Photometry and astrometry with anisoplanatic AO images

We present a new approach to address the anisoplanatism in very crowded regions. We studied photometric and astrometric measurements from adaptive optics (AO) observations of the Galactic Bulge, taken during ESO science verification runs in 2002. We compared H and K VLT/NACO observations of a crowded field with HST/NICMOS H-band data and NTT/SOFI K-band data for the same field. The AO image was affected by anisoplanatism, with the natural guide star just outside the 27.6" x 27.6" field of view in both the H and K bands. We wanted to address the question of the AO image photometric and astrometric precision, compared with analogous HST data taken as the "truth", even in presence of anisoplanatism. We showed that a subdivision of the entire region in subfields in which the PSF is constant produces reliable photometry and astrometry. The average PSFs retrieved for each subfield in both the H and K bands differ due to anisoplanatism, to contamination from the NGS halo and to the frame selection. Even so, the photometric and the astrometric results show very little sensitivity to these PSF variations between the subfields.