images the software presented by Anderson et al.(2008) for images collected with the Wide Field Channel of the Advanced Camera for Survey (WFC/ACS). In a nutshell, the software performs Point-Spread-Function fitting of all the sources in the field of view. It uses two different approaches that provide optimal results for bright and faint stars, respectively. The fluxes and positions of bright stars are measured in each image separately, and then averaged. Faint stars in a given patch of the sky are fitted simultaneously by using all the images, once transformed in a common reference frame. To do this, the software exploits library PSFs and the geometric-distortion solution by Jay Anderson[^3].

We calibrated our photometry to the Vega-magnitude system by following the procedure described by Bedin et al.(2005) and adopting the photometric zero points provided by STScI web page[^4] for WFC3/NIR. We used various diagnostics of the photometric and astrometric qualities to select a sample of relatively isolated stars that are well fitted by the PSF. These diagnostics comprise the position and magnitude rms, the fraction of flux in the aperture due to neighbours and the quality of the PSF fit. We plotted each parameter as a function of the stellar 