The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XIII. ACS/WFC parallel-field catalogues

The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XIII. ACS/WFC... Abstract As part of the Hubble Space Telescope UV Legacy Survey of Galactic globular clusters, 110 parallel fields were observed with the Wide Field Channel of the Advanced Camera for Surveys, in the outskirts of 48 globular clusters, plus the open cluster NGC 6791. Totalling about 0.3 deg2 of observed sky, this is the largest homogeneous Hubble Space Telescope photometric survey of Galalctic globular clusters outskirts to date. In particular, two distinct pointings have been obtained for each target on average, all centred at about 6.5 arcmin from the cluster centre, thus covering a mean area of about 23 arcmin2 for each globular cluster. For each field, at least one exposure in both F475W and F814W filters was collected. In this work, we publicly release the astrometric and photometric catalogues and the astrometrized atlases for each of these fields. atlases, catalogues, Hertzsprung–Russell and colour–magnitude diagrams, globular clusters: general 1 INTRODUCTION For almost three decades, the Milky Way globular clusters (GCs) have been the target of large CCD photometric surveys aimed at sampling their stellar populations in a homogeneous way (Rosenberg, Piotto & Saviane 2000a, Rosenberg, Aparicio & Saviane 2000b, Piotto et al. 2002, Sarajedini et al. 2007) using both space- and ground-based instruments. The growing sample of data, and the advent of increasingly sophisticated data-analysis techniques, have clearly demonstrated that GCs host distinct stellar populations with different chemical abundances. High-precision photometric measurements have revealed that the colour–magnitude diagrams (CMDs) show distinct sequences in various evolutionary stages (see e.g. Anderson 1997, Lee et al. 1999; Pancino et al. 2000; Bedin et al. 2004; Piotto et al. 2007; Milone et al. 2008; Bellini et al. 2010). These findings are also supported by spectroscopical evidence that the stellar populations of these systems are not as simple as thought (see e.g. Kraft 1994; Gratton et al. 2004; Marino et al. 2008; Yong & Grundahl 2008; Carretta et al. 2009a; Carretta, Bragaglia & Gratton 2009b; Gratton et al. 2012). The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters (GO-13297; PI: Piotto) has been specifically designed to further investigate this phenomenon and it now appears likely that all Galactic GCs host multiple stellar populations (Piotto et al. 2015 – hereafter Paper I; Milone et al. 2017 – hereafter Paper IX). In the context of this survey, parallel Advanced Camera for Surveys (ACS) observations have been obtained. While the main observations were taken using a combination of UV and optical filters of the Wide Field Camera 3 (WFC3), the lack of filters bluer than F435W dictated the use of the F475W and F814W filters of the Wide Field Channel of the ACS (ACS/WFC) in the parallel observations. The large colour baseline provided by this filter combination guarantees sensitivity to helium abundance differences, while being largely insensitive to star-to-star variations in light-element abundances (Sbordone, Salaris & Weiss 2011, Cassisi, Salaris & Pietrinferni 2017). One of the main objectives for which these observations were planned is to investigate how different stellar populations formed in GCs. Strong observational constraints come from the analysis of the radial distribution of each stellar population (D’Ercole et al. 2008; Bellini et al. 2009; Vesperini, McMillan & D’Antona 2013). As an example, Simioni et al. (2016) complemented WFC3 data of the central regions of NGC 2808 with ACS parallel observations and found evidence of different radial trends associated with distinct stellar populations hosted by the cluster. Thus, clusters with large helium variations among their stellar populations are the preferred target of investigation with the current data-sample. Other interesting targets, albeit extensively studied, are those defined as Type-II clusters in Paper IX, which displays multiple sub-giant branches in optical CMDs. We stress the fact that this is the first homogeneous HST photometric survey of the outskirts of Galactic GCs. The observations presented here represent a first epoch for future studies aimed at systematical measurements of absolute, relative and internal proper motions of stars in these regions. Archival HST observations matching a sub-sample of the observed fields exist, and proper motions will be published separately. In the imaged stellar fields, the stellar density is not as high as in the central regions. As a consequence, crowding is not a serious issue for these data. That makes them particularly suitable to be used as input catalogue for future spectroscopic surveys. The present catalogues can be used to perform several interesting analyses. For example, dynamical interactions between stars in GCs is at the origin of the mass segregation phenomenon. A precise estimate of its effects is fundamental for the derivation of a global mass function for a GC (Vesperini & Heggie 1997; Paust et al. 2010; Sollima & Baumgardt 2017). The measurement of the fraction of binaries is also fundamental for this kind of analysis and could provide useful constraints for dynamical models (Milone et al. 2012). We note, also, that in some cases white dwarf cooling sequences are visible in the obtained CMDs. Finally, it is interesting to note that due to the presence of many extragalactic objects in the observed field, other studies could benefit from these observations. In this work, we present the first photometric catalogues from the ACS/WFC parallel observations of the GO-13297 program. All data have been reduced in a homogeneous manner, making these catalogues particularly suitable for intercomparison. The article is organized as follows: in Section 2 the data are presented along with some information about the observing strategy, together with a detailed description of the data reduction. The extracted CMDs are presented in Section 3. Details on the selection of well-measured stars are given in Section 4. In Section 5 the catalogues and the released electronic material are described in detail. Finally, in Section 6, after a summary, we briefly discuss some of the main scientific questions we will address with these catalogues in subsequent papers. 2 OBSERVATIONS AND DATA REDUCTION Table reports the log of ACS/WFC observations used to construct the catalogues. For each target, we indicate the total number of orbits assigned to each observed field separated by the different position angles (PAs) of the V3 axis of the HST focal plane. Typically, one F475W and one F814W image were taken each orbit, with a dither between the two dictated by primary WFC3 observing strategy. For each field, right ascension and declination of the centre of ACS/WFC are provided along with exposure time in each filter. Table 1. Observation log. For each GC in the survey, and the open cluster NGC 6791, we show right ascension and declination of each distinct parallel field, referred to the centre of ACS/WFC. We also report the number of orbits, telescope orientation (V3 PA) for each orbit, and exposure time in each filter. #  Cluster  Orbits  Field  RA (J2000)  Dec. (J2000)  Epoch  Exp. time  Exp. time        [PA (deg)]  (h m s)  (° ΄ ΄)    F475W (s)  F814W (s)  01  NGC 1261  5  F1 [92]  03:12:49.68  −55:17:25.2  31/08/13  770  694        F2 [138]  03:12:16.96  −55:19:29.6  11/09/13  745  669        F3 [182]  03:11:44.35  −55:17:39.3  08/11/13  766  690        F4 [225]  03:11:30.53  −55:13:17.9  07/12/13  745  669        F5 [48]  03:13:01.92  −55:12:51.4  29/06/14  829  753  02  NGC 1851  7  F1 [195]  05:13:37.57  −40:06:44.0  27/12/10  2 × 40; 2 × 1277; 1237  6 × 488; 1 × 40        F2 [164]  05:13:51.57  −40:08:55.8  11/11/10  2 × 40; 2 × 1277; 2 × 1237  8 × 488; 2 × 40  03  NGC 2298  4  F1 [185]  06:48:36.04  −36:05:08.7  18/12/13  2 × 785  2 × 683        F2 [273]  06:48:35.14  −35:55:40.7  07/03/14  885  816              15/03/15  887  815  04  NGC 3201  2  F1 [25]  10:18:12.19  −46:21:47.1  13/09/13  685  612        F2 [115]  10:17:54.00  −46:30:51.0  01/01/14  689  616  05  NGC 4590  2  F1 [112]  12:39:42.64  −26:50:34.9  21/12/13  627  554        F2 [202]  12:39:01.10  −26:47:48.5  30/03/14  627  554  06  NGC 4833  4  F1 [113]  13:00:10.12  −70:58:29.4  17/01/14  2 × 840  2 × 771        F2 [202]  12:58:20.68  −70:55:42.6  09/04/14  2 × 806  2 × 730  07  NGC 5024  6  F1 [31]  13:13:21.59  +18:12:01.1  24/03/14  4 × 725; 2 × 723  3 × 370        F2 [120]  13:13:03.82  18:03:52.0  08/12/13  4 × 775; 2 × 774  3 × 375  08  NGC 5053  5  F1 [352]  13:16:42.03  +17:47:28.7  01/04/14  740  664        F2 [37]  13:16;53.97  +17:43:15.0  16/03/14  740  664        F3 [80]  13:16;50.28  +17:38:33.1  23/01/14  790  714        F4 [125]  13:16;33.21  +17:35:39.5  05/12/13  790  714        F5 [308]  13:16;21.88  +17:48:25.0  16/05/14  765  689  09  NGC 5286  2  F1 [73]  13:47:07.09  −51:25:00.2  14/12/13  728  655        F2 [162]  13:46:11.09  −51:28:46.7  15/03/14  603  559  10  NGC 5466  4  F1 [112]  14:05:41.08  +28:26:12.1  05/01/14  834; 835  763; 765        F2 [21]  14:05:54.29  +28:35:18.1  29/03/14  2 × 776  2 × 700  11  NGC 5897  4  F1 [112]  15:17:37.48  −21:06:27.5  12/02/14  830; 833  2 × 761        F2 [202]  15:16:58.75  −21:03:44.3  13/05/14  779; 781  710; 709  12  NGC 5904  2  F1 [323]  15:18:34.23  02:11:38.3  17/05/14  620  559        F2 [52]  15:19:00.34  02:04:36.4  08/04/14  621  559  13  NGC 5927  3  F1 [100]  15:28:28.73  −50:45:30.4  01/02/14  603  559        F2 [189]  15:27:28.26  −50:44:48.7  19/05/14  603  559              27/05/15  603  559  14  NGC 5986  3  F1 [92]  15:46:28.87  −37:51:38.6  27/01/14  676  603        F2 [180]  15:45:40.83  −37:52:21.9  17/05/14  603  559              10/05/15  603  559  15  NGC 6093  5  F1 [255]  16:16:35.04  −22:55:45.9  09/06/12  5 × 760; 5 × 845  5 × 539  16  NGC 6101  5  F1 [147]  16:25:36.08  −72:18:35.2  04/04/14  762  686        F2 [190]  16:24:40.57  −72:16:07.7  25/05/14  762  686        F3 [235]  16:24:23.48  −72:11:18.6  29/06/14  800  724        F4 [282]  16:24:58.26  −72:06:50.7  14/08/13  851  775        F5 [101]  16:26:39.71  −72:17:19.9  28/02/14  800  724  17  NGC 6121  2  F1 [272]  16:23:12.57  −26:27:02.1  06/07/14  739  666        F2 [98]  16:23:55.64  −26:36:33.4  17/02/15  666  593  18  NGC 6144  2  F1 [83]  16:27:39.42  −26:05:00.9  28/02/14  679  606        F2 [174]  16:26:57.33  −26:07:05.2  27/05/14  679  606  19  NGC 6171  4  F1 [342]  16:32:41.94  −12:56:56.1  31/05/14  830; 833  2 × 761        F2 [72]  16:32:42.20  −12:57:07.5  25/03/14  800; 802  731; 730  20  NGC 6218  2  F1 [276]  16:46:55.21  −01:52:03.7  16/08/13  721  648        F2 [6]  16:47:33.42  −01:52:07.7  27/05/14  645  572  21  NGC 6254  2  F1 [276]  16:56:50.04  −04:01:10.1  16/08/13  721  648        F2 [7]  16:57:28.66  −04:01:19.2  27/05/14  644  571  22  NGC 6304  2  F1 [184]  17:14:10.53  −29:32:35.1  07/06/14  624  559        F2 [274]  17:14:09.53  −29:23:04.8  26/08/13  731  658  23  NGC 6341  2  F1 [230]  17:16:30.21  +43:08:03.0  22/10/13  638  565        F2 [319]  17:17:06.19  +43:14:56.0  03/08/14  750  677  24  NGC 6352  2  F1 [161]  17:25:15.00  −48:31:41.7  27/05/14  637  564        F2 [251]  17:24:51.02  −48:22:52.5  13/08/13  731  658  25  NGC 6362  2  F1 [125]  17:32:14.07  −67:09:25.3  30/03/14  651  578        F2 [215]  17:30:47.67  −67:04:37.3  01/07/14  760  687  26  NGC 6366  2  F1 [293]  17:27:31.94  −04:58:44.4  26/08/13  726  653        F2 [351]  17:27:58.09  −04:58:57.1  07/06/15  644  571  27  NGC 6388  4  F1 [238]  17:35:40.80  −44:43:02.9  05/07/14  865; 906  796; 834        F2 [141]  17:36:17.23  −44:50:53.5  12/05/14  793; 795  724; 723  28  NGC 6397  2  F1 [90]  17:41:17.47  −53:44:46.1  27/03/14  683  610        F2 [180]  17:40:12.50  −53:45:38.3  11/06/14  640  567  29  NGC 6441  4  F1 [102]  17:50:34.23  −37:08:21.5  26/03/14  833; 835  764; 763        F2 [192]  17:49:46.24  −37:07:12.6  15/06/14  2 × 794  725; 722  30  NGC 6496  2  F1 [157]  17:58:53.12  −44:22:28.2  30/05/14  638  656        F2 [247]  17:58:27.50  −44:13:57.0  12/08/13  731  658  31  NGC 6535  2  F1 [319]  18:03:49.95  +00:11:04.6  19/07/14  724  651        F2 [50]  18:04:17.63  +00:17:50.2  26/05/14  644  571  32  NGC 6541  2  F1 [80]  18:08:35.16  −43:46:11.5  14/02/14  689  616        F2 [170]  18:07:44.04  −43:48:48.8  11/06/14  639  566  33  NGC 6584  2  F1 [155]  18:18:26.74  −52:19:31.3  30/05/14  640  567        F2 [245]  18:17:54.89  −52:11:09.9  18/08/13  726  653  34  NGC 6624  2  F1 [264]  18:23:14.50  −30:17:51.1  03/09/13  731  658        F2 [174]  18:23:23.29  −30:27:20.2  27/06/14  638  565  35  NGC 6637  4  F1 [84]  18:31:49.55  −32:24:40.9  18/02/14  827; 840  758; 768        F2 [174]  18:31:05.49  −32:26:31.5  30/06/14  792; 794  723; 722  36  NGC 6652  3  F1 [281]  18:35:25.09  −32:54:08.8  30/08/13  707  633        F2 [238]  18:35:14.87  −32:58:25.1  14/08/13  733  658        F3 [192]  18:35:20.06  −33:03:36.1  01/07/14  621  548  37  NGC 6656  4  F1 [266]  18:36:00.25  −23:50:16.5  23/09/10  656;644  2 × 389        F2 [85]  18:36:47.76  −23:58:08.0  17/03/11  2 × 656  2 × 389  38  NGC 6681  2  F1 [271]  18:42:48.78  −32:13:03.7  05/09/13  730  658        F2 [183]  18:42:51.19  −32:22:33.1  29/06/14  637  564  39  NGC 6715  6  F1 [153]  18:54:56.04  −30:35:08.9  29/06/14  2 × 736; 2 × 737; 2 × 734  3 × 370        F2 [243]  18:54:33.96  −30:27:06.5  05/09/13  4 × 819; 2 × 817  3 × 390  40  NGC 6717  3  F1 [80]  18:55:31.98  −22:45:22.7  06/05/14  619  535        F2 [125]  18:55:12.41  −22:48:27.0  03/07/14  619  544        F3 [171]  18:54:51.25  −22:47:57.0  03/07/14  617  544  41  NGC 6723  3  F1 [103]  18:59:54.19  −36:43:18.0  03/04/14  666  592        F2 [148]  18:59:27.49  −36:44:21.9  15/06/14  626  551        F3 [193]  18:59:06.07  −36:41:59.9  05/07/14  624  551  42  NGC 6779  2  F1 [241]  19:16:04.77  +30:12:18.9  13/10/13  731  658        F2 [333]  19:16:42.17  +30:17:38.2  19/07/14  637  564  43  NGC 6791  2  F1 [322]  19:20:53.69  +37:53:04.8  17/08/13  631  559        F2 [50]  19:21:27.31  +37:46:16.6  26/04/14  638  565  44  NGC 6809  2  F1 [262]  19:39:32.79  −30:54:19.4  21/08/14  753  680        F2 [82]  19:40:26.65  −31:01:26.4  29/03/14  677  604  45  NGC 6838  2  F1 [244]  19:53:18.63  +18:48:20.3  23/10/13  723  650        F2 [75]  19:54:12.48  +18:43:53.9  03/05/14  681  608  46  NGC 6934  2  F1 [245]  20:33:44.84  +07:25:55.5  08/10/13  723  650        F2 [334]  20:34:17.59  +07:30:52.3  18/08/14  644  571  47  NGC 6981  2  F1 [289]  20:53:13.45  −12:26:24.9  13/08/13  641  568        F2 [19]  20:53:51.29  −12:28:39.5  03/08/14  624  551  48  NGC 7089  3  F1 [237]  21:32:59.85  +00:48:42.0  18/10/13  717  643        F2 [281]  21:33:10.93  +00:44:15.0  29/08/13  668  593        F3 [327]  21:33:29.92  +00:42:39.1  14/08/13  611  534  49  NGC 7099  2  F1 [92]  21:40:44.34  −23:15:15.1  08/06/14  656  583        F2 [182]  21:40:02.30  −23:15:49.0  19/08/14  656  583  #  Cluster  Orbits  Field  RA (J2000)  Dec. (J2000)  Epoch  Exp. time  Exp. time        [PA (deg)]  (h m s)  (° ΄ ΄)    F475W (s)  F814W (s)  01  NGC 1261  5  F1 [92]  03:12:49.68  −55:17:25.2  31/08/13  770  694        F2 [138]  03:12:16.96  −55:19:29.6  11/09/13  745  669        F3 [182]  03:11:44.35  −55:17:39.3  08/11/13  766  690        F4 [225]  03:11:30.53  −55:13:17.9  07/12/13  745  669        F5 [48]  03:13:01.92  −55:12:51.4  29/06/14  829  753  02  NGC 1851  7  F1 [195]  05:13:37.57  −40:06:44.0  27/12/10  2 × 40; 2 × 1277; 1237  6 × 488; 1 × 40        F2 [164]  05:13:51.57  −40:08:55.8  11/11/10  2 × 40; 2 × 1277; 2 × 1237  8 × 488; 2 × 40  03  NGC 2298  4  F1 [185]  06:48:36.04  −36:05:08.7  18/12/13  2 × 785  2 × 683        F2 [273]  06:48:35.14  −35:55:40.7  07/03/14  885  816              15/03/15  887  815  04  NGC 3201  2  F1 [25]  10:18:12.19  −46:21:47.1  13/09/13  685  612        F2 [115]  10:17:54.00  −46:30:51.0  01/01/14  689  616  05  NGC 4590  2  F1 [112]  12:39:42.64  −26:50:34.9  21/12/13  627  554        F2 [202]  12:39:01.10  −26:47:48.5  30/03/14  627  554  06  NGC 4833  4  F1 [113]  13:00:10.12  −70:58:29.4  17/01/14  2 × 840  2 × 771        F2 [202]  12:58:20.68  −70:55:42.6  09/04/14  2 × 806  2 × 730  07  NGC 5024  6  F1 [31]  13:13:21.59  +18:12:01.1  24/03/14  4 × 725; 2 × 723  3 × 370        F2 [120]  13:13:03.82  18:03:52.0  08/12/13  4 × 775; 2 × 774  3 × 375  08  NGC 5053  5  F1 [352]  13:16:42.03  +17:47:28.7  01/04/14  740  664        F2 [37]  13:16;53.97  +17:43:15.0  16/03/14  740  664        F3 [80]  13:16;50.28  +17:38:33.1  23/01/14  790  714        F4 [125]  13:16;33.21  +17:35:39.5  05/12/13  790  714        F5 [308]  13:16;21.88  +17:48:25.0  16/05/14  765  689  09  NGC 5286  2  F1 [73]  13:47:07.09  −51:25:00.2  14/12/13  728  655        F2 [162]  13:46:11.09  −51:28:46.7  15/03/14  603  559  10  NGC 5466  4  F1 [112]  14:05:41.08  +28:26:12.1  05/01/14  834; 835  763; 765        F2 [21]  14:05:54.29  +28:35:18.1  29/03/14  2 × 776  2 × 700  11  NGC 5897  4  F1 [112]  15:17:37.48  −21:06:27.5  12/02/14  830; 833  2 × 761        F2 [202]  15:16:58.75  −21:03:44.3  13/05/14  779; 781  710; 709  12  NGC 5904  2  F1 [323]  15:18:34.23  02:11:38.3  17/05/14  620  559        F2 [52]  15:19:00.34  02:04:36.4  08/04/14  621  559  13  NGC 5927  3  F1 [100]  15:28:28.73  −50:45:30.4  01/02/14  603  559        F2 [189]  15:27:28.26  −50:44:48.7  19/05/14  603  559              27/05/15  603  559  14  NGC 5986  3  F1 [92]  15:46:28.87  −37:51:38.6  27/01/14  676  603        F2 [180]  15:45:40.83  −37:52:21.9  17/05/14  603  559              10/05/15  603  559  15  NGC 6093  5  F1 [255]  16:16:35.04  −22:55:45.9  09/06/12  5 × 760; 5 × 845  5 × 539  16  NGC 6101  5  F1 [147]  16:25:36.08  −72:18:35.2  04/04/14  762  686        F2 [190]  16:24:40.57  −72:16:07.7  25/05/14  762  686        F3 [235]  16:24:23.48  −72:11:18.6  29/06/14  800  724        F4 [282]  16:24:58.26  −72:06:50.7  14/08/13  851  775        F5 [101]  16:26:39.71  −72:17:19.9  28/02/14  800  724  17  NGC 6121  2  F1 [272]  16:23:12.57  −26:27:02.1  06/07/14  739  666        F2 [98]  16:23:55.64  −26:36:33.4  17/02/15  666  593  18  NGC 6144  2  F1 [83]  16:27:39.42  −26:05:00.9  28/02/14  679  606        F2 [174]  16:26:57.33  −26:07:05.2  27/05/14  679  606  19  NGC 6171  4  F1 [342]  16:32:41.94  −12:56:56.1  31/05/14  830; 833  2 × 761        F2 [72]  16:32:42.20  −12:57:07.5  25/03/14  800; 802  731; 730  20  NGC 6218  2  F1 [276]  16:46:55.21  −01:52:03.7  16/08/13  721  648        F2 [6]  16:47:33.42  −01:52:07.7  27/05/14  645  572  21  NGC 6254  2  F1 [276]  16:56:50.04  −04:01:10.1  16/08/13  721  648        F2 [7]  16:57:28.66  −04:01:19.2  27/05/14  644  571  22  NGC 6304  2  F1 [184]  17:14:10.53  −29:32:35.1  07/06/14  624  559        F2 [274]  17:14:09.53  −29:23:04.8  26/08/13  731  658  23  NGC 6341  2  F1 [230]  17:16:30.21  +43:08:03.0  22/10/13  638  565        F2 [319]  17:17:06.19  +43:14:56.0  03/08/14  750  677  24  NGC 6352  2  F1 [161]  17:25:15.00  −48:31:41.7  27/05/14  637  564        F2 [251]  17:24:51.02  −48:22:52.5  13/08/13  731  658  25  NGC 6362  2  F1 [125]  17:32:14.07  −67:09:25.3  30/03/14  651  578        F2 [215]  17:30:47.67  −67:04:37.3  01/07/14  760  687  26  NGC 6366  2  F1 [293]  17:27:31.94  −04:58:44.4  26/08/13  726  653        F2 [351]  17:27:58.09  −04:58:57.1  07/06/15  644  571  27  NGC 6388  4  F1 [238]  17:35:40.80  −44:43:02.9  05/07/14  865; 906  796; 834        F2 [141]  17:36:17.23  −44:50:53.5  12/05/14  793; 795  724; 723  28  NGC 6397  2  F1 [90]  17:41:17.47  −53:44:46.1  27/03/14  683  610        F2 [180]  17:40:12.50  −53:45:38.3  11/06/14  640  567  29  NGC 6441  4  F1 [102]  17:50:34.23  −37:08:21.5  26/03/14  833; 835  764; 763        F2 [192]  17:49:46.24  −37:07:12.6  15/06/14  2 × 794  725; 722  30  NGC 6496  2  F1 [157]  17:58:53.12  −44:22:28.2  30/05/14  638  656        F2 [247]  17:58:27.50  −44:13:57.0  12/08/13  731  658  31  NGC 6535  2  F1 [319]  18:03:49.95  +00:11:04.6  19/07/14  724  651        F2 [50]  18:04:17.63  +00:17:50.2  26/05/14  644  571  32  NGC 6541  2  F1 [80]  18:08:35.16  −43:46:11.5  14/02/14  689  616        F2 [170]  18:07:44.04  −43:48:48.8  11/06/14  639  566  33  NGC 6584  2  F1 [155]  18:18:26.74  −52:19:31.3  30/05/14  640  567        F2 [245]  18:17:54.89  −52:11:09.9  18/08/13  726  653  34  NGC 6624  2  F1 [264]  18:23:14.50  −30:17:51.1  03/09/13  731  658        F2 [174]  18:23:23.29  −30:27:20.2  27/06/14  638  565  35  NGC 6637  4  F1 [84]  18:31:49.55  −32:24:40.9  18/02/14  827; 840  758; 768        F2 [174]  18:31:05.49  −32:26:31.5  30/06/14  792; 794  723; 722  36  NGC 6652  3  F1 [281]  18:35:25.09  −32:54:08.8  30/08/13  707  633        F2 [238]  18:35:14.87  −32:58:25.1  14/08/13  733  658        F3 [192]  18:35:20.06  −33:03:36.1  01/07/14  621  548  37  NGC 6656  4  F1 [266]  18:36:00.25  −23:50:16.5  23/09/10  656;644  2 × 389        F2 [85]  18:36:47.76  −23:58:08.0  17/03/11  2 × 656  2 × 389  38  NGC 6681  2  F1 [271]  18:42:48.78  −32:13:03.7  05/09/13  730  658        F2 [183]  18:42:51.19  −32:22:33.1  29/06/14  637  564  39  NGC 6715  6  F1 [153]  18:54:56.04  −30:35:08.9  29/06/14  2 × 736; 2 × 737; 2 × 734  3 × 370        F2 [243]  18:54:33.96  −30:27:06.5  05/09/13  4 × 819; 2 × 817  3 × 390  40  NGC 6717  3  F1 [80]  18:55:31.98  −22:45:22.7  06/05/14  619  535        F2 [125]  18:55:12.41  −22:48:27.0  03/07/14  619  544        F3 [171]  18:54:51.25  −22:47:57.0  03/07/14  617  544  41  NGC 6723  3  F1 [103]  18:59:54.19  −36:43:18.0  03/04/14  666  592        F2 [148]  18:59:27.49  −36:44:21.9  15/06/14  626  551        F3 [193]  18:59:06.07  −36:41:59.9  05/07/14  624  551  42  NGC 6779  2  F1 [241]  19:16:04.77  +30:12:18.9  13/10/13  731  658        F2 [333]  19:16:42.17  +30:17:38.2  19/07/14  637  564  43  NGC 6791  2  F1 [322]  19:20:53.69  +37:53:04.8  17/08/13  631  559        F2 [50]  19:21:27.31  +37:46:16.6  26/04/14  638  565  44  NGC 6809  2  F1 [262]  19:39:32.79  −30:54:19.4  21/08/14  753  680        F2 [82]  19:40:26.65  −31:01:26.4  29/03/14  677  604  45  NGC 6838  2  F1 [244]  19:53:18.63  +18:48:20.3  23/10/13  723  650        F2 [75]  19:54:12.48  +18:43:53.9  03/05/14  681  608  46  NGC 6934  2  F1 [245]  20:33:44.84  +07:25:55.5  08/10/13  723  650        F2 [334]  20:34:17.59  +07:30:52.3  18/08/14  644  571  47  NGC 6981  2  F1 [289]  20:53:13.45  −12:26:24.9  13/08/13  641  568        F2 [19]  20:53:51.29  −12:28:39.5  03/08/14  624  551  48  NGC 7089  3  F1 [237]  21:32:59.85  +00:48:42.0  18/10/13  717  643        F2 [281]  21:33:10.93  +00:44:15.0  29/08/13  668  593        F3 [327]  21:33:29.92  +00:42:39.1  14/08/13  611  534  49  NGC 7099  2  F1 [92]  21:40:44.34  −23:15:15.1  08/06/14  656  583        F2 [182]  21:40:02.30  −23:15:49.0  19/08/14  656  583  View Large Table 2. Tabulated $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec} )}$$ values used in the aperture correction. #  Cluster  Field  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F475W}$$  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F814W}$$        (mag)  (mag)  01  NGC 1261  1  +0.016 ± 0.006  −0.017 ± 0.003      2  +0.030 ± 0.005  −0.021 ± 0.002      3  +0.021 ± 0.005  −0.013 ± 0.002      4  +0.038 ± 0.005  +0.117 ± 0.005      5  +0.053 ± 0.006  −0.020 ± 0.003  02  NGC 1851  1+2  −0.008 ± 0.002  −0.004 ± 0.001  03  NGC 2298  1  +0.057 ± 0.014  +0.148 ± 0.008      2  +0.016 ± 0.010  +0.013 ± 0.005  04  NGC 3201  1  +0.022 ± 0.001  +0.001 ± 0.001      2  +0.023 ± 0.001  +0.035 ± 0.001  05  NGC 4590  1  −0.003 ± 0.003  +0.002 ± 0.002      2  0.000 ± 0.003  +0.039 ± 0.002  06  NGC 4833  1  +0.004 ± 0.002  +0.002 ± 0.001      2  +0.009 ± 0.001  −0.008 ± 0.001  07  NGC 5024  1  +0.003 ± 0.001  +0.012 ± 0.001      2  +0.009 ± 0.003  −0.019 ± 0.001  08  NGC 5053  1  +0.012 ± 0.004  +0.001 ± 0.004      2  +0.003 ± 0.004  0.000 ± 0.003      3  +0.005 ± 0.005  −0.011 ± 0.003      4  +0.012 ± 0.005  +0.006 ± 0.003      5  +0.132 ± 0.006  +0.029 ± 0.003  09  NGC 5286  1  +0.036 ± 0.003  +0.019 ± 0.002      2  +0.031 ± 0.004  −0.011 ± 0.001  10  NGC 5466  1  +0.009 ± 0.003  −0.011 ± 0.003      2  +0.008 ± 0.003  −0.009 ± 0.002  11  NGC 5897  1  +0.029 ± 0.003  −0.012 ± 0.001      2  +0.028 ± 0.003  −0.014 ± 0.001  12  NGC 5904  1  +0.011 ± 0.001  −0.016 ± 0.001      2  +0.003 ± 0.001  −0.008 ± 0.001  13  NGC 5927  1  +0.023 ± 0.002  +0.017 ± 0.001      2  +0.022 ± 0.002  −0.003 ± 0.001  14  NGC 5986  1  −0.004 ± 0.002  −0.011 ± 0.001      2  +0.043 ± 0.004  −0.008 ± 0.001  15  NGC 6093  1  +0.090 ± 0.002  +0.136 ± 0.002  16  NGC 6101  1  +0.003 ± 0.003  +0.004 ± 0.002      2  +0.007 ± 0.003  −0.012 ± 0.001      3  +0.010 ± 0.004  +0.004 ± 0.002      4  +0.003 ± 0.002  −0.014 ± 0.001      5  +0.009 ± 0.002  −0.001 ± 0.001  17  NGC 6121  1  +0.003 ± 0.002  +0.002 ± 0.001      2  −0.004 ± 0.003  −0.005 ± 0.001  18  NGC 6144  1  +0.030 ± 0.004  +0.027 ± 0.001      2  +0.013 ± 0.004  −0.002 ± 0.001  19  NGC 6171  1  +0.021 ± 0.002  −0.007 ± 0.002      2  +0.022 ± 0.002  +0.004 ± 0.002  20  NGC 6218  1  −0.003 ± 0.002  −0.001 ± 0.001      2  +0.006 ± 0.002  −0.009 ± 0.001  21  NGC 6254  1  −0.006 ± 0.002  +0.002 ± 0.001      2  0.000 ± 0.002  −0.003 ± 0.001  22  NGC 6304  1  +0.010 ± 0.001  +0.036 ± 0.001      2  +0.012 ± 0.002  +0.016 ± 0.001  23  NGC 6341  1  +0.003 ± 0.002  −0.026 ± 0.001      2  +0.016 ± 0.002  −0.013 ± 0.001  24  NGC 6352  1  +0.001 ± 0.001  −0.004 ± 0.001      2  −0.010 ± 0.001  +0.011 ± 0.001  25  NGC 6362  1  −0.002 ± 0.002  −0.006 ± 0.001      2  +0.024 ± 0.003  +0.056 ± 0.001  26  NGC 6366  1  +0.023 ± 0.005  −0.003 ± 0.001      2  +0.003 ± 0.002  +0.007 ± 0.001  27  NGC 6388  1  +0.011 ± 0.001  +0.016 ± 0.001      2  +0.003 ± 0.001  +0.018 ± 0.001  28  NGC 6397  1  0.000 ± 0.001  +0.003 ± 0.001      2  +0.008 ± 0.002  +0.031 ± 0.001  29  NGC 6441  1  +0.010 ± 0.001  +0.027 ± 0.001      2  +0.040 ± 0.001  +0.057 ± 0.001  30  NGC 6496  1  −0.008 ± 0.002  −0.005 ± 0.001      2  −0.002 ± 0.001  −0.013 ± 0.001  31  NGC 6535  1  −0.002 ± 0.002  +0.030 ± 0.001      2  +0.010 ± 0.003  −0.007 ± 0.001  32  NGC 6541  1  +0.011 ± 0.001  +0.020 ± 0.001      2  0.000 ± 0.002  0.000 ± 0.001  33  NGC 6584  1  −0.011 ± 0.003  −0.011 ± 0.001      2  −0.005 ± 0.004  −0.016 ± 0.001  34  NGC 6624  1  +0.020 ± 0.001  +0.046 ± 0.001      2  +0.008 ± 0.001  +0.014 ± 0.001  35  NGC 6637  1  +0.003 ± 0.001  +0.009 ± 0.001      2  −0.008 ± 0.001  −0.003 ± 0.001  36  NGC 6652  1  −0.002 ± 0.002  +0.039 ± 0.001      2  −0.008 ± 0.002  +0.003 ± 0.001      3  +0.006 ± 0.001  +0.015 ± 0.001  37  NGC 6656  1  0.000 ± 0.001  +0.031 ± 0.001      2  +0.079 ± 0.001  +0.030 ± 0.001  38  NGC 6681  1  −0.007 ± 0.001  −0.007 ± 0.001      2  +0.005 ± 0.001  −0.005 ± 0.001  39  NGC 6715  1  +0.008 ± 0.001  +0.076 ± 0.001      2  +0.001 ± 0.001  +0.033 ± 0.001  40  NGC 6717  1  +0.006 ± 0.002  −0.013 ± 0.001      2  −0.002 ± 0.002  +0.007 ± 0.001      3  +0.005 ± 0.001  +0.011 ± 0.001  41  NGC 6723  1  −0.007 ± 0.003  0.000 ± 0.001      2  +0.007 ± 0.002  −0.007 ± 0.001      3  +0.016 ± 0.003  −0.008 ± 0.001  42  NGC 6779  1  +0.004 ± 0.003  0.000 ± 0.002      2  −0.005 ± 0.002  +0.025 ± 0.002  43  NGC 6791  1  −0.005 ± 0.004  +0.006 ± 0.002      2  −0.011 ± 0.003  −0.011 ± 0.001  44  NGC 6809  1  +0.037 ± 0.002  −0.006 ± 0.001      2  0.000 ± 0.001  −0.002 ± 0.001  45  NGC 6838  1  −0.014 ± 0.002  +0.016 ± 0.001      2  +0.009 ± 0.002  −0.009 ± 0.001  46  NGC 6934  1  −0.001 ± 0.005  +0.004 ± 0.002      2  +0.005 ± 0.007  −0.001 ± 0.003  47  NGC 6981  1  −0.002 ± 0.007  −0.009 ± 0.003      2  +0.008 ± 0.011  +0.003 ± 0.005  48  NGC 7089  1  +0.014 ± 0.003  +0.016 ± 0.001      2  +0.036 ± 0.002  −0.012 ± 0.001      3  +0.003 ± 0.002  −0.020 ± 0.001  49  NGC 7099  1  +0.013 ± 0.003  +0.019 ± 0.002      2  +0.016 ± 0.004  −0.010 ± 0.002  #  Cluster  Field  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F475W}$$  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F814W}$$        (mag)  (mag)  01  NGC 1261  1  +0.016 ± 0.006  −0.017 ± 0.003      2  +0.030 ± 0.005  −0.021 ± 0.002      3  +0.021 ± 0.005  −0.013 ± 0.002      4  +0.038 ± 0.005  +0.117 ± 0.005      5  +0.053 ± 0.006  −0.020 ± 0.003  02  NGC 1851  1+2  −0.008 ± 0.002  −0.004 ± 0.001  03  NGC 2298  1  +0.057 ± 0.014  +0.148 ± 0.008      2  +0.016 ± 0.010  +0.013 ± 0.005  04  NGC 3201  1  +0.022 ± 0.001  +0.001 ± 0.001      2  +0.023 ± 0.001  +0.035 ± 0.001  05  NGC 4590  1  −0.003 ± 0.003  +0.002 ± 0.002      2  0.000 ± 0.003  +0.039 ± 0.002  06  NGC 4833  1  +0.004 ± 0.002  +0.002 ± 0.001      2  +0.009 ± 0.001  −0.008 ± 0.001  07  NGC 5024  1  +0.003 ± 0.001  +0.012 ± 0.001      2  +0.009 ± 0.003  −0.019 ± 0.001  08  NGC 5053  1  +0.012 ± 0.004  +0.001 ± 0.004      2  +0.003 ± 0.004  0.000 ± 0.003      3  +0.005 ± 0.005  −0.011 ± 0.003      4  +0.012 ± 0.005  +0.006 ± 0.003      5  +0.132 ± 0.006  +0.029 ± 0.003  09  NGC 5286  1  +0.036 ± 0.003  +0.019 ± 0.002      2  +0.031 ± 0.004  −0.011 ± 0.001  10  NGC 5466  1  +0.009 ± 0.003  −0.011 ± 0.003      2  +0.008 ± 0.003  −0.009 ± 0.002  11  NGC 5897  1  +0.029 ± 0.003  −0.012 ± 0.001      2  +0.028 ± 0.003  −0.014 ± 0.001  12  NGC 5904  1  +0.011 ± 0.001  −0.016 ± 0.001      2  +0.003 ± 0.001  −0.008 ± 0.001  13  NGC 5927  1  +0.023 ± 0.002  +0.017 ± 0.001      2  +0.022 ± 0.002  −0.003 ± 0.001  14  NGC 5986  1  −0.004 ± 0.002  −0.011 ± 0.001      2  +0.043 ± 0.004  −0.008 ± 0.001  15  NGC 6093  1  +0.090 ± 0.002  +0.136 ± 0.002  16  NGC 6101  1  +0.003 ± 0.003  +0.004 ± 0.002      2  +0.007 ± 0.003  −0.012 ± 0.001      3  +0.010 ± 0.004  +0.004 ± 0.002      4  +0.003 ± 0.002  −0.014 ± 0.001      5  +0.009 ± 0.002  −0.001 ± 0.001  17  NGC 6121  1  +0.003 ± 0.002  +0.002 ± 0.001      2  −0.004 ± 0.003  −0.005 ± 0.001  18  NGC 6144  1  +0.030 ± 0.004  +0.027 ± 0.001      2  +0.013 ± 0.004  −0.002 ± 0.001  19  NGC 6171  1  +0.021 ± 0.002  −0.007 ± 0.002      2  +0.022 ± 0.002  +0.004 ± 0.002  20  NGC 6218  1  −0.003 ± 0.002  −0.001 ± 0.001      2  +0.006 ± 0.002  −0.009 ± 0.001  21  NGC 6254  1  −0.006 ± 0.002  +0.002 ± 0.001      2  0.000 ± 0.002  −0.003 ± 0.001  22  NGC 6304  1  +0.010 ± 0.001  +0.036 ± 0.001      2  +0.012 ± 0.002  +0.016 ± 0.001  23  NGC 6341  1  +0.003 ± 0.002  −0.026 ± 0.001      2  +0.016 ± 0.002  −0.013 ± 0.001  24  NGC 6352  1  +0.001 ± 0.001  −0.004 ± 0.001      2  −0.010 ± 0.001  +0.011 ± 0.001  25  NGC 6362  1  −0.002 ± 0.002  −0.006 ± 0.001      2  +0.024 ± 0.003  +0.056 ± 0.001  26  NGC 6366  1  +0.023 ± 0.005  −0.003 ± 0.001      2  +0.003 ± 0.002  +0.007 ± 0.001  27  NGC 6388  1  +0.011 ± 0.001  +0.016 ± 0.001      2  +0.003 ± 0.001  +0.018 ± 0.001  28  NGC 6397  1  0.000 ± 0.001  +0.003 ± 0.001      2  +0.008 ± 0.002  +0.031 ± 0.001  29  NGC 6441  1  +0.010 ± 0.001  +0.027 ± 0.001      2  +0.040 ± 0.001  +0.057 ± 0.001  30  NGC 6496  1  −0.008 ± 0.002  −0.005 ± 0.001      2  −0.002 ± 0.001  −0.013 ± 0.001  31  NGC 6535  1  −0.002 ± 0.002  +0.030 ± 0.001      2  +0.010 ± 0.003  −0.007 ± 0.001  32  NGC 6541  1  +0.011 ± 0.001  +0.020 ± 0.001      2  0.000 ± 0.002  0.000 ± 0.001  33  NGC 6584  1  −0.011 ± 0.003  −0.011 ± 0.001      2  −0.005 ± 0.004  −0.016 ± 0.001  34  NGC 6624  1  +0.020 ± 0.001  +0.046 ± 0.001      2  +0.008 ± 0.001  +0.014 ± 0.001  35  NGC 6637  1  +0.003 ± 0.001  +0.009 ± 0.001      2  −0.008 ± 0.001  −0.003 ± 0.001  36  NGC 6652  1  −0.002 ± 0.002  +0.039 ± 0.001      2  −0.008 ± 0.002  +0.003 ± 0.001      3  +0.006 ± 0.001  +0.015 ± 0.001  37  NGC 6656  1  0.000 ± 0.001  +0.031 ± 0.001      2  +0.079 ± 0.001  +0.030 ± 0.001  38  NGC 6681  1  −0.007 ± 0.001  −0.007 ± 0.001      2  +0.005 ± 0.001  −0.005 ± 0.001  39  NGC 6715  1  +0.008 ± 0.001  +0.076 ± 0.001      2  +0.001 ± 0.001  +0.033 ± 0.001  40  NGC 6717  1  +0.006 ± 0.002  −0.013 ± 0.001      2  −0.002 ± 0.002  +0.007 ± 0.001      3  +0.005 ± 0.001  +0.011 ± 0.001  41  NGC 6723  1  −0.007 ± 0.003  0.000 ± 0.001      2  +0.007 ± 0.002  −0.007 ± 0.001      3  +0.016 ± 0.003  −0.008 ± 0.001  42  NGC 6779  1  +0.004 ± 0.003  0.000 ± 0.002      2  −0.005 ± 0.002  +0.025 ± 0.002  43  NGC 6791  1  −0.005 ± 0.004  +0.006 ± 0.002      2  −0.011 ± 0.003  −0.011 ± 0.001  44  NGC 6809  1  +0.037 ± 0.002  −0.006 ± 0.001      2  0.000 ± 0.001  −0.002 ± 0.001  45  NGC 6838  1  −0.014 ± 0.002  +0.016 ± 0.001      2  +0.009 ± 0.002  −0.009 ± 0.001  46  NGC 6934  1  −0.001 ± 0.005  +0.004 ± 0.002      2  +0.005 ± 0.007  −0.001 ± 0.003  47  NGC 6981  1  −0.002 ± 0.007  −0.009 ± 0.003      2  +0.008 ± 0.011  +0.003 ± 0.005  48  NGC 7089  1  +0.014 ± 0.003  +0.016 ± 0.001      2  +0.036 ± 0.002  −0.012 ± 0.001      3  +0.003 ± 0.002  −0.020 ± 0.001  49  NGC 7099  1  +0.013 ± 0.003  +0.019 ± 0.002      2  +0.016 ± 0.004  −0.010 ± 0.002  View Large The physical position of ACS/WFC detectors in the focal plane of HST is such that its projected field of view (FoV) in the sky is located at a distance of about 6.5 arcmin from the centre of the WFC3 FoV. Depending on the number of orbits allocated to each GC, from two to a maximum of five non-overlapping fields were observed. This is because, in order to secure a good handling of charge-transfer-efficiency (CTE) systematic errors, primary WFC3 observations were taken by applying a different telescope rotation at each orbit (Paper I, Section 4). For the majority of clusters, which were allocated two orbits, a rotation of about 90° was performed between the first and second orbit; for clusters observed for more than two orbits, a minimum difference of ∼45° between the V3 PA of each orbit was required. Five distinct pointings were obtained for three clusters, namely NGC 1261, NGC 5053, and NGC 6101. Three distinct fields were obtained for four clusters: NGC 6652, NGC 6717, NGC 6723, and NGC 7089. For the other clusters, only two pointings were planned. M80 is an exception and was not observed as part of the Program GO-13297. For it, we make use of archival HST data from GO-12311. When possible, ACS parallel observations targeted pre-existing HST observations. Figs 1 and A1–A8 display all HST observations that sample the sky area in the vicinity of those covered in this survey. Three cameras onboard HST were considered in order to enhance the probability of an overlap between observations: Wide Field and Planetary Camera 2 (WFPC2), ACS, and WFC3 (both UVIS and IR channels). Taking two images per orbit, one in F475W and one in F814W filters, the typical exposure times for both filters are of the order of 700 s. Figure 1. View largeDownload slide Finding charts for NGC 1261, NGC 1851, NGC 2298, and NGC 3201. Red outlines represent ACS/WFC parallel observations of GO-13297, black outlines refer to WFC3/UVIS observations. Dark grey, dashed outlines represent archive HST observations in the same regions. Footprints are labelled as in Table 1. Observations are subdivided by epoch in the smaller panels. Blue circles mark the position of core radius, half-light radius, and tidal radius for each cluster from Harris 1996 (2010). Where the tidal radius could not be included in the image, its value has been indicated in the upper-left corner of the image. Yellow, dashed circles mark the distances of 1, 5, and 10 arcmin from cluster centres. Grey dots corresponds to 2MASS sources, with brighter sources being larger. Figure 1. View largeDownload slide Finding charts for NGC 1261, NGC 1851, NGC 2298, and NGC 3201. Red outlines represent ACS/WFC parallel observations of GO-13297, black outlines refer to WFC3/UVIS observations. Dark grey, dashed outlines represent archive HST observations in the same regions. Footprints are labelled as in Table 1. Observations are subdivided by epoch in the smaller panels. Blue circles mark the position of core radius, half-light radius, and tidal radius for each cluster from Harris 1996 (2010). Where the tidal radius could not be included in the image, its value has been indicated in the upper-left corner of the image. Yellow, dashed circles mark the distances of 1, 5, and 10 arcmin from cluster centres. Grey dots corresponds to 2MASS sources, with brighter sources being larger. All exposures have been corrected for CTE effects using the method described in Anderson & Bedin (2010). Photometric measurements of stellar objects in each field have been performed using a suite of fortran programs based on img2xym (Anderson & King 2006) and kitchen_sync presented in Anderson et al. (2008). The spatial variation of the point spread function (PSF) has been taken into account adopting a grid of 9 × 10 model PSFs distributed along each image. However focus changes/breathing of the telescope, imperfect guiding, residual noise related to CTE can produce image-to-image variations of the PSF. To mitigate these sources of systematic errors, we derived a set of spatially varying perturbations of the PSF models for each calibrated, non-drizzled (flc) image. Adopting the procedure presented in Bellini et al. (2013), each image is divided into a grid with a number of cells changing from 2 × 2 to 5 × 5. In each cell, a subset of well-measured stars is used to locally adjust the PSF models to the stellar profiles. Using an updated version of Anderson & King (2006) software, in combination with the newly created PSF models, we extracted raw catalogues of stellar positions and magnitudes in each image. We choose the grid refinement (between 2 × 2 to 5 × 5) that produces the best results, inspecting the distribution of the quality of fit parameters as a function of magnitude, and taking into account the number of reference stars used to tailor the PSF perturbations in each cell. Each exposure related to the same field has been subsequently referred to a common reference frame. Since we are mainly interested in the faint, red part of the cluster CMDs, we used the F814W images to construct the reference frames. Catalogues are finally produced using a version of the software presented in Anderson et al. (2008), specifically tuned for this project. In particular, the kitchen_sync routine has been modified in order to work properly with only one image per filter. In addition, the method presented in Gilliland (2004) was applied in order to provide reliable photometry also for saturated stars. The raw, instrumental magnitudes have been zero-pointed into ACS/WFC Vega-mag photometric system following the prescriptions of Bedin et al. (2005). The zero-points and aperture corrections from 0.5 arcsec to infinity of Bohlin (2016) have been used. Especially in the present case, the photometric calibration plays a critical role, and we put strong efforts to precisely evaluate zero-point differences between various observations. Crowding is not a serious problem in the outer cluster regions and high-precision photometric measurements are relatively easy to obtain. But field-to-field zero-point variations must be accounted for to have the external fields of the same cluster on the same photometric scale. The main source of this photometric offsets between catalogues of distinct fields, is related to PSF modelling. The PSF model by construction is normalized to a surface flux of unity within a radius of 10 ACS/WFC pixels (0.5 arcsec). Only the inner 5 × 5 pixel region of sources was used to fit to the PSF model in order to minimize the contaminating impact of nearby neighbours, but any mismatch between the adopted PSF model and the pixels beyond the small square fitting aperture would result in a slight zero-point shift. The fact that we perturb the library PSFs (above) minimizes this even further. However, the best way to regularize the photometry is empirically: following Bedin et al. (2005) and Anderson et al. (2008), we measured aperture corrections inside this aperture using the calibrated and drizzled (drc) images as reference. These corrections were sufficient to properly take into account the majority of the photometric biases in our catalogues. The obtained values of the aperture correction for an aperture of 0.5 arcsec are listed in Table 2. We maintain the same nomenclature as in Bedin et al. (2005). The aperture correction values, along with their associated uncertainties are also reported in Fig. 2. Black dots are referred to F814W observations, red dots to F475W ones. It can be noted that for the majority of cases corrections are small: typically smaller than 0.04 magnitudes, but in the most severe cases, they can reach values as high as 0.15 mag. Figure 2. View largeDownload slide Aperture corrections measured in each field for each cluster. Red squares refer to F475W observations, while black squares refer to F814W ones. Figure 2. View largeDownload slide Aperture corrections measured in each field for each cluster. Red squares refer to F475W observations, while black squares refer to F814W ones. Table 3. Precision reached with the re-derived astrometric solution using Gaia catalogues; for completeness we report the number of reference stars used. We also provide measures of the accuracy of the original STScI astrometric solution in the form of differences between RA and Dec. #  Cluster  F  Stars  Precision  ΔRA  $$\Delta {\rm \text{Dec.}}$$          (mas)  WFC px  WFC px  01  NGC 1261  1  32  15.5  1.66 ± 0.34  −4.50 ± 0.16      2  41  10.0  −1.89 ± 0.08  4.31 ± 0.21      3  37  8.0  −0.51 ± 0.06  1.61 ± 0.18      4  36  11.0  −0.50 ± 0.14  1.03 ± 0.31      5  46  10.0  0.07 ± 0.05  −4.81 ± 0.19  02  NGC 1851  1 + 2  232  10.0  3.54 ± 0.20  −4.25 ± 0.19  03  NGC 2298  1  79  16.5  −0.17 ± 0.04  5.62 ± 0.24      2  53  14.0  −0.11 ± 0.06  −1.37 ± 0.25  04  NGC 3201  1  635  13.0  0.14 ± 0.32  −0.57 ± 0.23      2  653  10.0  4.33 ± 0.33  8.39 ± 0.20  05  NGC 4590  1  110  11.0  −0.78 ± 0.10  4.90 ± 0.15      2  111  11.5  −0.12 ± 0.04  −3.86 ± 0.17  06  NGC 4833  1  492  7.0  −3.53 ± 0.35  6.86 ± 0.22      2  570  6.5  1.75 ± 0.15  −0.02 ± 0.20  07  NGC 5024  1  173  11.5  −0.14 ± 0.23  3.09 ± 0.20      2  142  10.0  −45.45 ± 0.77  1.35 ± 0.18  08  NGC 5053  1  44  9.5  0.72 ± 0.16  −0.62 ± 0.16      2  41  8.0  −0.07 ± 0.15  −1.86 ± 0.18      3  38  8.5  0.22 ± 0.13  0.29 ± 0.23      4  42  10.0  0.43 ± 0.09  0.23 ± 0.19      5  52  12.0  0.57 ± 0.16  −0.65 ± 0.16  09  NGC 5286  1  215  17.0  1.63 ± 0.20  1.85 ± 0.25      2  217  16.5  1.61 ± 0.20  1.10 ± 0.21  10  NGC 5466  1  56  7.5  −0.13 ± 0.27  1.37 ± 0.12      2  72  8.0  −0.79 ± 0.29  −2.22 ± 0.20  11  NGC 5897  1  164  9.0  0.12 ± 0.22  −0.37 ± 0.20      2  154  7.5  2.58 ± 0.17  −2.51 ± 0.18  12  NGC 5904  1  579  7.0  −1.29 ± 0.19  −2.69 ± 0.15      2  630  7.0  −0.16 ± 0.15  −1.08 ± 0.22  13  NGC 5927  1  848  8.5  3.65 ± 0.30  −0.38 ± 0.20      2  985  8.0  −9.20 ± 0.35  11.91 ± 0.21  14  NGC 5986  1  278  8.0  −0.54 ± 0.30  1.36 ± 0.22      2  268  6.0  −6.46 ± 0.26  4.12 ± 0.18  15  NGC 6093  1  236  22.0  1.81 ± 0.19  −4.47 ± 0.20  16  NGC 6101  1  235  7.0  −1.85 ± 0.76  1.09 ± 0.21      2  232  6.5  −0.80 ± 0.61  2.90 ± 0.20      3  229  8.0  −15.82 ± 0.73  0.55 ± 0.25      4  263  11.0  −1.83 ± 0.89  −0.52 ± 0.27      5  204  8.5  12.13 ± 0.85  −3.52 ± 0.24  17  NGC 6121  1  633  7.5  −4.81 ± 0.37  7.07 ± 0.23      2  649  7.5  −0.69 ± 0.14  1.90 ± 0.22  18  NGC 6144  1  147  7.0  −4.48 ± 0.18  −0.10 ± 0.19      2  147  9.5  −3.09 ± 0.15  −0.76 ± 0.20  19  NGC 6171  1  172  9.0  4.44 ± 0.20  −2.36 ± 0.16      2  196  7.5  −3.26 ± 0.19  −1.51 ± 0.21  20  NGC 6218  1  300  10.0  0.40 ± 0.07  −4.05 ± 0.17      2  362  6.5  −0.72 ± 0.09  0.13 ± 0.19  21  NGC 6254  1  525  7.5  −0.94 ± 0.11  2.02 ± 0.20      2  507  5.0  −0.08 ± 0.12  −0.52 ± 0.18  22  NGC 6304  1  1021  11.0  0.00 ± 0.09  7.29 ± 0.22      2  1137  11.0  −2.63 ± 0.15  3.66 ± 0.28  23  NGC 6341  1  272  7.0  3.31 ± 0.13  −1.46 ± 0.17      2  276  6.5  1.30 ± 0.16  2.01 ± 0.14  24  NGC 6352  1  676  8.0  0.44 ± 0.09  6.24 ± 0.18      2  658  9.5  0.20 ± 0.11  0.84 ± 0.21  25  NGC 6362  1  399  6.0  −1.98 ± 0.18  −1.19 ± 0.20      2  412  6.0  −0.15 ± 0.23  2.51 ± 0.17  26  NGC 6366  1  265  7.0  1.07 ± 0.11  −4.03 ± 0.22      2  263  6.0  2.89 ± 0.19  4.66 ± 0.18  27  NGC 6388  1  812  6.5  1.68 ± 0.24  4.01 ± 0.26      2  873  7.0  −0.86 ± 0.22  5.67 ± 0.22  28  NGC 6397  1  605  8.0  16.46 ± 0.47  12.42 ± 0.26      2  602  7.0  −4.15 ± 0.33  4.79 ± 0.30  29  NGC 6441  1  1075  10.0  34.98 ± 0.54  −3.77 ± 0.26      2  1114  9.5  4.46 ± 0.31  3.80 ± 0.27  30  NGC 6496  1  482  6.0  1.85 ± 0.30  7.03 ± 0.21      2  525  9.0  8.00 ± 0.28  1.65 ± 0.22  31  NGC 6535  1  304  6.5  −4.97 ± 0.19  0.75 ± 0.18      2  275  7.5  −1.78 ± 0.18  −9.16 ± 0.20  32  NGC 6541  1  385  7.5  −11.12 ± 0.28  7.28 ± 0.23      2  481  9.0  −7.23 ± 0.32  7.72 ± 0.22  33  NGC 6584  1  119  7.5  1.82 ± 0.19  6.15 ± 0.24      2  141  10.0  2.67 ± 0.14  2.71 ± 0.25  34  NGC 6624  1  360  8.5  0.83 ± 0.12  1.93 ± 0.23      2  774  9.5  0.61 ± 0.15  6.08 ± 0.21  35  NGC 6637  1  620  10.0  −3.66 ± 0.20  1.83 ± 0.25      2  615  11.5  −2.54 ± 0.23  −0.80 ± 0.29  36  NGC 6652  1  608  12.0  0.35 ± 0.09  5.17 ± 0.26      2  624  9.5  −0.06 ± 0.06  2.40 ± 0.25      3  682  7.0  0.49 ± 0.15  5.78 ± 0.21  37  NGC 6656  1  734  31.0  −1.20 ± 0.21  3.03 ± 0.25      2  1052  27.5  1.21 ± 0.30  6.93 ± 0.31  38  NGC 6681  1  450  14.0  −4.20 ± 0.25  1.76 ± 0.29      2  385  15.5  −2.37 ± 0.20  1.75 ± 0.32  39  NGC 6715  1  484  16.0  −0.69 ± 0.25  −4.67 ± 0.28      2  505  15.5  3.28 ± 0.18  −1.79 ± 0.27  40  NGC 6717  1  439  14.0  3.48 ± 0.21  0.07 ± 0.30      2  406  13.0  0.49 ± 0.20  1.11 ± 0.28      3  445  12.0  0.11 ± 0.18  4.99 ± 0.26  41  NGC 6723  1  256  14.5  0.14 ± 0.20  0.95 ± 0.30      2  144  11.5  −0.25 ± 0.19  4.59 ± 0.22      3  111  9.0  −0.33 ± 0.16  3.99 ± 0.18  42  NGC 6779  1  345  7.5  0.86 ± 0.07  1.07 ± 0.21      2  333  6.5  3.24 ± 0.16  −8.92 ± 0.19  43  NGC 6791  1  288  9.0  −1.92 ± 0.26  −7.50 ± 0.21      2  274  5.5  −4.98 ± 0.25  −6.18 ± 0.21  44  NGC 6809  1  584  7.0  −1.58 ± 0.19  0.99 ± 0.22      2  590  7.5  −9.19 ± 0.21  −0.68 ± 0.23  45  NGC 6838  1  816  7.0  −8.81 ± 0.24  −12.24 ± 0.22      2  800  6.5  −3.96 ± 0.22  −8.28 ± 0.21  46  NGC 6934  1  87  11.5  3.41 ± 0.20  2.62 ± 0.24      2  84  5.5  1.41 ± 0.10  −12.00 ± 0.17  47  NGC 6981  1  31  14.5  1.29 ± 0.26  −9.37 ± 0.19      2  27  9.5  1.73 ± 0.23  −8.07 ± 0.23  48  NGC 7089  1  92  5.5  −0.55 ± 0.12  −6.84 ± 0.15      2  77  10.5  2.52 ± 0.15  −1.40 ± 0.18      3  82  11.0  1.85 ± 0.12  −0.54 ± 0.20  49  NGC 7099  1  79  4.5  0.93 ± 0.08  −0.89 ± 0.17      2  89  6.5  0.66 ± 0.08  1.75 ± 0.21  #  Cluster  F  Stars  Precision  ΔRA  $$\Delta {\rm \text{Dec.}}$$          (mas)  WFC px  WFC px  01  NGC 1261  1  32  15.5  1.66 ± 0.34  −4.50 ± 0.16      2  41  10.0  −1.89 ± 0.08  4.31 ± 0.21      3  37  8.0  −0.51 ± 0.06  1.61 ± 0.18      4  36  11.0  −0.50 ± 0.14  1.03 ± 0.31      5  46  10.0  0.07 ± 0.05  −4.81 ± 0.19  02  NGC 1851  1 + 2  232  10.0  3.54 ± 0.20  −4.25 ± 0.19  03  NGC 2298  1  79  16.5  −0.17 ± 0.04  5.62 ± 0.24      2  53  14.0  −0.11 ± 0.06  −1.37 ± 0.25  04  NGC 3201  1  635  13.0  0.14 ± 0.32  −0.57 ± 0.23      2  653  10.0  4.33 ± 0.33  8.39 ± 0.20  05  NGC 4590  1  110  11.0  −0.78 ± 0.10  4.90 ± 0.15      2  111  11.5  −0.12 ± 0.04  −3.86 ± 0.17  06  NGC 4833  1  492  7.0  −3.53 ± 0.35  6.86 ± 0.22      2  570  6.5  1.75 ± 0.15  −0.02 ± 0.20  07  NGC 5024  1  173  11.5  −0.14 ± 0.23  3.09 ± 0.20      2  142  10.0  −45.45 ± 0.77  1.35 ± 0.18  08  NGC 5053  1  44  9.5  0.72 ± 0.16  −0.62 ± 0.16      2  41  8.0  −0.07 ± 0.15  −1.86 ± 0.18      3  38  8.5  0.22 ± 0.13  0.29 ± 0.23      4  42  10.0  0.43 ± 0.09  0.23 ± 0.19      5  52  12.0  0.57 ± 0.16  −0.65 ± 0.16  09  NGC 5286  1  215  17.0  1.63 ± 0.20  1.85 ± 0.25      2  217  16.5  1.61 ± 0.20  1.10 ± 0.21  10  NGC 5466  1  56  7.5  −0.13 ± 0.27  1.37 ± 0.12      2  72  8.0  −0.79 ± 0.29  −2.22 ± 0.20  11  NGC 5897  1  164  9.0  0.12 ± 0.22  −0.37 ± 0.20      2  154  7.5  2.58 ± 0.17  −2.51 ± 0.18  12  NGC 5904  1  579  7.0  −1.29 ± 0.19  −2.69 ± 0.15      2  630  7.0  −0.16 ± 0.15  −1.08 ± 0.22  13  NGC 5927  1  848  8.5  3.65 ± 0.30  −0.38 ± 0.20      2  985  8.0  −9.20 ± 0.35  11.91 ± 0.21  14  NGC 5986  1  278  8.0  −0.54 ± 0.30  1.36 ± 0.22      2  268  6.0  −6.46 ± 0.26  4.12 ± 0.18  15  NGC 6093  1  236  22.0  1.81 ± 0.19  −4.47 ± 0.20  16  NGC 6101  1  235  7.0  −1.85 ± 0.76  1.09 ± 0.21      2  232  6.5  −0.80 ± 0.61  2.90 ± 0.20      3  229  8.0  −15.82 ± 0.73  0.55 ± 0.25      4  263  11.0  −1.83 ± 0.89  −0.52 ± 0.27      5  204  8.5  12.13 ± 0.85  −3.52 ± 0.24  17  NGC 6121  1  633  7.5  −4.81 ± 0.37  7.07 ± 0.23      2  649  7.5  −0.69 ± 0.14  1.90 ± 0.22  18  NGC 6144  1  147  7.0  −4.48 ± 0.18  −0.10 ± 0.19      2  147  9.5  −3.09 ± 0.15  −0.76 ± 0.20  19  NGC 6171  1  172  9.0  4.44 ± 0.20  −2.36 ± 0.16      2  196  7.5  −3.26 ± 0.19  −1.51 ± 0.21  20  NGC 6218  1  300  10.0  0.40 ± 0.07  −4.05 ± 0.17      2  362  6.5  −0.72 ± 0.09  0.13 ± 0.19  21  NGC 6254  1  525  7.5  −0.94 ± 0.11  2.02 ± 0.20      2  507  5.0  −0.08 ± 0.12  −0.52 ± 0.18  22  NGC 6304  1  1021  11.0  0.00 ± 0.09  7.29 ± 0.22      2  1137  11.0  −2.63 ± 0.15  3.66 ± 0.28  23  NGC 6341  1  272  7.0  3.31 ± 0.13  −1.46 ± 0.17      2  276  6.5  1.30 ± 0.16  2.01 ± 0.14  24  NGC 6352  1  676  8.0  0.44 ± 0.09  6.24 ± 0.18      2  658  9.5  0.20 ± 0.11  0.84 ± 0.21  25  NGC 6362  1  399  6.0  −1.98 ± 0.18  −1.19 ± 0.20      2  412  6.0  −0.15 ± 0.23  2.51 ± 0.17  26  NGC 6366  1  265  7.0  1.07 ± 0.11  −4.03 ± 0.22      2  263  6.0  2.89 ± 0.19  4.66 ± 0.18  27  NGC 6388  1  812  6.5  1.68 ± 0.24  4.01 ± 0.26      2  873  7.0  −0.86 ± 0.22  5.67 ± 0.22  28  NGC 6397  1  605  8.0  16.46 ± 0.47  12.42 ± 0.26      2  602  7.0  −4.15 ± 0.33  4.79 ± 0.30  29  NGC 6441  1  1075  10.0  34.98 ± 0.54  −3.77 ± 0.26      2  1114  9.5  4.46 ± 0.31  3.80 ± 0.27  30  NGC 6496  1  482  6.0  1.85 ± 0.30  7.03 ± 0.21      2  525  9.0  8.00 ± 0.28  1.65 ± 0.22  31  NGC 6535  1  304  6.5  −4.97 ± 0.19  0.75 ± 0.18      2  275  7.5  −1.78 ± 0.18  −9.16 ± 0.20  32  NGC 6541  1  385  7.5  −11.12 ± 0.28  7.28 ± 0.23      2  481  9.0  −7.23 ± 0.32  7.72 ± 0.22  33  NGC 6584  1  119  7.5  1.82 ± 0.19  6.15 ± 0.24      2  141  10.0  2.67 ± 0.14  2.71 ± 0.25  34  NGC 6624  1  360  8.5  0.83 ± 0.12  1.93 ± 0.23      2  774  9.5  0.61 ± 0.15  6.08 ± 0.21  35  NGC 6637  1  620  10.0  −3.66 ± 0.20  1.83 ± 0.25      2  615  11.5  −2.54 ± 0.23  −0.80 ± 0.29  36  NGC 6652  1  608  12.0  0.35 ± 0.09  5.17 ± 0.26      2  624  9.5  −0.06 ± 0.06  2.40 ± 0.25      3  682  7.0  0.49 ± 0.15  5.78 ± 0.21  37  NGC 6656  1  734  31.0  −1.20 ± 0.21  3.03 ± 0.25      2  1052  27.5  1.21 ± 0.30  6.93 ± 0.31  38  NGC 6681  1  450  14.0  −4.20 ± 0.25  1.76 ± 0.29      2  385  15.5  −2.37 ± 0.20  1.75 ± 0.32  39  NGC 6715  1  484  16.0  −0.69 ± 0.25  −4.67 ± 0.28      2  505  15.5  3.28 ± 0.18  −1.79 ± 0.27  40  NGC 6717  1  439  14.0  3.48 ± 0.21  0.07 ± 0.30      2  406  13.0  0.49 ± 0.20  1.11 ± 0.28      3  445  12.0  0.11 ± 0.18  4.99 ± 0.26  41  NGC 6723  1  256  14.5  0.14 ± 0.20  0.95 ± 0.30      2  144  11.5  −0.25 ± 0.19  4.59 ± 0.22      3  111  9.0  −0.33 ± 0.16  3.99 ± 0.18  42  NGC 6779  1  345  7.5  0.86 ± 0.07  1.07 ± 0.21      2  333  6.5  3.24 ± 0.16  −8.92 ± 0.19  43  NGC 6791  1  288  9.0  −1.92 ± 0.26  −7.50 ± 0.21      2  274  5.5  −4.98 ± 0.25  −6.18 ± 0.21  44  NGC 6809  1  584  7.0  −1.58 ± 0.19  0.99 ± 0.22      2  590  7.5  −9.19 ± 0.21  −0.68 ± 0.23  45  NGC 6838  1  816  7.0  −8.81 ± 0.24  −12.24 ± 0.22      2  800  6.5  −3.96 ± 0.22  −8.28 ± 0.21  46  NGC 6934  1  87  11.5  3.41 ± 0.20  2.62 ± 0.24      2  84  5.5  1.41 ± 0.10  −12.00 ± 0.17  47  NGC 6981  1  31  14.5  1.29 ± 0.26  −9.37 ± 0.19      2  27  9.5  1.73 ± 0.23  −8.07 ± 0.23  48  NGC 7089  1  92  5.5  −0.55 ± 0.12  −6.84 ± 0.15      2  77  10.5  2.52 ± 0.15  −1.40 ± 0.18      3  82  11.0  1.85 ± 0.12  −0.54 ± 0.20  49  NGC 7099  1  79  4.5  0.93 ± 0.08  −0.89 ± 0.17      2  89  6.5  0.66 ± 0.08  1.75 ± 0.21  View Large Astrometrized, stacked images of each observed field have also been produced for both filters with a 1 × 1 pixel sampling. These have been created, for each field, combining all overlapping flc images using the same coordinate transformations that define the common reference frames. Astrometric solutions have been independently derived using the Gaia DR1 catalogue as a reference (Gaia Collaboration 2016). As a consequence, positions are given for Equinox J2000 at epoch 2015. Table 3 reports the precision reached by the new astrometric solution in the fifth column, which is the root mean square error of the offset between Gaia positions and those derived, for the same stars, in our astrometrized stacked images. The measured average value is 0.2 pixel, or ∼10 milliarcsec. These values are also visualized in Fig. 3. Figure 3. View largeDownload slide Precision of the redefined astrometric solution for each observed field, see the text for details. Figure 3. View largeDownload slide Precision of the redefined astrometric solution for each observed field, see the text for details. For completeness, we also measure the astrometric precision of the original astrometric solution of drc images. The position offset between common sources in the Gaia DR1 catalogue and the drc images is used to define this quantity. Offset values are referred to RA and Dec. distances in image pixels and are reported in columns six and seven of Table 3. The associated errors corresponds to the measured standard deviations of each sample. A visual representation is also given in Fig. 4. It can be noted that, in general, offsets are lower than 5 ACS/WFC pixels (0.25 arcsec), and, in many cases, below 1 pixel, with some notable exceptions. Figure 4. View largeDownload slide Offsets between star positions obtained using the original STScI astrometric solution (header of drc images) and Gaia ones. Points represent single fields; the semi-axes of each ellipse have length equal to the measured standard deviation. The grey circle represents an offset of 1 ACS/WFC pixel, which corresponds to ∼0.05 arcsec. Some points falls outside the used limits; we indicate their position along the edges of the figure, the arrows point in their direction. Figure 4. View largeDownload slide Offsets between star positions obtained using the original STScI astrometric solution (header of drc images) and Gaia ones. Points represent single fields; the semi-axes of each ellipse have length equal to the measured standard deviation. The grey circle represents an offset of 1 ACS/WFC pixel, which corresponds to ∼0.05 arcsec. Some points falls outside the used limits; we indicate their position along the edges of the figure, the arrows point in their direction. 3 THE COLOUR–MAGNITUDE DIAGRAMS AND TRICHROMATIC STACKED IMAGES In Fig. 5 we report the CMDs obtained for all five fields of NGC 1261. Magnitudes are given both in the instrumental and Vega-mag photometric systems. Dashed lines represent the saturation limit, black dots represent unsaturated stars, saturated stars are represented with crosses. The five CMDs are all merged in the bottom-right panel, where a different colour have been assigned to each different field. In this case, the instrumental magnitude scale, along with saturation level (dashed line), refer only to Field 1. Figure 5. View largeDownload slide CMDs of the parallel fields of NGC 1261. Both instrumental and calibrated magnitude scales are shown. Dashed lines represent the saturation levels. Saturated stars are marked with crosses. The bottom-right panel collects all the stars present in all the five fields. Stars are here colour-coded as follows. Black dots represent stars measured in F1, red dots stars of F2, green dots stars of F3, blue dots stars of F4, and orange dots represent stars measured in F5. The instrumental magnitude scale and the saturation limit refer to F1 only. Figure 5. View largeDownload slide CMDs of the parallel fields of NGC 1261. Both instrumental and calibrated magnitude scales are shown. Dashed lines represent the saturation levels. Saturated stars are marked with crosses. The bottom-right panel collects all the stars present in all the five fields. Stars are here colour-coded as follows. Black dots represent stars measured in F1, red dots stars of F2, green dots stars of F3, blue dots stars of F4, and orange dots represent stars measured in F5. The instrumental magnitude scale and the saturation limit refer to F1 only. The final CMDs for all other targets are presented in Figs A9–A16. The presented CMDs have been obtained by selecting only high-quality stars, according to quality parameters presented in Anderson et al. (2008). In Section 4 we describe the adopted selection procedure. It is important to mention here that, since in the majority of the cases only one image per filter has been taken, artefact rejection is not an easy task. In order to include faint sources, we have chosen not to limit in flux our raw catalogues. We none the less restricted the detections only to those sources observed in at least one F814W and one F475W image simultaneously, with positions in the common reference frame consistent within 0.8 pixels. No rejection of foreground/background contamination has been performed, nor any correction for differential reddening. The homogeneity of the data guarantees very similar results in every case. None the less, it is out of the scope of this work to characterize in detail the obtained results, which require taking into consideration several issues. For example, the number density of cluster members present in the observed fields depends on the properties of each GC: in some cases, especially for bulge clusters, or those that appear projected in this dense Galactic region, it is difficult to identify the cluster sequence. We recall that another interesting application of the present data is the study of the stellar populations, external to the clusters, that contaminate the observed fields. In particular, for at least six GCs, namely NGC 6624, NGC 6637, NGC 6652, NGC 6681, NGC 6715, and NGC 6809, traces of the Sagittarius Stream are visible in the obtained CMDs (Siegel et al. 2011). As an additional tool to explore and characterize observations, we have created colour images of each observed field, combining the astrometrized stacked images: an example is shown in Fig. 6. The F814W stacked images have been associated with red channel while the F475W images are associated with the blue channel. The images associated with the green channel have been obtained as a result of a 3:1 weighted mean of F475W and F814W counts, respectively. Figure 6. View largeDownload slide Mosaic of all trichromatic images created for NGC 1261; see the text for details. Figure 6. View largeDownload slide Mosaic of all trichromatic images created for NGC 1261; see the text for details. 4 SELECTION OF WELL-MEASURED STARS This section describes the procedure adopted to reject spurious or poorly measured sources in the catalogues and to obtain a sample of bona-fide stellar sources. In this example we refer to the catalogue associated with Field 1 of NGC 6121 (M4). The V3 PA is 272 deg and, for this field, two images were collected, one in F814W, and the other in F475W (666 and 739 s, respectively). For the selection, we have adopted a procedure similar to that described in Milone et al. (2012), defining limits in both q and o (quality) parameters. In addition, we also made use of the RADXS parameter (Bedin et al. 2008, 2009, 2010). As shown in the lower left panels of Fig. 7, the q parameter displays a characteristic trend with magnitude. This parameter is defined as the absolute value of the subtraction between the PSF model and the spatial distribution of light of a particular detection in the image, inside the fitting radius. For a perfectly modelled source, the q parameter assumes value 0. Figure 7. View largeDownload slide Selection procedure for Field 1 of NGC 6121. Left-hand panels: black points mark the stars that were selected as well-measured. Red lines mark the limits chosen for the q, o, and RADXS parameters. Second panel from right: CMD of rejected sources. Right-hand panel: CMD with only sources that survived the selection. All plots refer to instrumental magnitudes. Figure 7. View largeDownload slide Selection procedure for Field 1 of NGC 6121. Left-hand panels: black points mark the stars that were selected as well-measured. Red lines mark the limits chosen for the q, o, and RADXS parameters. Second panel from right: CMD of rejected sources. Right-hand panel: CMD with only sources that survived the selection. All plots refer to instrumental magnitudes. The o parameter quantifies the amount of light that falls on the aperture used for PSF fitting, due to neighbouring sources (Anderson et al. 2008). Unlike the q parameter, it does not show a clear trend with magnitude (middle-left panels of Fig. 7), for this reason we have used a fixed limit (Milone et al. 2012). Finally, the RADXS parameter is related to the spatial extent of the sources, and it is used to distinguish between point sources and extended sources. It is defined as the flux in excess of that predicted from the PSF fitting just outside the core of each source (Bedin et al. 2008). Positive values are expected for extended objects, while negative values indicate detections that are sharper than stellar. The introduction of this parameter in the selection process for the present case is particularly necessary: since only one image per filter is available for most cases, spurious detections due to cosmic rays are present in the catalogues. Moreover, these observations cover the external regions of GCs, where the stellar density is not as high as in the central regions. The observed fields are thus relatively populated by extragalactic, non-stellar objects. We started by taking a more stringent limit in position-consistency for each source. We selected only sources with an rms error in position less than 0.3 pixel. We adopted fixed limits in the o and RADXS parameters for both filters simultaneously. Finally, we measured, for the sources that survived this first selection, the median trend in the plane defined by magnitude and q parameter values. In this way, we removed from the sample non-stellar and poorly measured sources, while not a priori losing faint sources. Red lines in the left-hand panels of Fig. 7 represent the limits used for the case of Field 1 of NGC 6121 (M4). Black points represent sources that passed the selection process. The CMD corresponding to rejected sources (grey dots in the left-hand panels) is shown in the second panel from the right. The resulting CMD is shown in the right-hand panel of Fig. 7. Note how the cleaning process allows the clear detection of the white-dwarf cooling sequence. Saturated stars have been excluded from the selection procedure, but their fluxes have been recovered using the procedure described in Gilliland (2004). 5 RELEASED ELECTRONIC MATERIAL We release, for each ACS/WFC parallel field, the astrometric and photometric catalogues and trichromatic astrometrized stacked images. All the released material will be available for download at the website of the Exoplanets & Stellar Populations Group of the Università degli Studi di Padova.1 Table 4 shows the first 10 rows of the catalogue produced for Field 1 of NGC 6121. The content of each column is explained in detail in Table 5. The parameters wi and wb are the same as in Anderson et al. (2008). They are records that represent the level of saturation of each source in F814W and F475W images, respectively. The last column includes the results of the selection of well-measured stars presented in Section 4. Table 4. First 10 rows extracted from the catalogue referring to Field 1 of NGC 6121 (M4). A detailed description of each column is given in Table 5. #id  RA (2015)  Dec. (2015)  x  dx  y  dy  F814W  di  F475W  db  qi  qb  oi  ob  RADXSi  RADXSb  ni  nb  wi  wb  Good  00000001  245.78962440  −26.47823845  549.416  0.353  688.174  0.224  28.832  9.900  27.078  9.900  1.620  0.760  1.526  0.115  −0.9900  −0.9157  1  1  1  1  0  00000002  245.78961100  −26.47812901  557.336  0.452  687.260  0.382  26.948  9.900  29.764  9.900  1.384  1.274  0.772  9.900  6.2647  −0.9900  1  1  1  1  0  00000003  245.78968050  −26.47823281  549.844  0.632  691.807  0.152  27.164  9.900  28.433  9.900  1.285  1.234  0.179  0.998  −0.9900  −0.9900  1  1  1  1  0  00000004  245.78969050  −26.47817087  554.332  0.391  692.424  0.136  27.335  9.900  30.545  9.900  1.229  1.715  1.014  2.384  0.9430  −0.9900  1  1  1  1  0  00000005  245.78969230  −26.47815496  555.485  0.426  692.537  0.076  27.109  9.900  29.691  9.900  1.189  1.274  0.404  9.900  2.2225  −0.9900  1  1  1  1  0  00000006  245.78973530  −26.47813294  557.094  0.032  695.317  0.263  28.421  9.900  28.038  9.900  1.204  1.009  3.708  0.336  5.6081  −0.9900  1  1  1  1  0  00000007  245.78971500  −26.47807749  561.102  0.068  693.975  0.088  27.108  9.900  28.184  9.900  1.187  0.948  0.787  0.228  1.0828  −0.9900  1  1  1  1  0  00000008  245.78970010  −26.47806742  561.826  0.001  693.008  0.354  26.711  9.900  30.436  9.900  1.154  1.722  0.354  2.059  −0.9900  −0.9900  1  1  1  1  0  00000009  245.78977530  −26.47818364  553.437  0.442  697.928  0.434  27.300  9.900  28.887  9.900  1.330  1.371  0.459  1.208  −0.7255  9.9900  1  1  1  1  0  00000010  245.79005820  −26.47833427  542.628  0.483  716.320  0.322  26.991  9.900  28.863  9.900  1.025  1.193  0.699  5.580  1.8430  1.1484  1  1  1  1  0  #id  RA (2015)  Dec. (2015)  x  dx  y  dy  F814W  di  F475W  db  qi  qb  oi  ob  RADXSi  RADXSb  ni  nb  wi  wb  Good  00000001  245.78962440  −26.47823845  549.416  0.353  688.174  0.224  28.832  9.900  27.078  9.900  1.620  0.760  1.526  0.115  −0.9900  −0.9157  1  1  1  1  0  00000002  245.78961100  −26.47812901  557.336  0.452  687.260  0.382  26.948  9.900  29.764  9.900  1.384  1.274  0.772  9.900  6.2647  −0.9900  1  1  1  1  0  00000003  245.78968050  −26.47823281  549.844  0.632  691.807  0.152  27.164  9.900  28.433  9.900  1.285  1.234  0.179  0.998  −0.9900  −0.9900  1  1  1  1  0  00000004  245.78969050  −26.47817087  554.332  0.391  692.424  0.136  27.335  9.900  30.545  9.900  1.229  1.715  1.014  2.384  0.9430  −0.9900  1  1  1  1  0  00000005  245.78969230  −26.47815496  555.485  0.426  692.537  0.076  27.109  9.900  29.691  9.900  1.189  1.274  0.404  9.900  2.2225  −0.9900  1  1  1  1  0  00000006  245.78973530  −26.47813294  557.094  0.032  695.317  0.263  28.421  9.900  28.038  9.900  1.204  1.009  3.708  0.336  5.6081  −0.9900  1  1  1  1  0  00000007  245.78971500  −26.47807749  561.102  0.068  693.975  0.088  27.108  9.900  28.184  9.900  1.187  0.948  0.787  0.228  1.0828  −0.9900  1  1  1  1  0  00000008  245.78970010  −26.47806742  561.826  0.001  693.008  0.354  26.711  9.900  30.436  9.900  1.154  1.722  0.354  2.059  −0.9900  −0.9900  1  1  1  1  0  00000009  245.78977530  −26.47818364  553.437  0.442  697.928  0.434  27.300  9.900  28.887  9.900  1.330  1.371  0.459  1.208  −0.7255  9.9900  1  1  1  1  0  00000010  245.79005820  −26.47833427  542.628  0.483  716.320  0.322  26.991  9.900  28.863  9.900  1.025  1.193  0.699  5.580  1.8430  1.1484  1  1  1  1  0  View Large Table 5. Information provided by each catalogue. Col.  Name  Explanation  01  id  ID number for each star  02  RA  Right ascension for each star (in deg, epoch 2015)  03  Dec.  Declination for each star (in deg, epoch 2015)  04  x  x position of each star on the reference frame (in pixels)  05  dx  rms errors associated with x position (in pixels)  06  y  y position of each star on the reference frame (in pixels)  07  dy  rms errors associated with y position (in pixels)  08  F814W  F814W magnitude calibrated into Vegamag system  09  di  rms errors associated with F814W magnitude  10  F475W  F475W magnitude calibrated into Vegamag system  11  db  rms errors associated with F475W magnitude  12  qi  q parameter for F814W magnitudes  13  qb  q parameter for F475W magnitudes  14  oi  o parameter for F814W magnitudes  15  ob  o parameter for F475W magnitudes  16  RADXSi  RADXS parameter for F814W  17  RADXSb  RADXS parameter for F475W  18  ni  Number of F814W images the source has been detected in  19  nb  Number of F475W images the source has been detected in  20  wi  Source of F814W photometry      1: unsaturated in deep; 2: unsaturated in short;      3: saturated in short; 4: saturated in deep;  21  wb  Source of F475W photometry (same as wi)  22  good  The source has passed the selection process  Col.  Name  Explanation  01  id  ID number for each star  02  RA  Right ascension for each star (in deg, epoch 2015)  03  Dec.  Declination for each star (in deg, epoch 2015)  04  x  x position of each star on the reference frame (in pixels)  05  dx  rms errors associated with x position (in pixels)  06  y  y position of each star on the reference frame (in pixels)  07  dy  rms errors associated with y position (in pixels)  08  F814W  F814W magnitude calibrated into Vegamag system  09  di  rms errors associated with F814W magnitude  10  F475W  F475W magnitude calibrated into Vegamag system  11  db  rms errors associated with F475W magnitude  12  qi  q parameter for F814W magnitudes  13  qb  q parameter for F475W magnitudes  14  oi  o parameter for F814W magnitudes  15  ob  o parameter for F475W magnitudes  16  RADXSi  RADXS parameter for F814W  17  RADXSb  RADXS parameter for F475W  18  ni  Number of F814W images the source has been detected in  19  nb  Number of F475W images the source has been detected in  20  wi  Source of F814W photometry      1: unsaturated in deep; 2: unsaturated in short;      3: saturated in short; 4: saturated in deep;  21  wb  Source of F475W photometry (same as wi)  22  good  The source has passed the selection process  View Large Note that for many stars, we report a magnitude rms error of 9.900. This is because the routine empirically determines errors based on multiple observations in each filter. When there is only one observation per filter, the error is given a high default value. 6 SUMMARY AND CONCLUSIONS In the context of the Hubble Space Telescope UV Legacy Survey Treasury program of Galactic Globular Clusters (GO-13297; PI: Piotto, Paper I), we are releasing the photometric catalogues relative to the ACS/WFC parallel observations. They represent the first HST photometric survey of external regions of Galactic GCs and consist of 109 distinct stellar fields of 49 targets: 48 GCs and one open cluster, NGC 6791. In the majority of cases, only two images per field were taken, one in F814W and other in F475W, centred at about 6.5 arcmin from cluster centre. Exposure times were selected in order to obtain reliable photometry of the main sequence of target GCs. These observations complement the WFC3 observations of the central regions of the surveyed GCs, and represent valuable tools for different investigations as outlined in Paper I. These data represent a first epoch for future studies aimed at proper motions measurements in these regions. Even without proper motions, these catalogues are suitable to various interesting studies such as measurements of mass functions and binaries fractions in external regions of GCs. Furthermore, crowding is not an issue in these external cluster fields, as a result, this data base could be also used as an input list for spectroscopic follow-up, for example for precise chemical tagging of cluster members. Acknowledgements We thank the anonymous referee for his careful revision that improved the quality of the present manuscript. MS, AA, and GP acknowledge support from the Spanish Ministry of Economy and Competitiveness (MINECO) under grant AYA2013-42781. MS and AA acknowledge support from the Instituto de Astrofísica de Canarias (IAC) under grant 309403. GP acknowledges partial support by the Università degli Studi di Padova Progetto di Ateneo CPDA141214 ‘Towards understanding complex star formation in Galactic globular clusters’ and by INAF under the program PRIN-INAF2014. APM acknowledges support by the Australian Research Council through Discovery Early Career Researcher Award DE150101816 and by the ERC-StG 2016 716082 project ‘GALFOR’ funded by the European Research Council. This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. 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R., Aparicio A., Piotto G., Vesperini E., Hong J., 2016, MNRAS , 463, 449 CrossRef Search ADS   Sollima A., Baumgardt H., 2017, MNRAS , 471, 3668 https://doi.org/10.1093/mnras/stx1856 CrossRef Search ADS   Vesperini E., Heggie D. C., 1997, MNRAS , 289, 898 https://doi.org/10.1093/mnras/289.4.898 CrossRef Search ADS   Vesperini E., McMillan S. L. W., D’Antona F., D’Ercole A., 2013, MNRAS , 429, 1913 https://doi.org/10.1093/mnras/sts434 CrossRef Search ADS   Yong D., Grundahl F., 2008, ApJ , 672, L29 https://doi.org/10.1086/525850 CrossRef Search ADS   APPENDIX A: EXTRA MATERIAL In this section we complement Figs 1 and 5 with those referred to the rest of the sample. In particular, Figs A1–A8 complement the sample of finding charts presented in Fig. 1. Figs A9–A16 are analogues to the lower right panel of Fig. 5. Figure A1. View largeDownload slide As in Fig. 1 but for NGC 4590, NGC 4833, NGC 5024, NGC 5053, NGC 5286, and NGC 5466. Figure A1. View largeDownload slide As in Fig. 1 but for NGC 4590, NGC 4833, NGC 5024, NGC 5053, NGC 5286, and NGC 5466. Figure A2. View largeDownload slide As in Fig. 1 but for NGC 5897, NGC 5904, NGC 5927, NGC 5986, NGC 6093, and NGC 6101. Figure A2. View largeDownload slide As in Fig. 1 but for NGC 5897, NGC 5904, NGC 5927, NGC 5986, NGC 6093, and NGC 6101. Figure A3. View largeDownload slide As in Fig. 1 but for NGC 6121, NGC 6144, NGC 6171, NGC 6218, NGC 6254, and NGC 6304. Figure A3. View largeDownload slide As in Fig. 1 but for NGC 6121, NGC 6144, NGC 6171, NGC 6218, NGC 6254, and NGC 6304. Figure A4. View largeDownload slide As in Fig. 1 but for NGC 6341, NGC 6352, NGC 6362, NGC 6366, NGC 6388, and NGC 6397. Figure A4. View largeDownload slide As in Fig. 1 but for NGC 6341, NGC 6352, NGC 6362, NGC 6366, NGC 6388, and NGC 6397. Figure A5. View largeDownload slide As in Fig. 1 but for NGC 6441, NGC 6496, NGC 6535, NGC 6541, NGC 6584, and NGC 6624. Figure A5. View largeDownload slide As in Fig. 1 but for NGC 6441, NGC 6496, NGC 6535, NGC 6541, NGC 6584, and NGC 6624. Figure A6. View largeDownload slide As in Fig. 1 but for NGC 6637, NGC 6652, NGC 6656, NGC 6681, NGC 6715, and NGC 6717. Figure A6. View largeDownload slide As in Fig. 1 but for NGC 6637, NGC 6652, NGC 6656, NGC 6681, NGC 6715, and NGC 6717. Figure A7. View largeDownload slide As in Fig. 1 but for NGC 6723, NGC 6779, NGC 6791, NGC 6809, NGC 6838, and NGC 6934. Figure A7. View largeDownload slide As in Fig. 1 but for NGC 6723, NGC 6779, NGC 6791, NGC 6809, NGC 6838, and NGC 6934. Figure A8. View largeDownload slide As in Fig. 1 but for NGC 6981, NGC 7089, and NGC 7099. Figure A8. View largeDownload slide As in Fig. 1 but for NGC 6981, NGC 7089, and NGC 7099. Figure A9. View largeDownload slide Obtained CMDs for NGC 1851, NGC 2298, NGC 3201, NGC 4590, and NGC 5024. For each cluster, CMDs for all fields have been merged together. The colour-code is such that black dots represent stars measured in F1; red dots, stars of F2; green dots stars of F3; blue dots stars of F4 and orange dots represent stars measured in F5. Dashed lines represent the saturation level for all F1, saturated stars are represented by crosses. Figure A9. View largeDownload slide Obtained CMDs for NGC 1851, NGC 2298, NGC 3201, NGC 4590, and NGC 5024. For each cluster, CMDs for all fields have been merged together. The colour-code is such that black dots represent stars measured in F1; red dots, stars of F2; green dots stars of F3; blue dots stars of F4 and orange dots represent stars measured in F5. Dashed lines represent the saturation level for all F1, saturated stars are represented by crosses. Figure A10. View largeDownload slide As in Fig. A9 but for NGC 5053, NGC 5286, NGC 5466, NGC 5897, NGC 5904, and NGC 5927. Figure A10. View largeDownload slide As in Fig. A9 but for NGC 5053, NGC 5286, NGC 5466, NGC 5897, NGC 5904, and NGC 5927. Figure A11. View largeDownload slide As in Fig. A9 but for NGC 5986, NGC 6093, NGC 6101, NGC 6121, NGC 6144, and NGC 6171. Figure A11. View largeDownload slide As in Fig. A9 but for NGC 5986, NGC 6093, NGC 6101, NGC 6121, NGC 6144, and NGC 6171. Figure A12. View largeDownload slide As in Fig. A9 but for NGC 6218, NGC 6254, NGC 6304, NGC 6341, NGC 6352, and NGC 6362. Figure A12. View largeDownload slide As in Fig. A9 but for NGC 6218, NGC 6254, NGC 6304, NGC 6341, NGC 6352, and NGC 6362. Figure A13. View largeDownload slide As in Fig. A9 but for NGC 6366, NGC 6388, NGC 6397, NGC 6441, NGC 6496, and NGC 6535. Figure A13. View largeDownload slide As in Fig. A9 but for NGC 6366, NGC 6388, NGC 6397, NGC 6441, NGC 6496, and NGC 6535. Figure A14. View largeDownload slide As in Fig. A9 but for NGC 6541, NGC 6584, NGC 6624, NGC 6637, NGC 6652, and NGC 6656. Figure A14. View largeDownload slide As in Fig. A9 but for NGC 6541, NGC 6584, NGC 6624, NGC 6637, NGC 6652, and NGC 6656. Figure A15. View largeDownload slide As in Fig. A9 but for NGC 6681, NGC 6715, NGC 6717, NGC 6723, NGC 6779 and NGC 6791. Figure A15. View largeDownload slide As in Fig. A9 but for NGC 6681, NGC 6715, NGC 6717, NGC 6723, NGC 6779 and NGC 6791. Figure A16. View largeDownload slide As in Fig. A9 but for NGC 6809, NGC 6838, NGC 6934, NGC 6981, NGC 7089, and NGC 7099. Figure A16. View largeDownload slide As in Fig. A9 but for NGC 6809, NGC 6838, NGC 6934, NGC 6981, NGC 7089, and NGC 7099. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Monthly Notices of the Royal Astronomical Society Oxford University Press

The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XIII. ACS/WFC parallel-field catalogues

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Oxford University Press
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© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
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0035-8711
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1365-2966
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10.1093/mnras/sty177
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Abstract

Abstract As part of the Hubble Space Telescope UV Legacy Survey of Galactic globular clusters, 110 parallel fields were observed with the Wide Field Channel of the Advanced Camera for Surveys, in the outskirts of 48 globular clusters, plus the open cluster NGC 6791. Totalling about 0.3 deg2 of observed sky, this is the largest homogeneous Hubble Space Telescope photometric survey of Galalctic globular clusters outskirts to date. In particular, two distinct pointings have been obtained for each target on average, all centred at about 6.5 arcmin from the cluster centre, thus covering a mean area of about 23 arcmin2 for each globular cluster. For each field, at least one exposure in both F475W and F814W filters was collected. In this work, we publicly release the astrometric and photometric catalogues and the astrometrized atlases for each of these fields. atlases, catalogues, Hertzsprung–Russell and colour–magnitude diagrams, globular clusters: general 1 INTRODUCTION For almost three decades, the Milky Way globular clusters (GCs) have been the target of large CCD photometric surveys aimed at sampling their stellar populations in a homogeneous way (Rosenberg, Piotto & Saviane 2000a, Rosenberg, Aparicio & Saviane 2000b, Piotto et al. 2002, Sarajedini et al. 2007) using both space- and ground-based instruments. The growing sample of data, and the advent of increasingly sophisticated data-analysis techniques, have clearly demonstrated that GCs host distinct stellar populations with different chemical abundances. High-precision photometric measurements have revealed that the colour–magnitude diagrams (CMDs) show distinct sequences in various evolutionary stages (see e.g. Anderson 1997, Lee et al. 1999; Pancino et al. 2000; Bedin et al. 2004; Piotto et al. 2007; Milone et al. 2008; Bellini et al. 2010). These findings are also supported by spectroscopical evidence that the stellar populations of these systems are not as simple as thought (see e.g. Kraft 1994; Gratton et al. 2004; Marino et al. 2008; Yong & Grundahl 2008; Carretta et al. 2009a; Carretta, Bragaglia & Gratton 2009b; Gratton et al. 2012). The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters (GO-13297; PI: Piotto) has been specifically designed to further investigate this phenomenon and it now appears likely that all Galactic GCs host multiple stellar populations (Piotto et al. 2015 – hereafter Paper I; Milone et al. 2017 – hereafter Paper IX). In the context of this survey, parallel Advanced Camera for Surveys (ACS) observations have been obtained. While the main observations were taken using a combination of UV and optical filters of the Wide Field Camera 3 (WFC3), the lack of filters bluer than F435W dictated the use of the F475W and F814W filters of the Wide Field Channel of the ACS (ACS/WFC) in the parallel observations. The large colour baseline provided by this filter combination guarantees sensitivity to helium abundance differences, while being largely insensitive to star-to-star variations in light-element abundances (Sbordone, Salaris & Weiss 2011, Cassisi, Salaris & Pietrinferni 2017). One of the main objectives for which these observations were planned is to investigate how different stellar populations formed in GCs. Strong observational constraints come from the analysis of the radial distribution of each stellar population (D’Ercole et al. 2008; Bellini et al. 2009; Vesperini, McMillan & D’Antona 2013). As an example, Simioni et al. (2016) complemented WFC3 data of the central regions of NGC 2808 with ACS parallel observations and found evidence of different radial trends associated with distinct stellar populations hosted by the cluster. Thus, clusters with large helium variations among their stellar populations are the preferred target of investigation with the current data-sample. Other interesting targets, albeit extensively studied, are those defined as Type-II clusters in Paper IX, which displays multiple sub-giant branches in optical CMDs. We stress the fact that this is the first homogeneous HST photometric survey of the outskirts of Galactic GCs. The observations presented here represent a first epoch for future studies aimed at systematical measurements of absolute, relative and internal proper motions of stars in these regions. Archival HST observations matching a sub-sample of the observed fields exist, and proper motions will be published separately. In the imaged stellar fields, the stellar density is not as high as in the central regions. As a consequence, crowding is not a serious issue for these data. That makes them particularly suitable to be used as input catalogue for future spectroscopic surveys. The present catalogues can be used to perform several interesting analyses. For example, dynamical interactions between stars in GCs is at the origin of the mass segregation phenomenon. A precise estimate of its effects is fundamental for the derivation of a global mass function for a GC (Vesperini & Heggie 1997; Paust et al. 2010; Sollima & Baumgardt 2017). The measurement of the fraction of binaries is also fundamental for this kind of analysis and could provide useful constraints for dynamical models (Milone et al. 2012). We note, also, that in some cases white dwarf cooling sequences are visible in the obtained CMDs. Finally, it is interesting to note that due to the presence of many extragalactic objects in the observed field, other studies could benefit from these observations. In this work, we present the first photometric catalogues from the ACS/WFC parallel observations of the GO-13297 program. All data have been reduced in a homogeneous manner, making these catalogues particularly suitable for intercomparison. The article is organized as follows: in Section 2 the data are presented along with some information about the observing strategy, together with a detailed description of the data reduction. The extracted CMDs are presented in Section 3. Details on the selection of well-measured stars are given in Section 4. In Section 5 the catalogues and the released electronic material are described in detail. Finally, in Section 6, after a summary, we briefly discuss some of the main scientific questions we will address with these catalogues in subsequent papers. 2 OBSERVATIONS AND DATA REDUCTION Table reports the log of ACS/WFC observations used to construct the catalogues. For each target, we indicate the total number of orbits assigned to each observed field separated by the different position angles (PAs) of the V3 axis of the HST focal plane. Typically, one F475W and one F814W image were taken each orbit, with a dither between the two dictated by primary WFC3 observing strategy. For each field, right ascension and declination of the centre of ACS/WFC are provided along with exposure time in each filter. Table 1. Observation log. For each GC in the survey, and the open cluster NGC 6791, we show right ascension and declination of each distinct parallel field, referred to the centre of ACS/WFC. We also report the number of orbits, telescope orientation (V3 PA) for each orbit, and exposure time in each filter. #  Cluster  Orbits  Field  RA (J2000)  Dec. (J2000)  Epoch  Exp. time  Exp. time        [PA (deg)]  (h m s)  (° ΄ ΄)    F475W (s)  F814W (s)  01  NGC 1261  5  F1 [92]  03:12:49.68  −55:17:25.2  31/08/13  770  694        F2 [138]  03:12:16.96  −55:19:29.6  11/09/13  745  669        F3 [182]  03:11:44.35  −55:17:39.3  08/11/13  766  690        F4 [225]  03:11:30.53  −55:13:17.9  07/12/13  745  669        F5 [48]  03:13:01.92  −55:12:51.4  29/06/14  829  753  02  NGC 1851  7  F1 [195]  05:13:37.57  −40:06:44.0  27/12/10  2 × 40; 2 × 1277; 1237  6 × 488; 1 × 40        F2 [164]  05:13:51.57  −40:08:55.8  11/11/10  2 × 40; 2 × 1277; 2 × 1237  8 × 488; 2 × 40  03  NGC 2298  4  F1 [185]  06:48:36.04  −36:05:08.7  18/12/13  2 × 785  2 × 683        F2 [273]  06:48:35.14  −35:55:40.7  07/03/14  885  816              15/03/15  887  815  04  NGC 3201  2  F1 [25]  10:18:12.19  −46:21:47.1  13/09/13  685  612        F2 [115]  10:17:54.00  −46:30:51.0  01/01/14  689  616  05  NGC 4590  2  F1 [112]  12:39:42.64  −26:50:34.9  21/12/13  627  554        F2 [202]  12:39:01.10  −26:47:48.5  30/03/14  627  554  06  NGC 4833  4  F1 [113]  13:00:10.12  −70:58:29.4  17/01/14  2 × 840  2 × 771        F2 [202]  12:58:20.68  −70:55:42.6  09/04/14  2 × 806  2 × 730  07  NGC 5024  6  F1 [31]  13:13:21.59  +18:12:01.1  24/03/14  4 × 725; 2 × 723  3 × 370        F2 [120]  13:13:03.82  18:03:52.0  08/12/13  4 × 775; 2 × 774  3 × 375  08  NGC 5053  5  F1 [352]  13:16:42.03  +17:47:28.7  01/04/14  740  664        F2 [37]  13:16;53.97  +17:43:15.0  16/03/14  740  664        F3 [80]  13:16;50.28  +17:38:33.1  23/01/14  790  714        F4 [125]  13:16;33.21  +17:35:39.5  05/12/13  790  714        F5 [308]  13:16;21.88  +17:48:25.0  16/05/14  765  689  09  NGC 5286  2  F1 [73]  13:47:07.09  −51:25:00.2  14/12/13  728  655        F2 [162]  13:46:11.09  −51:28:46.7  15/03/14  603  559  10  NGC 5466  4  F1 [112]  14:05:41.08  +28:26:12.1  05/01/14  834; 835  763; 765        F2 [21]  14:05:54.29  +28:35:18.1  29/03/14  2 × 776  2 × 700  11  NGC 5897  4  F1 [112]  15:17:37.48  −21:06:27.5  12/02/14  830; 833  2 × 761        F2 [202]  15:16:58.75  −21:03:44.3  13/05/14  779; 781  710; 709  12  NGC 5904  2  F1 [323]  15:18:34.23  02:11:38.3  17/05/14  620  559        F2 [52]  15:19:00.34  02:04:36.4  08/04/14  621  559  13  NGC 5927  3  F1 [100]  15:28:28.73  −50:45:30.4  01/02/14  603  559        F2 [189]  15:27:28.26  −50:44:48.7  19/05/14  603  559              27/05/15  603  559  14  NGC 5986  3  F1 [92]  15:46:28.87  −37:51:38.6  27/01/14  676  603        F2 [180]  15:45:40.83  −37:52:21.9  17/05/14  603  559              10/05/15  603  559  15  NGC 6093  5  F1 [255]  16:16:35.04  −22:55:45.9  09/06/12  5 × 760; 5 × 845  5 × 539  16  NGC 6101  5  F1 [147]  16:25:36.08  −72:18:35.2  04/04/14  762  686        F2 [190]  16:24:40.57  −72:16:07.7  25/05/14  762  686        F3 [235]  16:24:23.48  −72:11:18.6  29/06/14  800  724        F4 [282]  16:24:58.26  −72:06:50.7  14/08/13  851  775        F5 [101]  16:26:39.71  −72:17:19.9  28/02/14  800  724  17  NGC 6121  2  F1 [272]  16:23:12.57  −26:27:02.1  06/07/14  739  666        F2 [98]  16:23:55.64  −26:36:33.4  17/02/15  666  593  18  NGC 6144  2  F1 [83]  16:27:39.42  −26:05:00.9  28/02/14  679  606        F2 [174]  16:26:57.33  −26:07:05.2  27/05/14  679  606  19  NGC 6171  4  F1 [342]  16:32:41.94  −12:56:56.1  31/05/14  830; 833  2 × 761        F2 [72]  16:32:42.20  −12:57:07.5  25/03/14  800; 802  731; 730  20  NGC 6218  2  F1 [276]  16:46:55.21  −01:52:03.7  16/08/13  721  648        F2 [6]  16:47:33.42  −01:52:07.7  27/05/14  645  572  21  NGC 6254  2  F1 [276]  16:56:50.04  −04:01:10.1  16/08/13  721  648        F2 [7]  16:57:28.66  −04:01:19.2  27/05/14  644  571  22  NGC 6304  2  F1 [184]  17:14:10.53  −29:32:35.1  07/06/14  624  559        F2 [274]  17:14:09.53  −29:23:04.8  26/08/13  731  658  23  NGC 6341  2  F1 [230]  17:16:30.21  +43:08:03.0  22/10/13  638  565        F2 [319]  17:17:06.19  +43:14:56.0  03/08/14  750  677  24  NGC 6352  2  F1 [161]  17:25:15.00  −48:31:41.7  27/05/14  637  564        F2 [251]  17:24:51.02  −48:22:52.5  13/08/13  731  658  25  NGC 6362  2  F1 [125]  17:32:14.07  −67:09:25.3  30/03/14  651  578        F2 [215]  17:30:47.67  −67:04:37.3  01/07/14  760  687  26  NGC 6366  2  F1 [293]  17:27:31.94  −04:58:44.4  26/08/13  726  653        F2 [351]  17:27:58.09  −04:58:57.1  07/06/15  644  571  27  NGC 6388  4  F1 [238]  17:35:40.80  −44:43:02.9  05/07/14  865; 906  796; 834        F2 [141]  17:36:17.23  −44:50:53.5  12/05/14  793; 795  724; 723  28  NGC 6397  2  F1 [90]  17:41:17.47  −53:44:46.1  27/03/14  683  610        F2 [180]  17:40:12.50  −53:45:38.3  11/06/14  640  567  29  NGC 6441  4  F1 [102]  17:50:34.23  −37:08:21.5  26/03/14  833; 835  764; 763        F2 [192]  17:49:46.24  −37:07:12.6  15/06/14  2 × 794  725; 722  30  NGC 6496  2  F1 [157]  17:58:53.12  −44:22:28.2  30/05/14  638  656        F2 [247]  17:58:27.50  −44:13:57.0  12/08/13  731  658  31  NGC 6535  2  F1 [319]  18:03:49.95  +00:11:04.6  19/07/14  724  651        F2 [50]  18:04:17.63  +00:17:50.2  26/05/14  644  571  32  NGC 6541  2  F1 [80]  18:08:35.16  −43:46:11.5  14/02/14  689  616        F2 [170]  18:07:44.04  −43:48:48.8  11/06/14  639  566  33  NGC 6584  2  F1 [155]  18:18:26.74  −52:19:31.3  30/05/14  640  567        F2 [245]  18:17:54.89  −52:11:09.9  18/08/13  726  653  34  NGC 6624  2  F1 [264]  18:23:14.50  −30:17:51.1  03/09/13  731  658        F2 [174]  18:23:23.29  −30:27:20.2  27/06/14  638  565  35  NGC 6637  4  F1 [84]  18:31:49.55  −32:24:40.9  18/02/14  827; 840  758; 768        F2 [174]  18:31:05.49  −32:26:31.5  30/06/14  792; 794  723; 722  36  NGC 6652  3  F1 [281]  18:35:25.09  −32:54:08.8  30/08/13  707  633        F2 [238]  18:35:14.87  −32:58:25.1  14/08/13  733  658        F3 [192]  18:35:20.06  −33:03:36.1  01/07/14  621  548  37  NGC 6656  4  F1 [266]  18:36:00.25  −23:50:16.5  23/09/10  656;644  2 × 389        F2 [85]  18:36:47.76  −23:58:08.0  17/03/11  2 × 656  2 × 389  38  NGC 6681  2  F1 [271]  18:42:48.78  −32:13:03.7  05/09/13  730  658        F2 [183]  18:42:51.19  −32:22:33.1  29/06/14  637  564  39  NGC 6715  6  F1 [153]  18:54:56.04  −30:35:08.9  29/06/14  2 × 736; 2 × 737; 2 × 734  3 × 370        F2 [243]  18:54:33.96  −30:27:06.5  05/09/13  4 × 819; 2 × 817  3 × 390  40  NGC 6717  3  F1 [80]  18:55:31.98  −22:45:22.7  06/05/14  619  535        F2 [125]  18:55:12.41  −22:48:27.0  03/07/14  619  544        F3 [171]  18:54:51.25  −22:47:57.0  03/07/14  617  544  41  NGC 6723  3  F1 [103]  18:59:54.19  −36:43:18.0  03/04/14  666  592        F2 [148]  18:59:27.49  −36:44:21.9  15/06/14  626  551        F3 [193]  18:59:06.07  −36:41:59.9  05/07/14  624  551  42  NGC 6779  2  F1 [241]  19:16:04.77  +30:12:18.9  13/10/13  731  658        F2 [333]  19:16:42.17  +30:17:38.2  19/07/14  637  564  43  NGC 6791  2  F1 [322]  19:20:53.69  +37:53:04.8  17/08/13  631  559        F2 [50]  19:21:27.31  +37:46:16.6  26/04/14  638  565  44  NGC 6809  2  F1 [262]  19:39:32.79  −30:54:19.4  21/08/14  753  680        F2 [82]  19:40:26.65  −31:01:26.4  29/03/14  677  604  45  NGC 6838  2  F1 [244]  19:53:18.63  +18:48:20.3  23/10/13  723  650        F2 [75]  19:54:12.48  +18:43:53.9  03/05/14  681  608  46  NGC 6934  2  F1 [245]  20:33:44.84  +07:25:55.5  08/10/13  723  650        F2 [334]  20:34:17.59  +07:30:52.3  18/08/14  644  571  47  NGC 6981  2  F1 [289]  20:53:13.45  −12:26:24.9  13/08/13  641  568        F2 [19]  20:53:51.29  −12:28:39.5  03/08/14  624  551  48  NGC 7089  3  F1 [237]  21:32:59.85  +00:48:42.0  18/10/13  717  643        F2 [281]  21:33:10.93  +00:44:15.0  29/08/13  668  593        F3 [327]  21:33:29.92  +00:42:39.1  14/08/13  611  534  49  NGC 7099  2  F1 [92]  21:40:44.34  −23:15:15.1  08/06/14  656  583        F2 [182]  21:40:02.30  −23:15:49.0  19/08/14  656  583  #  Cluster  Orbits  Field  RA (J2000)  Dec. (J2000)  Epoch  Exp. time  Exp. time        [PA (deg)]  (h m s)  (° ΄ ΄)    F475W (s)  F814W (s)  01  NGC 1261  5  F1 [92]  03:12:49.68  −55:17:25.2  31/08/13  770  694        F2 [138]  03:12:16.96  −55:19:29.6  11/09/13  745  669        F3 [182]  03:11:44.35  −55:17:39.3  08/11/13  766  690        F4 [225]  03:11:30.53  −55:13:17.9  07/12/13  745  669        F5 [48]  03:13:01.92  −55:12:51.4  29/06/14  829  753  02  NGC 1851  7  F1 [195]  05:13:37.57  −40:06:44.0  27/12/10  2 × 40; 2 × 1277; 1237  6 × 488; 1 × 40        F2 [164]  05:13:51.57  −40:08:55.8  11/11/10  2 × 40; 2 × 1277; 2 × 1237  8 × 488; 2 × 40  03  NGC 2298  4  F1 [185]  06:48:36.04  −36:05:08.7  18/12/13  2 × 785  2 × 683        F2 [273]  06:48:35.14  −35:55:40.7  07/03/14  885  816              15/03/15  887  815  04  NGC 3201  2  F1 [25]  10:18:12.19  −46:21:47.1  13/09/13  685  612        F2 [115]  10:17:54.00  −46:30:51.0  01/01/14  689  616  05  NGC 4590  2  F1 [112]  12:39:42.64  −26:50:34.9  21/12/13  627  554        F2 [202]  12:39:01.10  −26:47:48.5  30/03/14  627  554  06  NGC 4833  4  F1 [113]  13:00:10.12  −70:58:29.4  17/01/14  2 × 840  2 × 771        F2 [202]  12:58:20.68  −70:55:42.6  09/04/14  2 × 806  2 × 730  07  NGC 5024  6  F1 [31]  13:13:21.59  +18:12:01.1  24/03/14  4 × 725; 2 × 723  3 × 370        F2 [120]  13:13:03.82  18:03:52.0  08/12/13  4 × 775; 2 × 774  3 × 375  08  NGC 5053  5  F1 [352]  13:16:42.03  +17:47:28.7  01/04/14  740  664        F2 [37]  13:16;53.97  +17:43:15.0  16/03/14  740  664        F3 [80]  13:16;50.28  +17:38:33.1  23/01/14  790  714        F4 [125]  13:16;33.21  +17:35:39.5  05/12/13  790  714        F5 [308]  13:16;21.88  +17:48:25.0  16/05/14  765  689  09  NGC 5286  2  F1 [73]  13:47:07.09  −51:25:00.2  14/12/13  728  655        F2 [162]  13:46:11.09  −51:28:46.7  15/03/14  603  559  10  NGC 5466  4  F1 [112]  14:05:41.08  +28:26:12.1  05/01/14  834; 835  763; 765        F2 [21]  14:05:54.29  +28:35:18.1  29/03/14  2 × 776  2 × 700  11  NGC 5897  4  F1 [112]  15:17:37.48  −21:06:27.5  12/02/14  830; 833  2 × 761        F2 [202]  15:16:58.75  −21:03:44.3  13/05/14  779; 781  710; 709  12  NGC 5904  2  F1 [323]  15:18:34.23  02:11:38.3  17/05/14  620  559        F2 [52]  15:19:00.34  02:04:36.4  08/04/14  621  559  13  NGC 5927  3  F1 [100]  15:28:28.73  −50:45:30.4  01/02/14  603  559        F2 [189]  15:27:28.26  −50:44:48.7  19/05/14  603  559              27/05/15  603  559  14  NGC 5986  3  F1 [92]  15:46:28.87  −37:51:38.6  27/01/14  676  603        F2 [180]  15:45:40.83  −37:52:21.9  17/05/14  603  559              10/05/15  603  559  15  NGC 6093  5  F1 [255]  16:16:35.04  −22:55:45.9  09/06/12  5 × 760; 5 × 845  5 × 539  16  NGC 6101  5  F1 [147]  16:25:36.08  −72:18:35.2  04/04/14  762  686        F2 [190]  16:24:40.57  −72:16:07.7  25/05/14  762  686        F3 [235]  16:24:23.48  −72:11:18.6  29/06/14  800  724        F4 [282]  16:24:58.26  −72:06:50.7  14/08/13  851  775        F5 [101]  16:26:39.71  −72:17:19.9  28/02/14  800  724  17  NGC 6121  2  F1 [272]  16:23:12.57  −26:27:02.1  06/07/14  739  666        F2 [98]  16:23:55.64  −26:36:33.4  17/02/15  666  593  18  NGC 6144  2  F1 [83]  16:27:39.42  −26:05:00.9  28/02/14  679  606        F2 [174]  16:26:57.33  −26:07:05.2  27/05/14  679  606  19  NGC 6171  4  F1 [342]  16:32:41.94  −12:56:56.1  31/05/14  830; 833  2 × 761        F2 [72]  16:32:42.20  −12:57:07.5  25/03/14  800; 802  731; 730  20  NGC 6218  2  F1 [276]  16:46:55.21  −01:52:03.7  16/08/13  721  648        F2 [6]  16:47:33.42  −01:52:07.7  27/05/14  645  572  21  NGC 6254  2  F1 [276]  16:56:50.04  −04:01:10.1  16/08/13  721  648        F2 [7]  16:57:28.66  −04:01:19.2  27/05/14  644  571  22  NGC 6304  2  F1 [184]  17:14:10.53  −29:32:35.1  07/06/14  624  559        F2 [274]  17:14:09.53  −29:23:04.8  26/08/13  731  658  23  NGC 6341  2  F1 [230]  17:16:30.21  +43:08:03.0  22/10/13  638  565        F2 [319]  17:17:06.19  +43:14:56.0  03/08/14  750  677  24  NGC 6352  2  F1 [161]  17:25:15.00  −48:31:41.7  27/05/14  637  564        F2 [251]  17:24:51.02  −48:22:52.5  13/08/13  731  658  25  NGC 6362  2  F1 [125]  17:32:14.07  −67:09:25.3  30/03/14  651  578        F2 [215]  17:30:47.67  −67:04:37.3  01/07/14  760  687  26  NGC 6366  2  F1 [293]  17:27:31.94  −04:58:44.4  26/08/13  726  653        F2 [351]  17:27:58.09  −04:58:57.1  07/06/15  644  571  27  NGC 6388  4  F1 [238]  17:35:40.80  −44:43:02.9  05/07/14  865; 906  796; 834        F2 [141]  17:36:17.23  −44:50:53.5  12/05/14  793; 795  724; 723  28  NGC 6397  2  F1 [90]  17:41:17.47  −53:44:46.1  27/03/14  683  610        F2 [180]  17:40:12.50  −53:45:38.3  11/06/14  640  567  29  NGC 6441  4  F1 [102]  17:50:34.23  −37:08:21.5  26/03/14  833; 835  764; 763        F2 [192]  17:49:46.24  −37:07:12.6  15/06/14  2 × 794  725; 722  30  NGC 6496  2  F1 [157]  17:58:53.12  −44:22:28.2  30/05/14  638  656        F2 [247]  17:58:27.50  −44:13:57.0  12/08/13  731  658  31  NGC 6535  2  F1 [319]  18:03:49.95  +00:11:04.6  19/07/14  724  651        F2 [50]  18:04:17.63  +00:17:50.2  26/05/14  644  571  32  NGC 6541  2  F1 [80]  18:08:35.16  −43:46:11.5  14/02/14  689  616        F2 [170]  18:07:44.04  −43:48:48.8  11/06/14  639  566  33  NGC 6584  2  F1 [155]  18:18:26.74  −52:19:31.3  30/05/14  640  567        F2 [245]  18:17:54.89  −52:11:09.9  18/08/13  726  653  34  NGC 6624  2  F1 [264]  18:23:14.50  −30:17:51.1  03/09/13  731  658        F2 [174]  18:23:23.29  −30:27:20.2  27/06/14  638  565  35  NGC 6637  4  F1 [84]  18:31:49.55  −32:24:40.9  18/02/14  827; 840  758; 768        F2 [174]  18:31:05.49  −32:26:31.5  30/06/14  792; 794  723; 722  36  NGC 6652  3  F1 [281]  18:35:25.09  −32:54:08.8  30/08/13  707  633        F2 [238]  18:35:14.87  −32:58:25.1  14/08/13  733  658        F3 [192]  18:35:20.06  −33:03:36.1  01/07/14  621  548  37  NGC 6656  4  F1 [266]  18:36:00.25  −23:50:16.5  23/09/10  656;644  2 × 389        F2 [85]  18:36:47.76  −23:58:08.0  17/03/11  2 × 656  2 × 389  38  NGC 6681  2  F1 [271]  18:42:48.78  −32:13:03.7  05/09/13  730  658        F2 [183]  18:42:51.19  −32:22:33.1  29/06/14  637  564  39  NGC 6715  6  F1 [153]  18:54:56.04  −30:35:08.9  29/06/14  2 × 736; 2 × 737; 2 × 734  3 × 370        F2 [243]  18:54:33.96  −30:27:06.5  05/09/13  4 × 819; 2 × 817  3 × 390  40  NGC 6717  3  F1 [80]  18:55:31.98  −22:45:22.7  06/05/14  619  535        F2 [125]  18:55:12.41  −22:48:27.0  03/07/14  619  544        F3 [171]  18:54:51.25  −22:47:57.0  03/07/14  617  544  41  NGC 6723  3  F1 [103]  18:59:54.19  −36:43:18.0  03/04/14  666  592        F2 [148]  18:59:27.49  −36:44:21.9  15/06/14  626  551        F3 [193]  18:59:06.07  −36:41:59.9  05/07/14  624  551  42  NGC 6779  2  F1 [241]  19:16:04.77  +30:12:18.9  13/10/13  731  658        F2 [333]  19:16:42.17  +30:17:38.2  19/07/14  637  564  43  NGC 6791  2  F1 [322]  19:20:53.69  +37:53:04.8  17/08/13  631  559        F2 [50]  19:21:27.31  +37:46:16.6  26/04/14  638  565  44  NGC 6809  2  F1 [262]  19:39:32.79  −30:54:19.4  21/08/14  753  680        F2 [82]  19:40:26.65  −31:01:26.4  29/03/14  677  604  45  NGC 6838  2  F1 [244]  19:53:18.63  +18:48:20.3  23/10/13  723  650        F2 [75]  19:54:12.48  +18:43:53.9  03/05/14  681  608  46  NGC 6934  2  F1 [245]  20:33:44.84  +07:25:55.5  08/10/13  723  650        F2 [334]  20:34:17.59  +07:30:52.3  18/08/14  644  571  47  NGC 6981  2  F1 [289]  20:53:13.45  −12:26:24.9  13/08/13  641  568        F2 [19]  20:53:51.29  −12:28:39.5  03/08/14  624  551  48  NGC 7089  3  F1 [237]  21:32:59.85  +00:48:42.0  18/10/13  717  643        F2 [281]  21:33:10.93  +00:44:15.0  29/08/13  668  593        F3 [327]  21:33:29.92  +00:42:39.1  14/08/13  611  534  49  NGC 7099  2  F1 [92]  21:40:44.34  −23:15:15.1  08/06/14  656  583        F2 [182]  21:40:02.30  −23:15:49.0  19/08/14  656  583  View Large Table 2. Tabulated $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec} )}$$ values used in the aperture correction. #  Cluster  Field  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F475W}$$  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F814W}$$        (mag)  (mag)  01  NGC 1261  1  +0.016 ± 0.006  −0.017 ± 0.003      2  +0.030 ± 0.005  −0.021 ± 0.002      3  +0.021 ± 0.005  −0.013 ± 0.002      4  +0.038 ± 0.005  +0.117 ± 0.005      5  +0.053 ± 0.006  −0.020 ± 0.003  02  NGC 1851  1+2  −0.008 ± 0.002  −0.004 ± 0.001  03  NGC 2298  1  +0.057 ± 0.014  +0.148 ± 0.008      2  +0.016 ± 0.010  +0.013 ± 0.005  04  NGC 3201  1  +0.022 ± 0.001  +0.001 ± 0.001      2  +0.023 ± 0.001  +0.035 ± 0.001  05  NGC 4590  1  −0.003 ± 0.003  +0.002 ± 0.002      2  0.000 ± 0.003  +0.039 ± 0.002  06  NGC 4833  1  +0.004 ± 0.002  +0.002 ± 0.001      2  +0.009 ± 0.001  −0.008 ± 0.001  07  NGC 5024  1  +0.003 ± 0.001  +0.012 ± 0.001      2  +0.009 ± 0.003  −0.019 ± 0.001  08  NGC 5053  1  +0.012 ± 0.004  +0.001 ± 0.004      2  +0.003 ± 0.004  0.000 ± 0.003      3  +0.005 ± 0.005  −0.011 ± 0.003      4  +0.012 ± 0.005  +0.006 ± 0.003      5  +0.132 ± 0.006  +0.029 ± 0.003  09  NGC 5286  1  +0.036 ± 0.003  +0.019 ± 0.002      2  +0.031 ± 0.004  −0.011 ± 0.001  10  NGC 5466  1  +0.009 ± 0.003  −0.011 ± 0.003      2  +0.008 ± 0.003  −0.009 ± 0.002  11  NGC 5897  1  +0.029 ± 0.003  −0.012 ± 0.001      2  +0.028 ± 0.003  −0.014 ± 0.001  12  NGC 5904  1  +0.011 ± 0.001  −0.016 ± 0.001      2  +0.003 ± 0.001  −0.008 ± 0.001  13  NGC 5927  1  +0.023 ± 0.002  +0.017 ± 0.001      2  +0.022 ± 0.002  −0.003 ± 0.001  14  NGC 5986  1  −0.004 ± 0.002  −0.011 ± 0.001      2  +0.043 ± 0.004  −0.008 ± 0.001  15  NGC 6093  1  +0.090 ± 0.002  +0.136 ± 0.002  16  NGC 6101  1  +0.003 ± 0.003  +0.004 ± 0.002      2  +0.007 ± 0.003  −0.012 ± 0.001      3  +0.010 ± 0.004  +0.004 ± 0.002      4  +0.003 ± 0.002  −0.014 ± 0.001      5  +0.009 ± 0.002  −0.001 ± 0.001  17  NGC 6121  1  +0.003 ± 0.002  +0.002 ± 0.001      2  −0.004 ± 0.003  −0.005 ± 0.001  18  NGC 6144  1  +0.030 ± 0.004  +0.027 ± 0.001      2  +0.013 ± 0.004  −0.002 ± 0.001  19  NGC 6171  1  +0.021 ± 0.002  −0.007 ± 0.002      2  +0.022 ± 0.002  +0.004 ± 0.002  20  NGC 6218  1  −0.003 ± 0.002  −0.001 ± 0.001      2  +0.006 ± 0.002  −0.009 ± 0.001  21  NGC 6254  1  −0.006 ± 0.002  +0.002 ± 0.001      2  0.000 ± 0.002  −0.003 ± 0.001  22  NGC 6304  1  +0.010 ± 0.001  +0.036 ± 0.001      2  +0.012 ± 0.002  +0.016 ± 0.001  23  NGC 6341  1  +0.003 ± 0.002  −0.026 ± 0.001      2  +0.016 ± 0.002  −0.013 ± 0.001  24  NGC 6352  1  +0.001 ± 0.001  −0.004 ± 0.001      2  −0.010 ± 0.001  +0.011 ± 0.001  25  NGC 6362  1  −0.002 ± 0.002  −0.006 ± 0.001      2  +0.024 ± 0.003  +0.056 ± 0.001  26  NGC 6366  1  +0.023 ± 0.005  −0.003 ± 0.001      2  +0.003 ± 0.002  +0.007 ± 0.001  27  NGC 6388  1  +0.011 ± 0.001  +0.016 ± 0.001      2  +0.003 ± 0.001  +0.018 ± 0.001  28  NGC 6397  1  0.000 ± 0.001  +0.003 ± 0.001      2  +0.008 ± 0.002  +0.031 ± 0.001  29  NGC 6441  1  +0.010 ± 0.001  +0.027 ± 0.001      2  +0.040 ± 0.001  +0.057 ± 0.001  30  NGC 6496  1  −0.008 ± 0.002  −0.005 ± 0.001      2  −0.002 ± 0.001  −0.013 ± 0.001  31  NGC 6535  1  −0.002 ± 0.002  +0.030 ± 0.001      2  +0.010 ± 0.003  −0.007 ± 0.001  32  NGC 6541  1  +0.011 ± 0.001  +0.020 ± 0.001      2  0.000 ± 0.002  0.000 ± 0.001  33  NGC 6584  1  −0.011 ± 0.003  −0.011 ± 0.001      2  −0.005 ± 0.004  −0.016 ± 0.001  34  NGC 6624  1  +0.020 ± 0.001  +0.046 ± 0.001      2  +0.008 ± 0.001  +0.014 ± 0.001  35  NGC 6637  1  +0.003 ± 0.001  +0.009 ± 0.001      2  −0.008 ± 0.001  −0.003 ± 0.001  36  NGC 6652  1  −0.002 ± 0.002  +0.039 ± 0.001      2  −0.008 ± 0.002  +0.003 ± 0.001      3  +0.006 ± 0.001  +0.015 ± 0.001  37  NGC 6656  1  0.000 ± 0.001  +0.031 ± 0.001      2  +0.079 ± 0.001  +0.030 ± 0.001  38  NGC 6681  1  −0.007 ± 0.001  −0.007 ± 0.001      2  +0.005 ± 0.001  −0.005 ± 0.001  39  NGC 6715  1  +0.008 ± 0.001  +0.076 ± 0.001      2  +0.001 ± 0.001  +0.033 ± 0.001  40  NGC 6717  1  +0.006 ± 0.002  −0.013 ± 0.001      2  −0.002 ± 0.002  +0.007 ± 0.001      3  +0.005 ± 0.001  +0.011 ± 0.001  41  NGC 6723  1  −0.007 ± 0.003  0.000 ± 0.001      2  +0.007 ± 0.002  −0.007 ± 0.001      3  +0.016 ± 0.003  −0.008 ± 0.001  42  NGC 6779  1  +0.004 ± 0.003  0.000 ± 0.002      2  −0.005 ± 0.002  +0.025 ± 0.002  43  NGC 6791  1  −0.005 ± 0.004  +0.006 ± 0.002      2  −0.011 ± 0.003  −0.011 ± 0.001  44  NGC 6809  1  +0.037 ± 0.002  −0.006 ± 0.001      2  0.000 ± 0.001  −0.002 ± 0.001  45  NGC 6838  1  −0.014 ± 0.002  +0.016 ± 0.001      2  +0.009 ± 0.002  −0.009 ± 0.001  46  NGC 6934  1  −0.001 ± 0.005  +0.004 ± 0.002      2  +0.005 ± 0.007  −0.001 ± 0.003  47  NGC 6981  1  −0.002 ± 0.007  −0.009 ± 0.003      2  +0.008 ± 0.011  +0.003 ± 0.005  48  NGC 7089  1  +0.014 ± 0.003  +0.016 ± 0.001      2  +0.036 ± 0.002  −0.012 ± 0.001      3  +0.003 ± 0.002  −0.020 ± 0.001  49  NGC 7099  1  +0.013 ± 0.003  +0.019 ± 0.002      2  +0.016 ± 0.004  −0.010 ± 0.002  #  Cluster  Field  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F475W}$$  $$\Delta m_{\rm PSF\text{--}AP(0.5\,\text{arcsec})}^{\rm F814W}$$        (mag)  (mag)  01  NGC 1261  1  +0.016 ± 0.006  −0.017 ± 0.003      2  +0.030 ± 0.005  −0.021 ± 0.002      3  +0.021 ± 0.005  −0.013 ± 0.002      4  +0.038 ± 0.005  +0.117 ± 0.005      5  +0.053 ± 0.006  −0.020 ± 0.003  02  NGC 1851  1+2  −0.008 ± 0.002  −0.004 ± 0.001  03  NGC 2298  1  +0.057 ± 0.014  +0.148 ± 0.008      2  +0.016 ± 0.010  +0.013 ± 0.005  04  NGC 3201  1  +0.022 ± 0.001  +0.001 ± 0.001      2  +0.023 ± 0.001  +0.035 ± 0.001  05  NGC 4590  1  −0.003 ± 0.003  +0.002 ± 0.002      2  0.000 ± 0.003  +0.039 ± 0.002  06  NGC 4833  1  +0.004 ± 0.002  +0.002 ± 0.001      2  +0.009 ± 0.001  −0.008 ± 0.001  07  NGC 5024  1  +0.003 ± 0.001  +0.012 ± 0.001      2  +0.009 ± 0.003  −0.019 ± 0.001  08  NGC 5053  1  +0.012 ± 0.004  +0.001 ± 0.004      2  +0.003 ± 0.004  0.000 ± 0.003      3  +0.005 ± 0.005  −0.011 ± 0.003      4  +0.012 ± 0.005  +0.006 ± 0.003      5  +0.132 ± 0.006  +0.029 ± 0.003  09  NGC 5286  1  +0.036 ± 0.003  +0.019 ± 0.002      2  +0.031 ± 0.004  −0.011 ± 0.001  10  NGC 5466  1  +0.009 ± 0.003  −0.011 ± 0.003      2  +0.008 ± 0.003  −0.009 ± 0.002  11  NGC 5897  1  +0.029 ± 0.003  −0.012 ± 0.001      2  +0.028 ± 0.003  −0.014 ± 0.001  12  NGC 5904  1  +0.011 ± 0.001  −0.016 ± 0.001      2  +0.003 ± 0.001  −0.008 ± 0.001  13  NGC 5927  1  +0.023 ± 0.002  +0.017 ± 0.001      2  +0.022 ± 0.002  −0.003 ± 0.001  14  NGC 5986  1  −0.004 ± 0.002  −0.011 ± 0.001      2  +0.043 ± 0.004  −0.008 ± 0.001  15  NGC 6093  1  +0.090 ± 0.002  +0.136 ± 0.002  16  NGC 6101  1  +0.003 ± 0.003  +0.004 ± 0.002      2  +0.007 ± 0.003  −0.012 ± 0.001      3  +0.010 ± 0.004  +0.004 ± 0.002      4  +0.003 ± 0.002  −0.014 ± 0.001      5  +0.009 ± 0.002  −0.001 ± 0.001  17  NGC 6121  1  +0.003 ± 0.002  +0.002 ± 0.001      2  −0.004 ± 0.003  −0.005 ± 0.001  18  NGC 6144  1  +0.030 ± 0.004  +0.027 ± 0.001      2  +0.013 ± 0.004  −0.002 ± 0.001  19  NGC 6171  1  +0.021 ± 0.002  −0.007 ± 0.002      2  +0.022 ± 0.002  +0.004 ± 0.002  20  NGC 6218  1  −0.003 ± 0.002  −0.001 ± 0.001      2  +0.006 ± 0.002  −0.009 ± 0.001  21  NGC 6254  1  −0.006 ± 0.002  +0.002 ± 0.001      2  0.000 ± 0.002  −0.003 ± 0.001  22  NGC 6304  1  +0.010 ± 0.001  +0.036 ± 0.001      2  +0.012 ± 0.002  +0.016 ± 0.001  23  NGC 6341  1  +0.003 ± 0.002  −0.026 ± 0.001      2  +0.016 ± 0.002  −0.013 ± 0.001  24  NGC 6352  1  +0.001 ± 0.001  −0.004 ± 0.001      2  −0.010 ± 0.001  +0.011 ± 0.001  25  NGC 6362  1  −0.002 ± 0.002  −0.006 ± 0.001      2  +0.024 ± 0.003  +0.056 ± 0.001  26  NGC 6366  1  +0.023 ± 0.005  −0.003 ± 0.001      2  +0.003 ± 0.002  +0.007 ± 0.001  27  NGC 6388  1  +0.011 ± 0.001  +0.016 ± 0.001      2  +0.003 ± 0.001  +0.018 ± 0.001  28  NGC 6397  1  0.000 ± 0.001  +0.003 ± 0.001      2  +0.008 ± 0.002  +0.031 ± 0.001  29  NGC 6441  1  +0.010 ± 0.001  +0.027 ± 0.001      2  +0.040 ± 0.001  +0.057 ± 0.001  30  NGC 6496  1  −0.008 ± 0.002  −0.005 ± 0.001      2  −0.002 ± 0.001  −0.013 ± 0.001  31  NGC 6535  1  −0.002 ± 0.002  +0.030 ± 0.001      2  +0.010 ± 0.003  −0.007 ± 0.001  32  NGC 6541  1  +0.011 ± 0.001  +0.020 ± 0.001      2  0.000 ± 0.002  0.000 ± 0.001  33  NGC 6584  1  −0.011 ± 0.003  −0.011 ± 0.001      2  −0.005 ± 0.004  −0.016 ± 0.001  34  NGC 6624  1  +0.020 ± 0.001  +0.046 ± 0.001      2  +0.008 ± 0.001  +0.014 ± 0.001  35  NGC 6637  1  +0.003 ± 0.001  +0.009 ± 0.001      2  −0.008 ± 0.001  −0.003 ± 0.001  36  NGC 6652  1  −0.002 ± 0.002  +0.039 ± 0.001      2  −0.008 ± 0.002  +0.003 ± 0.001      3  +0.006 ± 0.001  +0.015 ± 0.001  37  NGC 6656  1  0.000 ± 0.001  +0.031 ± 0.001      2  +0.079 ± 0.001  +0.030 ± 0.001  38  NGC 6681  1  −0.007 ± 0.001  −0.007 ± 0.001      2  +0.005 ± 0.001  −0.005 ± 0.001  39  NGC 6715  1  +0.008 ± 0.001  +0.076 ± 0.001      2  +0.001 ± 0.001  +0.033 ± 0.001  40  NGC 6717  1  +0.006 ± 0.002  −0.013 ± 0.001      2  −0.002 ± 0.002  +0.007 ± 0.001      3  +0.005 ± 0.001  +0.011 ± 0.001  41  NGC 6723  1  −0.007 ± 0.003  0.000 ± 0.001      2  +0.007 ± 0.002  −0.007 ± 0.001      3  +0.016 ± 0.003  −0.008 ± 0.001  42  NGC 6779  1  +0.004 ± 0.003  0.000 ± 0.002      2  −0.005 ± 0.002  +0.025 ± 0.002  43  NGC 6791  1  −0.005 ± 0.004  +0.006 ± 0.002      2  −0.011 ± 0.003  −0.011 ± 0.001  44  NGC 6809  1  +0.037 ± 0.002  −0.006 ± 0.001      2  0.000 ± 0.001  −0.002 ± 0.001  45  NGC 6838  1  −0.014 ± 0.002  +0.016 ± 0.001      2  +0.009 ± 0.002  −0.009 ± 0.001  46  NGC 6934  1  −0.001 ± 0.005  +0.004 ± 0.002      2  +0.005 ± 0.007  −0.001 ± 0.003  47  NGC 6981  1  −0.002 ± 0.007  −0.009 ± 0.003      2  +0.008 ± 0.011  +0.003 ± 0.005  48  NGC 7089  1  +0.014 ± 0.003  +0.016 ± 0.001      2  +0.036 ± 0.002  −0.012 ± 0.001      3  +0.003 ± 0.002  −0.020 ± 0.001  49  NGC 7099  1  +0.013 ± 0.003  +0.019 ± 0.002      2  +0.016 ± 0.004  −0.010 ± 0.002  View Large The physical position of ACS/WFC detectors in the focal plane of HST is such that its projected field of view (FoV) in the sky is located at a distance of about 6.5 arcmin from the centre of the WFC3 FoV. Depending on the number of orbits allocated to each GC, from two to a maximum of five non-overlapping fields were observed. This is because, in order to secure a good handling of charge-transfer-efficiency (CTE) systematic errors, primary WFC3 observations were taken by applying a different telescope rotation at each orbit (Paper I, Section 4). For the majority of clusters, which were allocated two orbits, a rotation of about 90° was performed between the first and second orbit; for clusters observed for more than two orbits, a minimum difference of ∼45° between the V3 PA of each orbit was required. Five distinct pointings were obtained for three clusters, namely NGC 1261, NGC 5053, and NGC 6101. Three distinct fields were obtained for four clusters: NGC 6652, NGC 6717, NGC 6723, and NGC 7089. For the other clusters, only two pointings were planned. M80 is an exception and was not observed as part of the Program GO-13297. For it, we make use of archival HST data from GO-12311. When possible, ACS parallel observations targeted pre-existing HST observations. Figs 1 and A1–A8 display all HST observations that sample the sky area in the vicinity of those covered in this survey. Three cameras onboard HST were considered in order to enhance the probability of an overlap between observations: Wide Field and Planetary Camera 2 (WFPC2), ACS, and WFC3 (both UVIS and IR channels). Taking two images per orbit, one in F475W and one in F814W filters, the typical exposure times for both filters are of the order of 700 s. Figure 1. View largeDownload slide Finding charts for NGC 1261, NGC 1851, NGC 2298, and NGC 3201. Red outlines represent ACS/WFC parallel observations of GO-13297, black outlines refer to WFC3/UVIS observations. Dark grey, dashed outlines represent archive HST observations in the same regions. Footprints are labelled as in Table 1. Observations are subdivided by epoch in the smaller panels. Blue circles mark the position of core radius, half-light radius, and tidal radius for each cluster from Harris 1996 (2010). Where the tidal radius could not be included in the image, its value has been indicated in the upper-left corner of the image. Yellow, dashed circles mark the distances of 1, 5, and 10 arcmin from cluster centres. Grey dots corresponds to 2MASS sources, with brighter sources being larger. Figure 1. View largeDownload slide Finding charts for NGC 1261, NGC 1851, NGC 2298, and NGC 3201. Red outlines represent ACS/WFC parallel observations of GO-13297, black outlines refer to WFC3/UVIS observations. Dark grey, dashed outlines represent archive HST observations in the same regions. Footprints are labelled as in Table 1. Observations are subdivided by epoch in the smaller panels. Blue circles mark the position of core radius, half-light radius, and tidal radius for each cluster from Harris 1996 (2010). Where the tidal radius could not be included in the image, its value has been indicated in the upper-left corner of the image. Yellow, dashed circles mark the distances of 1, 5, and 10 arcmin from cluster centres. Grey dots corresponds to 2MASS sources, with brighter sources being larger. All exposures have been corrected for CTE effects using the method described in Anderson & Bedin (2010). Photometric measurements of stellar objects in each field have been performed using a suite of fortran programs based on img2xym (Anderson & King 2006) and kitchen_sync presented in Anderson et al. (2008). The spatial variation of the point spread function (PSF) has been taken into account adopting a grid of 9 × 10 model PSFs distributed along each image. However focus changes/breathing of the telescope, imperfect guiding, residual noise related to CTE can produce image-to-image variations of the PSF. To mitigate these sources of systematic errors, we derived a set of spatially varying perturbations of the PSF models for each calibrated, non-drizzled (flc) image. Adopting the procedure presented in Bellini et al. (2013), each image is divided into a grid with a number of cells changing from 2 × 2 to 5 × 5. In each cell, a subset of well-measured stars is used to locally adjust the PSF models to the stellar profiles. Using an updated version of Anderson & King (2006) software, in combination with the newly created PSF models, we extracted raw catalogues of stellar positions and magnitudes in each image. We choose the grid refinement (between 2 × 2 to 5 × 5) that produces the best results, inspecting the distribution of the quality of fit parameters as a function of magnitude, and taking into account the number of reference stars used to tailor the PSF perturbations in each cell. Each exposure related to the same field has been subsequently referred to a common reference frame. Since we are mainly interested in the faint, red part of the cluster CMDs, we used the F814W images to construct the reference frames. Catalogues are finally produced using a version of the software presented in Anderson et al. (2008), specifically tuned for this project. In particular, the kitchen_sync routine has been modified in order to work properly with only one image per filter. In addition, the method presented in Gilliland (2004) was applied in order to provide reliable photometry also for saturated stars. The raw, instrumental magnitudes have been zero-pointed into ACS/WFC Vega-mag photometric system following the prescriptions of Bedin et al. (2005). The zero-points and aperture corrections from 0.5 arcsec to infinity of Bohlin (2016) have been used. Especially in the present case, the photometric calibration plays a critical role, and we put strong efforts to precisely evaluate zero-point differences between various observations. Crowding is not a serious problem in the outer cluster regions and high-precision photometric measurements are relatively easy to obtain. But field-to-field zero-point variations must be accounted for to have the external fields of the same cluster on the same photometric scale. The main source of this photometric offsets between catalogues of distinct fields, is related to PSF modelling. The PSF model by construction is normalized to a surface flux of unity within a radius of 10 ACS/WFC pixels (0.5 arcsec). Only the inner 5 × 5 pixel region of sources was used to fit to the PSF model in order to minimize the contaminating impact of nearby neighbours, but any mismatch between the adopted PSF model and the pixels beyond the small square fitting aperture would result in a slight zero-point shift. The fact that we perturb the library PSFs (above) minimizes this even further. However, the best way to regularize the photometry is empirically: following Bedin et al. (2005) and Anderson et al. (2008), we measured aperture corrections inside this aperture using the calibrated and drizzled (drc) images as reference. These corrections were sufficient to properly take into account the majority of the photometric biases in our catalogues. The obtained values of the aperture correction for an aperture of 0.5 arcsec are listed in Table 2. We maintain the same nomenclature as in Bedin et al. (2005). The aperture correction values, along with their associated uncertainties are also reported in Fig. 2. Black dots are referred to F814W observations, red dots to F475W ones. It can be noted that for the majority of cases corrections are small: typically smaller than 0.04 magnitudes, but in the most severe cases, they can reach values as high as 0.15 mag. Figure 2. View largeDownload slide Aperture corrections measured in each field for each cluster. Red squares refer to F475W observations, while black squares refer to F814W ones. Figure 2. View largeDownload slide Aperture corrections measured in each field for each cluster. Red squares refer to F475W observations, while black squares refer to F814W ones. Table 3. Precision reached with the re-derived astrometric solution using Gaia catalogues; for completeness we report the number of reference stars used. We also provide measures of the accuracy of the original STScI astrometric solution in the form of differences between RA and Dec. #  Cluster  F  Stars  Precision  ΔRA  $$\Delta {\rm \text{Dec.}}$$          (mas)  WFC px  WFC px  01  NGC 1261  1  32  15.5  1.66 ± 0.34  −4.50 ± 0.16      2  41  10.0  −1.89 ± 0.08  4.31 ± 0.21      3  37  8.0  −0.51 ± 0.06  1.61 ± 0.18      4  36  11.0  −0.50 ± 0.14  1.03 ± 0.31      5  46  10.0  0.07 ± 0.05  −4.81 ± 0.19  02  NGC 1851  1 + 2  232  10.0  3.54 ± 0.20  −4.25 ± 0.19  03  NGC 2298  1  79  16.5  −0.17 ± 0.04  5.62 ± 0.24      2  53  14.0  −0.11 ± 0.06  −1.37 ± 0.25  04  NGC 3201  1  635  13.0  0.14 ± 0.32  −0.57 ± 0.23      2  653  10.0  4.33 ± 0.33  8.39 ± 0.20  05  NGC 4590  1  110  11.0  −0.78 ± 0.10  4.90 ± 0.15      2  111  11.5  −0.12 ± 0.04  −3.86 ± 0.17  06  NGC 4833  1  492  7.0  −3.53 ± 0.35  6.86 ± 0.22      2  570  6.5  1.75 ± 0.15  −0.02 ± 0.20  07  NGC 5024  1  173  11.5  −0.14 ± 0.23  3.09 ± 0.20      2  142  10.0  −45.45 ± 0.77  1.35 ± 0.18  08  NGC 5053  1  44  9.5  0.72 ± 0.16  −0.62 ± 0.16      2  41  8.0  −0.07 ± 0.15  −1.86 ± 0.18      3  38  8.5  0.22 ± 0.13  0.29 ± 0.23      4  42  10.0  0.43 ± 0.09  0.23 ± 0.19      5  52  12.0  0.57 ± 0.16  −0.65 ± 0.16  09  NGC 5286  1  215  17.0  1.63 ± 0.20  1.85 ± 0.25      2  217  16.5  1.61 ± 0.20  1.10 ± 0.21  10  NGC 5466  1  56  7.5  −0.13 ± 0.27  1.37 ± 0.12      2  72  8.0  −0.79 ± 0.29  −2.22 ± 0.20  11  NGC 5897  1  164  9.0  0.12 ± 0.22  −0.37 ± 0.20      2  154  7.5  2.58 ± 0.17  −2.51 ± 0.18  12  NGC 5904  1  579  7.0  −1.29 ± 0.19  −2.69 ± 0.15      2  630  7.0  −0.16 ± 0.15  −1.08 ± 0.22  13  NGC 5927  1  848  8.5  3.65 ± 0.30  −0.38 ± 0.20      2  985  8.0  −9.20 ± 0.35  11.91 ± 0.21  14  NGC 5986  1  278  8.0  −0.54 ± 0.30  1.36 ± 0.22      2  268  6.0  −6.46 ± 0.26  4.12 ± 0.18  15  NGC 6093  1  236  22.0  1.81 ± 0.19  −4.47 ± 0.20  16  NGC 6101  1  235  7.0  −1.85 ± 0.76  1.09 ± 0.21      2  232  6.5  −0.80 ± 0.61  2.90 ± 0.20      3  229  8.0  −15.82 ± 0.73  0.55 ± 0.25      4  263  11.0  −1.83 ± 0.89  −0.52 ± 0.27      5  204  8.5  12.13 ± 0.85  −3.52 ± 0.24  17  NGC 6121  1  633  7.5  −4.81 ± 0.37  7.07 ± 0.23      2  649  7.5  −0.69 ± 0.14  1.90 ± 0.22  18  NGC 6144  1  147  7.0  −4.48 ± 0.18  −0.10 ± 0.19      2  147  9.5  −3.09 ± 0.15  −0.76 ± 0.20  19  NGC 6171  1  172  9.0  4.44 ± 0.20  −2.36 ± 0.16      2  196  7.5  −3.26 ± 0.19  −1.51 ± 0.21  20  NGC 6218  1  300  10.0  0.40 ± 0.07  −4.05 ± 0.17      2  362  6.5  −0.72 ± 0.09  0.13 ± 0.19  21  NGC 6254  1  525  7.5  −0.94 ± 0.11  2.02 ± 0.20      2  507  5.0  −0.08 ± 0.12  −0.52 ± 0.18  22  NGC 6304  1  1021  11.0  0.00 ± 0.09  7.29 ± 0.22      2  1137  11.0  −2.63 ± 0.15  3.66 ± 0.28  23  NGC 6341  1  272  7.0  3.31 ± 0.13  −1.46 ± 0.17      2  276  6.5  1.30 ± 0.16  2.01 ± 0.14  24  NGC 6352  1  676  8.0  0.44 ± 0.09  6.24 ± 0.18      2  658  9.5  0.20 ± 0.11  0.84 ± 0.21  25  NGC 6362  1  399  6.0  −1.98 ± 0.18  −1.19 ± 0.20      2  412  6.0  −0.15 ± 0.23  2.51 ± 0.17  26  NGC 6366  1  265  7.0  1.07 ± 0.11  −4.03 ± 0.22      2  263  6.0  2.89 ± 0.19  4.66 ± 0.18  27  NGC 6388  1  812  6.5  1.68 ± 0.24  4.01 ± 0.26      2  873  7.0  −0.86 ± 0.22  5.67 ± 0.22  28  NGC 6397  1  605  8.0  16.46 ± 0.47  12.42 ± 0.26      2  602  7.0  −4.15 ± 0.33  4.79 ± 0.30  29  NGC 6441  1  1075  10.0  34.98 ± 0.54  −3.77 ± 0.26      2  1114  9.5  4.46 ± 0.31  3.80 ± 0.27  30  NGC 6496  1  482  6.0  1.85 ± 0.30  7.03 ± 0.21      2  525  9.0  8.00 ± 0.28  1.65 ± 0.22  31  NGC 6535  1  304  6.5  −4.97 ± 0.19  0.75 ± 0.18      2  275  7.5  −1.78 ± 0.18  −9.16 ± 0.20  32  NGC 6541  1  385  7.5  −11.12 ± 0.28  7.28 ± 0.23      2  481  9.0  −7.23 ± 0.32  7.72 ± 0.22  33  NGC 6584  1  119  7.5  1.82 ± 0.19  6.15 ± 0.24      2  141  10.0  2.67 ± 0.14  2.71 ± 0.25  34  NGC 6624  1  360  8.5  0.83 ± 0.12  1.93 ± 0.23      2  774  9.5  0.61 ± 0.15  6.08 ± 0.21  35  NGC 6637  1  620  10.0  −3.66 ± 0.20  1.83 ± 0.25      2  615  11.5  −2.54 ± 0.23  −0.80 ± 0.29  36  NGC 6652  1  608  12.0  0.35 ± 0.09  5.17 ± 0.26      2  624  9.5  −0.06 ± 0.06  2.40 ± 0.25      3  682  7.0  0.49 ± 0.15  5.78 ± 0.21  37  NGC 6656  1  734  31.0  −1.20 ± 0.21  3.03 ± 0.25      2  1052  27.5  1.21 ± 0.30  6.93 ± 0.31  38  NGC 6681  1  450  14.0  −4.20 ± 0.25  1.76 ± 0.29      2  385  15.5  −2.37 ± 0.20  1.75 ± 0.32  39  NGC 6715  1  484  16.0  −0.69 ± 0.25  −4.67 ± 0.28      2  505  15.5  3.28 ± 0.18  −1.79 ± 0.27  40  NGC 6717  1  439  14.0  3.48 ± 0.21  0.07 ± 0.30      2  406  13.0  0.49 ± 0.20  1.11 ± 0.28      3  445  12.0  0.11 ± 0.18  4.99 ± 0.26  41  NGC 6723  1  256  14.5  0.14 ± 0.20  0.95 ± 0.30      2  144  11.5  −0.25 ± 0.19  4.59 ± 0.22      3  111  9.0  −0.33 ± 0.16  3.99 ± 0.18  42  NGC 6779  1  345  7.5  0.86 ± 0.07  1.07 ± 0.21      2  333  6.5  3.24 ± 0.16  −8.92 ± 0.19  43  NGC 6791  1  288  9.0  −1.92 ± 0.26  −7.50 ± 0.21      2  274  5.5  −4.98 ± 0.25  −6.18 ± 0.21  44  NGC 6809  1  584  7.0  −1.58 ± 0.19  0.99 ± 0.22      2  590  7.5  −9.19 ± 0.21  −0.68 ± 0.23  45  NGC 6838  1  816  7.0  −8.81 ± 0.24  −12.24 ± 0.22      2  800  6.5  −3.96 ± 0.22  −8.28 ± 0.21  46  NGC 6934  1  87  11.5  3.41 ± 0.20  2.62 ± 0.24      2  84  5.5  1.41 ± 0.10  −12.00 ± 0.17  47  NGC 6981  1  31  14.5  1.29 ± 0.26  −9.37 ± 0.19      2  27  9.5  1.73 ± 0.23  −8.07 ± 0.23  48  NGC 7089  1  92  5.5  −0.55 ± 0.12  −6.84 ± 0.15      2  77  10.5  2.52 ± 0.15  −1.40 ± 0.18      3  82  11.0  1.85 ± 0.12  −0.54 ± 0.20  49  NGC 7099  1  79  4.5  0.93 ± 0.08  −0.89 ± 0.17      2  89  6.5  0.66 ± 0.08  1.75 ± 0.21  #  Cluster  F  Stars  Precision  ΔRA  $$\Delta {\rm \text{Dec.}}$$          (mas)  WFC px  WFC px  01  NGC 1261  1  32  15.5  1.66 ± 0.34  −4.50 ± 0.16      2  41  10.0  −1.89 ± 0.08  4.31 ± 0.21      3  37  8.0  −0.51 ± 0.06  1.61 ± 0.18      4  36  11.0  −0.50 ± 0.14  1.03 ± 0.31      5  46  10.0  0.07 ± 0.05  −4.81 ± 0.19  02  NGC 1851  1 + 2  232  10.0  3.54 ± 0.20  −4.25 ± 0.19  03  NGC 2298  1  79  16.5  −0.17 ± 0.04  5.62 ± 0.24      2  53  14.0  −0.11 ± 0.06  −1.37 ± 0.25  04  NGC 3201  1  635  13.0  0.14 ± 0.32  −0.57 ± 0.23      2  653  10.0  4.33 ± 0.33  8.39 ± 0.20  05  NGC 4590  1  110  11.0  −0.78 ± 0.10  4.90 ± 0.15      2  111  11.5  −0.12 ± 0.04  −3.86 ± 0.17  06  NGC 4833  1  492  7.0  −3.53 ± 0.35  6.86 ± 0.22      2  570  6.5  1.75 ± 0.15  −0.02 ± 0.20  07  NGC 5024  1  173  11.5  −0.14 ± 0.23  3.09 ± 0.20      2  142  10.0  −45.45 ± 0.77  1.35 ± 0.18  08  NGC 5053  1  44  9.5  0.72 ± 0.16  −0.62 ± 0.16      2  41  8.0  −0.07 ± 0.15  −1.86 ± 0.18      3  38  8.5  0.22 ± 0.13  0.29 ± 0.23      4  42  10.0  0.43 ± 0.09  0.23 ± 0.19      5  52  12.0  0.57 ± 0.16  −0.65 ± 0.16  09  NGC 5286  1  215  17.0  1.63 ± 0.20  1.85 ± 0.25      2  217  16.5  1.61 ± 0.20  1.10 ± 0.21  10  NGC 5466  1  56  7.5  −0.13 ± 0.27  1.37 ± 0.12      2  72  8.0  −0.79 ± 0.29  −2.22 ± 0.20  11  NGC 5897  1  164  9.0  0.12 ± 0.22  −0.37 ± 0.20      2  154  7.5  2.58 ± 0.17  −2.51 ± 0.18  12  NGC 5904  1  579  7.0  −1.29 ± 0.19  −2.69 ± 0.15      2  630  7.0  −0.16 ± 0.15  −1.08 ± 0.22  13  NGC 5927  1  848  8.5  3.65 ± 0.30  −0.38 ± 0.20      2  985  8.0  −9.20 ± 0.35  11.91 ± 0.21  14  NGC 5986  1  278  8.0  −0.54 ± 0.30  1.36 ± 0.22      2  268  6.0  −6.46 ± 0.26  4.12 ± 0.18  15  NGC 6093  1  236  22.0  1.81 ± 0.19  −4.47 ± 0.20  16  NGC 6101  1  235  7.0  −1.85 ± 0.76  1.09 ± 0.21      2  232  6.5  −0.80 ± 0.61  2.90 ± 0.20      3  229  8.0  −15.82 ± 0.73  0.55 ± 0.25      4  263  11.0  −1.83 ± 0.89  −0.52 ± 0.27      5  204  8.5  12.13 ± 0.85  −3.52 ± 0.24  17  NGC 6121  1  633  7.5  −4.81 ± 0.37  7.07 ± 0.23      2  649  7.5  −0.69 ± 0.14  1.90 ± 0.22  18  NGC 6144  1  147  7.0  −4.48 ± 0.18  −0.10 ± 0.19      2  147  9.5  −3.09 ± 0.15  −0.76 ± 0.20  19  NGC 6171  1  172  9.0  4.44 ± 0.20  −2.36 ± 0.16      2  196  7.5  −3.26 ± 0.19  −1.51 ± 0.21  20  NGC 6218  1  300  10.0  0.40 ± 0.07  −4.05 ± 0.17      2  362  6.5  −0.72 ± 0.09  0.13 ± 0.19  21  NGC 6254  1  525  7.5  −0.94 ± 0.11  2.02 ± 0.20      2  507  5.0  −0.08 ± 0.12  −0.52 ± 0.18  22  NGC 6304  1  1021  11.0  0.00 ± 0.09  7.29 ± 0.22      2  1137  11.0  −2.63 ± 0.15  3.66 ± 0.28  23  NGC 6341  1  272  7.0  3.31 ± 0.13  −1.46 ± 0.17      2  276  6.5  1.30 ± 0.16  2.01 ± 0.14  24  NGC 6352  1  676  8.0  0.44 ± 0.09  6.24 ± 0.18      2  658  9.5  0.20 ± 0.11  0.84 ± 0.21  25  NGC 6362  1  399  6.0  −1.98 ± 0.18  −1.19 ± 0.20      2  412  6.0  −0.15 ± 0.23  2.51 ± 0.17  26  NGC 6366  1  265  7.0  1.07 ± 0.11  −4.03 ± 0.22      2  263  6.0  2.89 ± 0.19  4.66 ± 0.18  27  NGC 6388  1  812  6.5  1.68 ± 0.24  4.01 ± 0.26      2  873  7.0  −0.86 ± 0.22  5.67 ± 0.22  28  NGC 6397  1  605  8.0  16.46 ± 0.47  12.42 ± 0.26      2  602  7.0  −4.15 ± 0.33  4.79 ± 0.30  29  NGC 6441  1  1075  10.0  34.98 ± 0.54  −3.77 ± 0.26      2  1114  9.5  4.46 ± 0.31  3.80 ± 0.27  30  NGC 6496  1  482  6.0  1.85 ± 0.30  7.03 ± 0.21      2  525  9.0  8.00 ± 0.28  1.65 ± 0.22  31  NGC 6535  1  304  6.5  −4.97 ± 0.19  0.75 ± 0.18      2  275  7.5  −1.78 ± 0.18  −9.16 ± 0.20  32  NGC 6541  1  385  7.5  −11.12 ± 0.28  7.28 ± 0.23      2  481  9.0  −7.23 ± 0.32  7.72 ± 0.22  33  NGC 6584  1  119  7.5  1.82 ± 0.19  6.15 ± 0.24      2  141  10.0  2.67 ± 0.14  2.71 ± 0.25  34  NGC 6624  1  360  8.5  0.83 ± 0.12  1.93 ± 0.23      2  774  9.5  0.61 ± 0.15  6.08 ± 0.21  35  NGC 6637  1  620  10.0  −3.66 ± 0.20  1.83 ± 0.25      2  615  11.5  −2.54 ± 0.23  −0.80 ± 0.29  36  NGC 6652  1  608  12.0  0.35 ± 0.09  5.17 ± 0.26      2  624  9.5  −0.06 ± 0.06  2.40 ± 0.25      3  682  7.0  0.49 ± 0.15  5.78 ± 0.21  37  NGC 6656  1  734  31.0  −1.20 ± 0.21  3.03 ± 0.25      2  1052  27.5  1.21 ± 0.30  6.93 ± 0.31  38  NGC 6681  1  450  14.0  −4.20 ± 0.25  1.76 ± 0.29      2  385  15.5  −2.37 ± 0.20  1.75 ± 0.32  39  NGC 6715  1  484  16.0  −0.69 ± 0.25  −4.67 ± 0.28      2  505  15.5  3.28 ± 0.18  −1.79 ± 0.27  40  NGC 6717  1  439  14.0  3.48 ± 0.21  0.07 ± 0.30      2  406  13.0  0.49 ± 0.20  1.11 ± 0.28      3  445  12.0  0.11 ± 0.18  4.99 ± 0.26  41  NGC 6723  1  256  14.5  0.14 ± 0.20  0.95 ± 0.30      2  144  11.5  −0.25 ± 0.19  4.59 ± 0.22      3  111  9.0  −0.33 ± 0.16  3.99 ± 0.18  42  NGC 6779  1  345  7.5  0.86 ± 0.07  1.07 ± 0.21      2  333  6.5  3.24 ± 0.16  −8.92 ± 0.19  43  NGC 6791  1  288  9.0  −1.92 ± 0.26  −7.50 ± 0.21      2  274  5.5  −4.98 ± 0.25  −6.18 ± 0.21  44  NGC 6809  1  584  7.0  −1.58 ± 0.19  0.99 ± 0.22      2  590  7.5  −9.19 ± 0.21  −0.68 ± 0.23  45  NGC 6838  1  816  7.0  −8.81 ± 0.24  −12.24 ± 0.22      2  800  6.5  −3.96 ± 0.22  −8.28 ± 0.21  46  NGC 6934  1  87  11.5  3.41 ± 0.20  2.62 ± 0.24      2  84  5.5  1.41 ± 0.10  −12.00 ± 0.17  47  NGC 6981  1  31  14.5  1.29 ± 0.26  −9.37 ± 0.19      2  27  9.5  1.73 ± 0.23  −8.07 ± 0.23  48  NGC 7089  1  92  5.5  −0.55 ± 0.12  −6.84 ± 0.15      2  77  10.5  2.52 ± 0.15  −1.40 ± 0.18      3  82  11.0  1.85 ± 0.12  −0.54 ± 0.20  49  NGC 7099  1  79  4.5  0.93 ± 0.08  −0.89 ± 0.17      2  89  6.5  0.66 ± 0.08  1.75 ± 0.21  View Large Astrometrized, stacked images of each observed field have also been produced for both filters with a 1 × 1 pixel sampling. These have been created, for each field, combining all overlapping flc images using the same coordinate transformations that define the common reference frames. Astrometric solutions have been independently derived using the Gaia DR1 catalogue as a reference (Gaia Collaboration 2016). As a consequence, positions are given for Equinox J2000 at epoch 2015. Table 3 reports the precision reached by the new astrometric solution in the fifth column, which is the root mean square error of the offset between Gaia positions and those derived, for the same stars, in our astrometrized stacked images. The measured average value is 0.2 pixel, or ∼10 milliarcsec. These values are also visualized in Fig. 3. Figure 3. View largeDownload slide Precision of the redefined astrometric solution for each observed field, see the text for details. Figure 3. View largeDownload slide Precision of the redefined astrometric solution for each observed field, see the text for details. For completeness, we also measure the astrometric precision of the original astrometric solution of drc images. The position offset between common sources in the Gaia DR1 catalogue and the drc images is used to define this quantity. Offset values are referred to RA and Dec. distances in image pixels and are reported in columns six and seven of Table 3. The associated errors corresponds to the measured standard deviations of each sample. A visual representation is also given in Fig. 4. It can be noted that, in general, offsets are lower than 5 ACS/WFC pixels (0.25 arcsec), and, in many cases, below 1 pixel, with some notable exceptions. Figure 4. View largeDownload slide Offsets between star positions obtained using the original STScI astrometric solution (header of drc images) and Gaia ones. Points represent single fields; the semi-axes of each ellipse have length equal to the measured standard deviation. The grey circle represents an offset of 1 ACS/WFC pixel, which corresponds to ∼0.05 arcsec. Some points falls outside the used limits; we indicate their position along the edges of the figure, the arrows point in their direction. Figure 4. View largeDownload slide Offsets between star positions obtained using the original STScI astrometric solution (header of drc images) and Gaia ones. Points represent single fields; the semi-axes of each ellipse have length equal to the measured standard deviation. The grey circle represents an offset of 1 ACS/WFC pixel, which corresponds to ∼0.05 arcsec. Some points falls outside the used limits; we indicate their position along the edges of the figure, the arrows point in their direction. 3 THE COLOUR–MAGNITUDE DIAGRAMS AND TRICHROMATIC STACKED IMAGES In Fig. 5 we report the CMDs obtained for all five fields of NGC 1261. Magnitudes are given both in the instrumental and Vega-mag photometric systems. Dashed lines represent the saturation limit, black dots represent unsaturated stars, saturated stars are represented with crosses. The five CMDs are all merged in the bottom-right panel, where a different colour have been assigned to each different field. In this case, the instrumental magnitude scale, along with saturation level (dashed line), refer only to Field 1. Figure 5. View largeDownload slide CMDs of the parallel fields of NGC 1261. Both instrumental and calibrated magnitude scales are shown. Dashed lines represent the saturation levels. Saturated stars are marked with crosses. The bottom-right panel collects all the stars present in all the five fields. Stars are here colour-coded as follows. Black dots represent stars measured in F1, red dots stars of F2, green dots stars of F3, blue dots stars of F4, and orange dots represent stars measured in F5. The instrumental magnitude scale and the saturation limit refer to F1 only. Figure 5. View largeDownload slide CMDs of the parallel fields of NGC 1261. Both instrumental and calibrated magnitude scales are shown. Dashed lines represent the saturation levels. Saturated stars are marked with crosses. The bottom-right panel collects all the stars present in all the five fields. Stars are here colour-coded as follows. Black dots represent stars measured in F1, red dots stars of F2, green dots stars of F3, blue dots stars of F4, and orange dots represent stars measured in F5. The instrumental magnitude scale and the saturation limit refer to F1 only. The final CMDs for all other targets are presented in Figs A9–A16. The presented CMDs have been obtained by selecting only high-quality stars, according to quality parameters presented in Anderson et al. (2008). In Section 4 we describe the adopted selection procedure. It is important to mention here that, since in the majority of the cases only one image per filter has been taken, artefact rejection is not an easy task. In order to include faint sources, we have chosen not to limit in flux our raw catalogues. We none the less restricted the detections only to those sources observed in at least one F814W and one F475W image simultaneously, with positions in the common reference frame consistent within 0.8 pixels. No rejection of foreground/background contamination has been performed, nor any correction for differential reddening. The homogeneity of the data guarantees very similar results in every case. None the less, it is out of the scope of this work to characterize in detail the obtained results, which require taking into consideration several issues. For example, the number density of cluster members present in the observed fields depends on the properties of each GC: in some cases, especially for bulge clusters, or those that appear projected in this dense Galactic region, it is difficult to identify the cluster sequence. We recall that another interesting application of the present data is the study of the stellar populations, external to the clusters, that contaminate the observed fields. In particular, for at least six GCs, namely NGC 6624, NGC 6637, NGC 6652, NGC 6681, NGC 6715, and NGC 6809, traces of the Sagittarius Stream are visible in the obtained CMDs (Siegel et al. 2011). As an additional tool to explore and characterize observations, we have created colour images of each observed field, combining the astrometrized stacked images: an example is shown in Fig. 6. The F814W stacked images have been associated with red channel while the F475W images are associated with the blue channel. The images associated with the green channel have been obtained as a result of a 3:1 weighted mean of F475W and F814W counts, respectively. Figure 6. View largeDownload slide Mosaic of all trichromatic images created for NGC 1261; see the text for details. Figure 6. View largeDownload slide Mosaic of all trichromatic images created for NGC 1261; see the text for details. 4 SELECTION OF WELL-MEASURED STARS This section describes the procedure adopted to reject spurious or poorly measured sources in the catalogues and to obtain a sample of bona-fide stellar sources. In this example we refer to the catalogue associated with Field 1 of NGC 6121 (M4). The V3 PA is 272 deg and, for this field, two images were collected, one in F814W, and the other in F475W (666 and 739 s, respectively). For the selection, we have adopted a procedure similar to that described in Milone et al. (2012), defining limits in both q and o (quality) parameters. In addition, we also made use of the RADXS parameter (Bedin et al. 2008, 2009, 2010). As shown in the lower left panels of Fig. 7, the q parameter displays a characteristic trend with magnitude. This parameter is defined as the absolute value of the subtraction between the PSF model and the spatial distribution of light of a particular detection in the image, inside the fitting radius. For a perfectly modelled source, the q parameter assumes value 0. Figure 7. View largeDownload slide Selection procedure for Field 1 of NGC 6121. Left-hand panels: black points mark the stars that were selected as well-measured. Red lines mark the limits chosen for the q, o, and RADXS parameters. Second panel from right: CMD of rejected sources. Right-hand panel: CMD with only sources that survived the selection. All plots refer to instrumental magnitudes. Figure 7. View largeDownload slide Selection procedure for Field 1 of NGC 6121. Left-hand panels: black points mark the stars that were selected as well-measured. Red lines mark the limits chosen for the q, o, and RADXS parameters. Second panel from right: CMD of rejected sources. Right-hand panel: CMD with only sources that survived the selection. All plots refer to instrumental magnitudes. The o parameter quantifies the amount of light that falls on the aperture used for PSF fitting, due to neighbouring sources (Anderson et al. 2008). Unlike the q parameter, it does not show a clear trend with magnitude (middle-left panels of Fig. 7), for this reason we have used a fixed limit (Milone et al. 2012). Finally, the RADXS parameter is related to the spatial extent of the sources, and it is used to distinguish between point sources and extended sources. It is defined as the flux in excess of that predicted from the PSF fitting just outside the core of each source (Bedin et al. 2008). Positive values are expected for extended objects, while negative values indicate detections that are sharper than stellar. The introduction of this parameter in the selection process for the present case is particularly necessary: since only one image per filter is available for most cases, spurious detections due to cosmic rays are present in the catalogues. Moreover, these observations cover the external regions of GCs, where the stellar density is not as high as in the central regions. The observed fields are thus relatively populated by extragalactic, non-stellar objects. We started by taking a more stringent limit in position-consistency for each source. We selected only sources with an rms error in position less than 0.3 pixel. We adopted fixed limits in the o and RADXS parameters for both filters simultaneously. Finally, we measured, for the sources that survived this first selection, the median trend in the plane defined by magnitude and q parameter values. In this way, we removed from the sample non-stellar and poorly measured sources, while not a priori losing faint sources. Red lines in the left-hand panels of Fig. 7 represent the limits used for the case of Field 1 of NGC 6121 (M4). Black points represent sources that passed the selection process. The CMD corresponding to rejected sources (grey dots in the left-hand panels) is shown in the second panel from the right. The resulting CMD is shown in the right-hand panel of Fig. 7. Note how the cleaning process allows the clear detection of the white-dwarf cooling sequence. Saturated stars have been excluded from the selection procedure, but their fluxes have been recovered using the procedure described in Gilliland (2004). 5 RELEASED ELECTRONIC MATERIAL We release, for each ACS/WFC parallel field, the astrometric and photometric catalogues and trichromatic astrometrized stacked images. All the released material will be available for download at the website of the Exoplanets & Stellar Populations Group of the Università degli Studi di Padova.1 Table 4 shows the first 10 rows of the catalogue produced for Field 1 of NGC 6121. The content of each column is explained in detail in Table 5. The parameters wi and wb are the same as in Anderson et al. (2008). They are records that represent the level of saturation of each source in F814W and F475W images, respectively. The last column includes the results of the selection of well-measured stars presented in Section 4. Table 4. First 10 rows extracted from the catalogue referring to Field 1 of NGC 6121 (M4). A detailed description of each column is given in Table 5. #id  RA (2015)  Dec. (2015)  x  dx  y  dy  F814W  di  F475W  db  qi  qb  oi  ob  RADXSi  RADXSb  ni  nb  wi  wb  Good  00000001  245.78962440  −26.47823845  549.416  0.353  688.174  0.224  28.832  9.900  27.078  9.900  1.620  0.760  1.526  0.115  −0.9900  −0.9157  1  1  1  1  0  00000002  245.78961100  −26.47812901  557.336  0.452  687.260  0.382  26.948  9.900  29.764  9.900  1.384  1.274  0.772  9.900  6.2647  −0.9900  1  1  1  1  0  00000003  245.78968050  −26.47823281  549.844  0.632  691.807  0.152  27.164  9.900  28.433  9.900  1.285  1.234  0.179  0.998  −0.9900  −0.9900  1  1  1  1  0  00000004  245.78969050  −26.47817087  554.332  0.391  692.424  0.136  27.335  9.900  30.545  9.900  1.229  1.715  1.014  2.384  0.9430  −0.9900  1  1  1  1  0  00000005  245.78969230  −26.47815496  555.485  0.426  692.537  0.076  27.109  9.900  29.691  9.900  1.189  1.274  0.404  9.900  2.2225  −0.9900  1  1  1  1  0  00000006  245.78973530  −26.47813294  557.094  0.032  695.317  0.263  28.421  9.900  28.038  9.900  1.204  1.009  3.708  0.336  5.6081  −0.9900  1  1  1  1  0  00000007  245.78971500  −26.47807749  561.102  0.068  693.975  0.088  27.108  9.900  28.184  9.900  1.187  0.948  0.787  0.228  1.0828  −0.9900  1  1  1  1  0  00000008  245.78970010  −26.47806742  561.826  0.001  693.008  0.354  26.711  9.900  30.436  9.900  1.154  1.722  0.354  2.059  −0.9900  −0.9900  1  1  1  1  0  00000009  245.78977530  −26.47818364  553.437  0.442  697.928  0.434  27.300  9.900  28.887  9.900  1.330  1.371  0.459  1.208  −0.7255  9.9900  1  1  1  1  0  00000010  245.79005820  −26.47833427  542.628  0.483  716.320  0.322  26.991  9.900  28.863  9.900  1.025  1.193  0.699  5.580  1.8430  1.1484  1  1  1  1  0  #id  RA (2015)  Dec. (2015)  x  dx  y  dy  F814W  di  F475W  db  qi  qb  oi  ob  RADXSi  RADXSb  ni  nb  wi  wb  Good  00000001  245.78962440  −26.47823845  549.416  0.353  688.174  0.224  28.832  9.900  27.078  9.900  1.620  0.760  1.526  0.115  −0.9900  −0.9157  1  1  1  1  0  00000002  245.78961100  −26.47812901  557.336  0.452  687.260  0.382  26.948  9.900  29.764  9.900  1.384  1.274  0.772  9.900  6.2647  −0.9900  1  1  1  1  0  00000003  245.78968050  −26.47823281  549.844  0.632  691.807  0.152  27.164  9.900  28.433  9.900  1.285  1.234  0.179  0.998  −0.9900  −0.9900  1  1  1  1  0  00000004  245.78969050  −26.47817087  554.332  0.391  692.424  0.136  27.335  9.900  30.545  9.900  1.229  1.715  1.014  2.384  0.9430  −0.9900  1  1  1  1  0  00000005  245.78969230  −26.47815496  555.485  0.426  692.537  0.076  27.109  9.900  29.691  9.900  1.189  1.274  0.404  9.900  2.2225  −0.9900  1  1  1  1  0  00000006  245.78973530  −26.47813294  557.094  0.032  695.317  0.263  28.421  9.900  28.038  9.900  1.204  1.009  3.708  0.336  5.6081  −0.9900  1  1  1  1  0  00000007  245.78971500  −26.47807749  561.102  0.068  693.975  0.088  27.108  9.900  28.184  9.900  1.187  0.948  0.787  0.228  1.0828  −0.9900  1  1  1  1  0  00000008  245.78970010  −26.47806742  561.826  0.001  693.008  0.354  26.711  9.900  30.436  9.900  1.154  1.722  0.354  2.059  −0.9900  −0.9900  1  1  1  1  0  00000009  245.78977530  −26.47818364  553.437  0.442  697.928  0.434  27.300  9.900  28.887  9.900  1.330  1.371  0.459  1.208  −0.7255  9.9900  1  1  1  1  0  00000010  245.79005820  −26.47833427  542.628  0.483  716.320  0.322  26.991  9.900  28.863  9.900  1.025  1.193  0.699  5.580  1.8430  1.1484  1  1  1  1  0  View Large Table 5. Information provided by each catalogue. Col.  Name  Explanation  01  id  ID number for each star  02  RA  Right ascension for each star (in deg, epoch 2015)  03  Dec.  Declination for each star (in deg, epoch 2015)  04  x  x position of each star on the reference frame (in pixels)  05  dx  rms errors associated with x position (in pixels)  06  y  y position of each star on the reference frame (in pixels)  07  dy  rms errors associated with y position (in pixels)  08  F814W  F814W magnitude calibrated into Vegamag system  09  di  rms errors associated with F814W magnitude  10  F475W  F475W magnitude calibrated into Vegamag system  11  db  rms errors associated with F475W magnitude  12  qi  q parameter for F814W magnitudes  13  qb  q parameter for F475W magnitudes  14  oi  o parameter for F814W magnitudes  15  ob  o parameter for F475W magnitudes  16  RADXSi  RADXS parameter for F814W  17  RADXSb  RADXS parameter for F475W  18  ni  Number of F814W images the source has been detected in  19  nb  Number of F475W images the source has been detected in  20  wi  Source of F814W photometry      1: unsaturated in deep; 2: unsaturated in short;      3: saturated in short; 4: saturated in deep;  21  wb  Source of F475W photometry (same as wi)  22  good  The source has passed the selection process  Col.  Name  Explanation  01  id  ID number for each star  02  RA  Right ascension for each star (in deg, epoch 2015)  03  Dec.  Declination for each star (in deg, epoch 2015)  04  x  x position of each star on the reference frame (in pixels)  05  dx  rms errors associated with x position (in pixels)  06  y  y position of each star on the reference frame (in pixels)  07  dy  rms errors associated with y position (in pixels)  08  F814W  F814W magnitude calibrated into Vegamag system  09  di  rms errors associated with F814W magnitude  10  F475W  F475W magnitude calibrated into Vegamag system  11  db  rms errors associated with F475W magnitude  12  qi  q parameter for F814W magnitudes  13  qb  q parameter for F475W magnitudes  14  oi  o parameter for F814W magnitudes  15  ob  o parameter for F475W magnitudes  16  RADXSi  RADXS parameter for F814W  17  RADXSb  RADXS parameter for F475W  18  ni  Number of F814W images the source has been detected in  19  nb  Number of F475W images the source has been detected in  20  wi  Source of F814W photometry      1: unsaturated in deep; 2: unsaturated in short;      3: saturated in short; 4: saturated in deep;  21  wb  Source of F475W photometry (same as wi)  22  good  The source has passed the selection process  View Large Note that for many stars, we report a magnitude rms error of 9.900. This is because the routine empirically determines errors based on multiple observations in each filter. When there is only one observation per filter, the error is given a high default value. 6 SUMMARY AND CONCLUSIONS In the context of the Hubble Space Telescope UV Legacy Survey Treasury program of Galactic Globular Clusters (GO-13297; PI: Piotto, Paper I), we are releasing the photometric catalogues relative to the ACS/WFC parallel observations. They represent the first HST photometric survey of external regions of Galactic GCs and consist of 109 distinct stellar fields of 49 targets: 48 GCs and one open cluster, NGC 6791. In the majority of cases, only two images per field were taken, one in F814W and other in F475W, centred at about 6.5 arcmin from cluster centre. Exposure times were selected in order to obtain reliable photometry of the main sequence of target GCs. These observations complement the WFC3 observations of the central regions of the surveyed GCs, and represent valuable tools for different investigations as outlined in Paper I. These data represent a first epoch for future studies aimed at proper motions measurements in these regions. Even without proper motions, these catalogues are suitable to various interesting studies such as measurements of mass functions and binaries fractions in external regions of GCs. Furthermore, crowding is not an issue in these external cluster fields, as a result, this data base could be also used as an input list for spectroscopic follow-up, for example for precise chemical tagging of cluster members. Acknowledgements We thank the anonymous referee for his careful revision that improved the quality of the present manuscript. MS, AA, and GP acknowledge support from the Spanish Ministry of Economy and Competitiveness (MINECO) under grant AYA2013-42781. MS and AA acknowledge support from the Instituto de Astrofísica de Canarias (IAC) under grant 309403. GP acknowledges partial support by the Università degli Studi di Padova Progetto di Ateneo CPDA141214 ‘Towards understanding complex star formation in Galactic globular clusters’ and by INAF under the program PRIN-INAF2014. APM acknowledges support by the Australian Research Council through Discovery Early Career Researcher Award DE150101816 and by the ERC-StG 2016 716082 project ‘GALFOR’ funded by the European Research Council. This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. 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View largeDownload slide As in Fig. 1 but for NGC 4590, NGC 4833, NGC 5024, NGC 5053, NGC 5286, and NGC 5466. Figure A2. View largeDownload slide As in Fig. 1 but for NGC 5897, NGC 5904, NGC 5927, NGC 5986, NGC 6093, and NGC 6101. Figure A2. View largeDownload slide As in Fig. 1 but for NGC 5897, NGC 5904, NGC 5927, NGC 5986, NGC 6093, and NGC 6101. Figure A3. View largeDownload slide As in Fig. 1 but for NGC 6121, NGC 6144, NGC 6171, NGC 6218, NGC 6254, and NGC 6304. Figure A3. View largeDownload slide As in Fig. 1 but for NGC 6121, NGC 6144, NGC 6171, NGC 6218, NGC 6254, and NGC 6304. Figure A4. View largeDownload slide As in Fig. 1 but for NGC 6341, NGC 6352, NGC 6362, NGC 6366, NGC 6388, and NGC 6397. Figure A4. View largeDownload slide As in Fig. 1 but for NGC 6341, NGC 6352, NGC 6362, NGC 6366, NGC 6388, and NGC 6397. Figure A5. View largeDownload slide As in Fig. 1 but for NGC 6441, NGC 6496, NGC 6535, NGC 6541, NGC 6584, and NGC 6624. Figure A5. View largeDownload slide As in Fig. 1 but for NGC 6441, NGC 6496, NGC 6535, NGC 6541, NGC 6584, and NGC 6624. Figure A6. View largeDownload slide As in Fig. 1 but for NGC 6637, NGC 6652, NGC 6656, NGC 6681, NGC 6715, and NGC 6717. Figure A6. View largeDownload slide As in Fig. 1 but for NGC 6637, NGC 6652, NGC 6656, NGC 6681, NGC 6715, and NGC 6717. Figure A7. View largeDownload slide As in Fig. 1 but for NGC 6723, NGC 6779, NGC 6791, NGC 6809, NGC 6838, and NGC 6934. Figure A7. View largeDownload slide As in Fig. 1 but for NGC 6723, NGC 6779, NGC 6791, NGC 6809, NGC 6838, and NGC 6934. Figure A8. View largeDownload slide As in Fig. 1 but for NGC 6981, NGC 7089, and NGC 7099. Figure A8. View largeDownload slide As in Fig. 1 but for NGC 6981, NGC 7089, and NGC 7099. Figure A9. View largeDownload slide Obtained CMDs for NGC 1851, NGC 2298, NGC 3201, NGC 4590, and NGC 5024. For each cluster, CMDs for all fields have been merged together. The colour-code is such that black dots represent stars measured in F1; red dots, stars of F2; green dots stars of F3; blue dots stars of F4 and orange dots represent stars measured in F5. Dashed lines represent the saturation level for all F1, saturated stars are represented by crosses. Figure A9. View largeDownload slide Obtained CMDs for NGC 1851, NGC 2298, NGC 3201, NGC 4590, and NGC 5024. For each cluster, CMDs for all fields have been merged together. The colour-code is such that black dots represent stars measured in F1; red dots, stars of F2; green dots stars of F3; blue dots stars of F4 and orange dots represent stars measured in F5. Dashed lines represent the saturation level for all F1, saturated stars are represented by crosses. Figure A10. View largeDownload slide As in Fig. A9 but for NGC 5053, NGC 5286, NGC 5466, NGC 5897, NGC 5904, and NGC 5927. Figure A10. View largeDownload slide As in Fig. A9 but for NGC 5053, NGC 5286, NGC 5466, NGC 5897, NGC 5904, and NGC 5927. Figure A11. View largeDownload slide As in Fig. A9 but for NGC 5986, NGC 6093, NGC 6101, NGC 6121, NGC 6144, and NGC 6171. Figure A11. View largeDownload slide As in Fig. A9 but for NGC 5986, NGC 6093, NGC 6101, NGC 6121, NGC 6144, and NGC 6171. Figure A12. View largeDownload slide As in Fig. A9 but for NGC 6218, NGC 6254, NGC 6304, NGC 6341, NGC 6352, and NGC 6362. Figure A12. View largeDownload slide As in Fig. A9 but for NGC 6218, NGC 6254, NGC 6304, NGC 6341, NGC 6352, and NGC 6362. Figure A13. View largeDownload slide As in Fig. A9 but for NGC 6366, NGC 6388, NGC 6397, NGC 6441, NGC 6496, and NGC 6535. Figure A13. View largeDownload slide As in Fig. A9 but for NGC 6366, NGC 6388, NGC 6397, NGC 6441, NGC 6496, and NGC 6535. Figure A14. View largeDownload slide As in Fig. A9 but for NGC 6541, NGC 6584, NGC 6624, NGC 6637, NGC 6652, and NGC 6656. Figure A14. View largeDownload slide As in Fig. A9 but for NGC 6541, NGC 6584, NGC 6624, NGC 6637, NGC 6652, and NGC 6656. Figure A15. View largeDownload slide As in Fig. A9 but for NGC 6681, NGC 6715, NGC 6717, NGC 6723, NGC 6779 and NGC 6791. Figure A15. View largeDownload slide As in Fig. A9 but for NGC 6681, NGC 6715, NGC 6717, NGC 6723, NGC 6779 and NGC 6791. Figure A16. View largeDownload slide As in Fig. A9 but for NGC 6809, NGC 6838, NGC 6934, NGC 6981, NGC 7089, and NGC 7099. Figure A16. View largeDownload slide As in Fig. A9 but for NGC 6809, NGC 6838, NGC 6934, NGC 6981, NGC 7089, and NGC 7099. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society

Journal

Monthly Notices of the Royal Astronomical SocietyOxford University Press

Published: May 1, 2018

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