GeMS/GSAOI photometric and astrometric performance in dense stellar
  fields
  Ground-based imagers at 8m class telescopes assisted by Multi conjugate
Adaptive Optics are primary facilities 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 Full Width
at Half Maximum (FWHM), Strehl Ratio (SR) and Encircled Energy (EE), over the
field of view. We found that, for sub-arcsec seeing at the observed airmass,
diffraction limit PSF FWHM ($\approx$ 80 mas), SR $\sim40\%$ and EE $\ge50\%$
with a dispersion around $10\%$ over the 85" x 85" field of view, can be
obtained 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 worsen 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 $\sim$1 mas in a stellar field with high
crowding.
