Simulations of ELT-GMCAO performance for deep field observations
  The Global-Multi Conjugated Adaptive Optics (GMCAO) approach offers an
alternative way to correct an adequate scientific Field of View (FoV) using
only natural guide stars (NGSs) to extremely large ground-based telescopes.
Thus, even in the absence of laser guide stars, a GMCAO-equipped ELT-like
telescope can achieve optimal performance in terms of Strehl Ratio (SR),
retrieving impressive results in studying star-poor fields, as in the cases of
the deep field observations. The benefits and usability of GMCAO have been
demonstrated by studying 6000 mock high redshift galaxies in the Chandra Deep
Field South region. However, a systematic study simulating observations in
several portions of the sky is mandatory to have a robust statistic of the
GMCAO performance. Technical, tomographic and astrophysical parameters,
discussed here, are given as inputs to GIUSTO, an IDL-based code that estimates
the SR over the considered field, and the results are analyzed with statistical
considerations. The best performance is obtained using stars that are
relatively close to the Scientific FoV; therefore, the SR correlates with the
mean off-axis position of NGSs, as expected, while their magnitude plays a
secondary role. This study concludes that the SRs correlate linearly with the
galactic latitude, as also expected. Because of the lack of natural guide stars
needed for low-order aberration sensing, the GMCAO confirms as a promising
technique to observe regions that can not be studied without the use of laser
beacons. It represents a robust alternative way or a risk mitigation strategy
for laser approaches on the ELTs.
