini South Adaptive Optics Imager (GSAOI) currently available at the Gemini South telescope, under different observing conditions. We characterized the stellar point-spread function (PSF) in terms of FWHM, Strehl ratio (SR), and encircled energy (EE), over the field of view (FOV). We found that, for sub-arcsecond seeing at the observed airmass, we can obtain the diffraction-limited PSF (FWHM ~80 mas), SR ∼ 40%, and EE >= 50% with a dispersion around 10% over the FOV of 85'' x 85'', in the K_s band. In the J band the best images provide FWHMs between 60 and 80 mas, SR >10%, and EE>40%. For seeing at the observed airmass exceeding 1'', the performance worsens but it is still possible to perform PSF fitting photometry with 25% EE in J and 40% in K_s. We also computed the geometric distortions of GeMS/GSAOI and we obtained corrected images with an astrometric accuracy of ∼1 mas in a stellar field with high crowding.