magnitude and verified that most stars follow a clear trend. Outliers include stars with poor astrometry and photometry and are excluded from our investigation of multiple populations. We refer to papers by Milone et al.(2009) and Bedin et al.(2009) for details.

To account for the spatial variations of the photometric zero point across the field that are due to small inaccuracies in the PSF model and in the background determination we used the procedure by Milone et al.(2012b, see their Section. 3.2). Briefly, we derived the MS fiducial line and examined the color residuals relative to this sequence. We corrected the color of each star by the median color residual of its 45 best-measured neighbors. These corrections are typically smaller than 0.007 mag, and never exceed 0.015 mag.

Artificial stars {#subsec:ASs}
----------------

Artificial stars (ASs) are used to estimate photometric errors and to derive the completeness level of our sample and are run by using the procedure by Anderson et al.(2008). Briefly, we generated a catalog including the coordinates and the F110W and F160W magnitudes of 100,000 stars randomly distributed along the field of view. The ASs have instrumental magnitudes, $-2.5$log$_{10}$(flux), ranging from $-10.0$ to 0.0 in the F160W band, 