GeMS/GSAOI PHOTOMETRIC and ASTROMETRIC PERFORMANCE in DENSE STELLAR FIELDS

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 subarcsecond seeing at the observed airmass, we can obtain the diffraction-limited PSF (FWHM 7ap;80 mas), SR ∼ 40%, and EE ≫50% with a dispersion around 10% over the FOV of 85" × 85?" in the Ks 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 Ks. 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.