Precise photometry and astrometry in the core of the globular cluster NGC 6121 using PSF-R techniques

Precise stellar photometry and astrometry require the best possible modelling of the point spread function (PSF). To date, the best performances have been obtained when building the PSF a posteriori, meaning directly from the image of dense stellar fields, by exploiting the fact that each star represents a different realisation of the same PSF. The recent advent of the Adaptive Optics technique makes this method more challenging, because of the strong PSF variations across the field of view. One alternative is to use a priori PSF-modelling techniques such as PSF-reconstruction (PSF-R), that rely on Adaptive Optics control loop data to determine the shape of the PSF at any spatial location. Despite being theoretically well established, so far a-priori methods have never surpassed the performance obtained by standard methods when applied to real astronomical imaging. Here we report on the successful use of PRIME, a new technique that combines both PSF-R and image fitting, to perform precise photometry and astrometry on real data of the Galactic globular cluster NGC6121, observed with SPHERE/ZIMPOL. Compared to the results obtained using standard techniques, PRIME achieves improvement in precision by up to a factor of four, and ensures a photometric accuracy within ∼ 0.1 mag. A similar performance is also achieved when using the analytical PSF method described by F´etick et al. 2019, which is specifically designed to model AO-assisted data. These results thus pave the way for the exploitation of innovative techniques to investigate resolved stellar population science cases with the new generation of Adaptive Optics-assisted instrumentation at the ESO’s Very Large Telescope, Keck or the Extremely Large Telescopes.