tokens: × 5 pixel 2 raster pixd (after neighbour subtraction) centred on the target stars, and PSF( i, j ) is the value of the local PSF-model expected in the pixel ( i, j ). Introduced by Bedin et al. ( 2008 , see also Bedin et al. 2009 , 2010 ; Simioni et al. 2018 ), the parameter RADXS is a shape parameter that allows us to distinguish sources that deviate from a PSF shape. It is a comparison between the measured flux of the source outside the core (in an annulus 1.0 r 2.5 pixels) and the flux expected from the PSF-model. For RADXS >0, the source is broader than that expected from the model (i.e. galaxies), while for negative values of RADXS the source is sharper than the PSF (i.e. cosmic rays and artefacts). Finally, the KS2 routine gives the number of images in which a star is found ( N ) and the number of consistent measurements of the star used to compute its average position and flux ( N ). 3 COLOUR–MAGNITUDE DIAGRAMS In Fig. 2 , we show an example of selection of well-measured stars for the case of NGC 6752 and the photometric method 1. Similar plots can be made for the methods 2 and 3. The top and middle panels of Fig. 2 show the selection of the stars based on the distribution of the photometric errors ( RMS , left-hand panels), the quality of fit ( QFIT , centre panels), and the shape of the sources ( RADXS , right-hand panels), in the case of F275W (top panels) and F336W (middle panels) filters. The selection based on RMS and QFIT are performed as done by Milone et al. ( 2012 ): we divided the distributions in 12 magnitude bins and, in each bin, we calculated the 3.5σ-clipped average of the magnitude and of the parameter, where σ is the standard deviation associated with the average value in the given bin. We added to the mean parameter of each bin 3.5 × σ, and we linearly interpolated these points (green line). We excluded all the points above (in the case of the RMS ) or below (in the case of the QFIT ) the green line (azure points). For the sharp parameter, we selected all the stars that satisfy the condition: −0.15 RADXS +0.15. Bottom panels show the m F275W versus ( m F275W − m F336W ) colour-magnitude diagram (CMD) for the stars that pass the selection criteria in both filters (left-hand panel) and for the stars that were rejected in at least one filter (right-hand panel). In red, the stars that are saturated in at least one filter and that have been recovered from the ‘first-pass’ photometry. From the CMDs, it is clear that many stars with poor photometric quality are rejected. Figure 2. Procedure adopted for selecting well-measured stars in the cluster NGC 6752. Top and middle panels show the selection of the stars based on RMS (left-hand panels), QFIT (central panels), and RADXS (right-hand panels) parameters: in azure the stars that are rejected. Bottom panels show the m F275W versus ( m F275W − m F336W ) CMD cleaned by rejected stars in both filters (left-hand panel) and the CMD of the stars rejected in at least one filter. Red
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