tokens: We, in particular FN, NB, VKP and IP, gratefully acknowledge financial support for this project provided by NASA through grant HST -GO-14069 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS526555. NB gratefully acknowledges financial support from the Royal Society (University Research Fellowship) and the European Research Council ( ERC-CoG-646928 , Multi-Pop). DG gratefully acknowledges support from the Chilean BASAL Centro de Excelencia en Astrofísica y Tecnologías Afines (CATA) grant PFB-06/2007 . MJC gratefully acknowledges support from the Sonderforschungsbereich SFB 881 ‘The Milky Way System’ (subproject A8) of the German Research Foundation (DFG) . We are grateful to Jay Anderson for sharing with us his ePSF software. We thank the anonymous referee for useful comments and suggestions. Based on observations made with the NASA/ESA HST , and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA).
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