MGCS
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The Hubble Missing Globular Cluster Survey. III. Astro-photometric catalogs,
artificial-star tests, and improved absolute proper motions
M. Libralato, A. Bellini, D. Massari, M. Bellazzini, F. Aguado-Agelet,
S. Cassisi, E. Ceccarelli, E. Dalessandro, E. Dodd, F. R. Ferraro,
C. Gallart, B. Lanzoni, M. Monelli, A. Mucciarelli, E. Pancino, R. Pascale,
L. Rosignoli, M. Salaris, S. Saracino, and C. Zerbinati
Astron. Astrophys. 709, A140 (2026)
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Keywords: astrometry – photometry – proper motions – stellar clusters – globular
clusters
Abstract:
The Hubble Missing Globular Cluster Survey (MGCS) has taken one of the last
opportunities to complete the census of Galactic globular clusters (GCs)
started by past Hubble Space Telescope (HST) programs, securing
high-resolution data for 34 GCs never observed before by HST. The previous
papers of the series have highlighted the astrometric and photometric
potential of the project by analyzing a subsample of targets. We present, and
release to the community, the official astro-photometric catalogs of the MGCS
for all GCs imaged by this project. We describe the data reduction using
state-of-the-art techniques designed for HST. We discuss the photometric
calibration and show, for the first time, the synergy with the Gaia catalog to
ensure homogeneous photometry across our data set. We compute artificial-star
tests that can be used to assess systematics and the completeness level of our
data. We combined HST and Gaia data to refine the absolute proper motions of
our GCs, reaching a precision ~3 times better than that of Gaia alone. We used
these new proper motions to update (and to determine for the first time for
six systems) the associations between GCs and their putative galaxy
progenitors. This work continues decades-long efforts of large Treasury
programs in sharing precise and accurate atlases to the community for studying
GCs across a wide range of scientific endeavors.
Astro-photometric catalogs, artificial-star catalogs and stacked images of the
34 globular targets part of the MGCS. Fluxes in the astrometrized stacked
images are not properly resampled. Thus, these images are meant for visual
reference only and not for scientific investigations.
Description of file: */*.fits
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Explanations
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Individual astrometrized stacked images, one for each field, camera and filter.
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Byte-by-byte Description of file: */*ASTRO
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Bytes Format Units Label Explanations
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1- 13 F13.9 --- ra Right Ascension at reference epoch
15- 25 F11.9 --- dec Declination at reference epoch
27- 36 F10.4 --- x X master-frame position (1) (2)
38- 47 F10.4 --- y Y master-frame position (1) (2)
49- 54 I6 --- id ID number of the source
56- 62 F7.3 --- pmra HST-Gaia raw proper motion along R.A.
cos(Dec.) (mas/yr) (3)
64- 68 F5.3 --- epmra Error on HST-Gaia raw proper motion along
R.A. cos(Dec.) (mas/yr) (3)
70- 76 F7.3 --- pmdec HST-Gaia raw proper motion along Dec.
(mas/yr) (3)
78- 82 F5.3 --- epmdec Error on HST-Gaia raw proper motion along
Dec. (mas/yr) (3)
84- 90 F7.3 --- pmra_c HST-Gaia corrected proper motion along R.A.
cos(Dec.) (mas/yr) (3)
92- 96 F5.3 --- epmra_c Error on HST-Gaia corrected proper motion
along R.A. cos(Dec.) (mas/yr) (3)
98-104 F7.3 --- pmdec_c HST-Gaia corrected proper motion along Dec.
(mas/yr) (3)
106-110 F5.3 --- epmdec_c Error on HST-Gaia corrected proper motion
along Dec. (mas/yr) (3)
112-130 I19 --- source_id SOURCE_ID from the Gaia DR3 catalog
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Note (1): The coordinates of the center of the cluster used as a reference are
provided in the header of the catalog.
Note (2): The pixel scale is provided in the header of the catalog.
Note (3): This column is included in the catalog only if proper motions were
computed using GaiaHub (see Section 4 of the paper).
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Byte-by-byte Description of file: */*PHOT
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Bytes Format Units Label Explanations
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1- 1 I1 --- sat Saturation flag (1) (2)
3- 9 F7.4 --- m Calibrated VEGA magnitude (1) (3)
11-17 F7.4 --- sm Photometric rms (1)
19-23 F5.3 --- q Quality-of-PSF-fit (QFIT) parameter (1)
25-34 F10.4 --- o Fractional flux within the fitting radius prior
to neighbor subtraction (1)
36-36 I1 --- f Number of exposures in which a source was found
(1)
38-38 I1 --- g Number of exposures used to measure the flux of
a source (1)
40-47 F8.4 --- rxs Excess or deficiency of flux outside the core of
the star (1)
49-55 F7.2 --- sky Sky in electrons (optical data) or electron/s
(IR data) (1) (4)
57-63 F7.2 --- rmssky Sky rms in electrons (optical data) or electron/s
(IR data) (1) (4)
65-70 I6 --- id ID number of the source (5)
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Note (1): All values for a source are set to 0 if the star is not measured in
that filter and camera, and all values but the calibrated magnitude
are set to 0 if a source is saturated in the catalog.
Note (2): The saturation flag is set to 0 for a measurement obtained from a long
exposure, 1 for a measurement obtained from a short exposure, and 9 if
the star is saturated (or unsaturated but missed by the second-pass
photometry tool, i.e., likely a spurious detection only included for
completeness; see Section 2 of the paper).
Note (3): The VEGA-magnitude zero-point is provided in the header of the
catalog.
Note (4): To estimate the significance of a source over the sky, first convert
calibrated VEGA magnitudes into instrumental fluxes (in unit of
electrons or electron s−1 for optical and IR data, respectively):
flux = 10^(-0.4*(m-ZP_VEGAmag)).
Note (5): The ID number is the same as in the corresponding astrometric catalog.
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Byte-by-byte Description of file: */*ASTRO.IO
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Bytes Format Units Label Explanations
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1- 13 F13.9 --- rai Input right ascension (1)
15- 25 F11.9 --- deci Input declination (1)
27- 36 F10.4 --- xi Input X master-frame position (1)
38- 47 F10.4 --- yi Input Y master-frame position (1)
49- 61 F13.9 --- rao Output right ascension (1)
63- 73 F11.9 --- deco Output declination (1)
75- 84 F10.4 --- xo Output X master-frame position (1)
86- 95 F10.4 --- yo Output Y master-frame position (1)
97-102 I6 --- id ID number of the source
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Note (1): The coordinates of the center of the cluster and the pixel scale are
the same as in the analog astrometric catalog with real sources.
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Byte-by-byte Description of file: */*ASTRO.PHOT
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Bytes Format Units Label Explanations
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1- 7 F7.4 --- mi Input calibrated VEGA magnitude (1) (2)
9-15 F7.4 --- mo Output calibrated VEGA magnitude (1) (2)
17-26 F10.4 --- smo Output photometric rms (1)
28-34 F7.3 --- qo Output quality-of-PSF-fit (QFIT) parameter (1)
36-46 F11.4 --- oo Output fractional flux within the fitting radius
prior to neighbor subtraction (1)
48-48 I1 --- fo Output number of exposures in which a source was
found (1)
50-50 I1 --- go Output number of exposures used to measure the
flux of a source (1)
52-59 F8.4 --- rxso Output excess or deficiency of flux outside the
core of the star (1)
61-67 F7.2 --- skyo Output sky in electrons (optical data) or
electron/s (IR data) (1)
69-75 F7.2 --- rmsskyo Output sky rms in electrons (optical data) or
electron/s (IR data) (1)
77-82 I6 --- id ID number of the source (3)
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Note (1): All values for a source are set to 0 if the star is not measured in
that filter and camera.
Note (2): The VEGA-magnitude zero-point is provided in the header of the catalog
and it is the same as for the analog photometric catalog with real
stars.
Note (3): The ID number is the same as in the corresponding artificial-star
astrometric catalog.
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Reference:
Libralato, M., Bellini, A., Massari, D., et al. 2026, A&A, 709, A140
See also:
Massari, D., Bellazzini, M., Libralato, M., et al. 2025, A&A, 698, A197
Rosignoli, L., Libralato, M., Pascale, R., et al. 2026, A&A, 707, A258
Acknowledgements:
Mattia Libralato, mattia.libralato(at)inaf.it
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