II/391 KMTNet Synoptic Survey of Southern Sky (KS4) DR1 (Chang+, 2026)
KMTNet Synoptic Survey of Southern Sky III: The First Data Release
Chang S.-W., Im M., Jeong M., Kim J., Park B., Lee J., Buckley D.A.H.,
Cooke J., Jung S., Kim D.-J., Kim J.H., Kim Y., Lee C.-U., Lee S.-K.,
Paek G.S.H., Shin J.
<Journal of the Korean Astronomical Society, vol. 59, pp. 179-198 (2026)>
=2026JKAS...59..179C 2026JKAS...59..179C
=2026arXiv260328089C 2026arXiv260328089C
ADC_Keywords: Photometry, UBVRI; Surveys
Keywords: techniques: photometric; surveys; catalogues;
virtual observatory tools
Abstract:
We present the first public data release (DR1) of the KMTNet Synoptic
Survey of Southern Sky (KS4). This deep, wide-field imaging survey
covers a southern footprint of -85°<DEC←28.8° in the B, V,
R, and I bands using a network of three 1.6-m telescopes. Although
primarily designed to secure reference imaging for gravitational wave
counterpart identification, DR1 delivers science-ready data for
∼4000deg2 to enable a broad range of astrophysical research. The
release includes deep co-added images reaching median 5σ depths
of 22.0-23.5 AB mag. It is accompanied by two source catalogs
containing over 200 million sources with SNR>5: an I-band-selected
forced-photometry catalog optimized for consistent colors, and a
band-merged catalog offering enhanced completeness. Validation
demonstrates robust data quality, characterized by mean astrometric
offsets of +0.054±0.129arcsec in RA and -0.015±0.120arcsec in DEC
relative to Gaia DR3. Photometric uniformity for point sources is
maintained within ±0.03mag relative to Gaia XP for 97.5-99.8% of the
footprint across all four bands. A key advantage of KS4 is its uniform
and contiguous spatial coverage. It extends to fainter magnitudes than
other uniform surveys while filling irregular gaps in existing deep
datasets.
Description:
The KMTNet Synoptic Survey of Southern Sky (KS4) DR1 dataset includes
observations acquired from the commencement of KS4 operations through
December 21, 2023. It covers 979 distinct survey tiles, representing a
subset of the 2,748 total fields defined in the survey strategy (see
Im et al., in preparation). Each tile corresponds to the KMTNet
field-of-view (2x2deg2) and is typically observed with a 120s exposure
in each filter, yielding a cumulative integration time of at least
480s across the four BVRI bands.
The dataset includes 17,626 individual exposures obtained across the
three KMTNet sites. The contribution from each site varies
significantly: SAAO provided 50.5% of the total exposures
(8,922 images), followed by CTIO with 31.5% (5,570 images) and SSO
with 17.7% (3,134 images). The total integration time amounts to
589.3hr, distributed approximately uniformly across the photometric
bands: B (142.0hr), V (143.8hr), R (153.1hr), and I (150.4hr).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
ks4dr1.sam 875 1000 *Sample of the band-merged single_master catalog
(1000 among 279,909,360 sources)
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Note on ks4dr1.sam: We construct the band-merged single_master catalog to
incorporate all objects reliably detected in at least one of the BVRI
bands. See Section 4.3.1.
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See also:
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
II/367 : The VISTA Hemisphere Survey (VHS) catalog DR5 (McMahon+, 2020)
II/336 : AAVSO Photometric All Sky Survey (APASS) DR9 (Henden+, 2016)
II/371 : The Dark Energy Survey (DES): Data Release 2 (Abbott+, 2021)
J/PASP/110/863 : A Stellar Spectral Flux Library: 1150 - 25000 A (Pickles 1998)
J/PASP/124/140 : UBVRI Hp BT and VT photonic responses (Bessell+, 2012)
J/ApJ/916/47 : GECKO follow-up obs. of 3 binary BH GW events (Kim+, 2021)
J/ApJS/275/46 : Unobscured QSO cand. in the southern sky with KS4 (Kim+, 2024)
http://datalab.noirlab.edu/data/ks4 : KS4 data at NOIRLab's
http://alasky.cds.unistra.fr/KS4/DR1 : KS4 images in Aladin
Byte-by-byte Description of file: ks4dr1.sam
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 I18 --- CoaddID Unique object identifier (coaddobjectid)
20- 30 F11.7 deg RAdeg Right Ascension (J2000) (ra)
32- 42 F11.7 deg DEdeg [-85.1/-28.7] Declination (J2000) (dec)
44- 52 I9 --- Healpix HEALPix index for sky position (healpix_index)
54- 55 I2 --- Ndet [1/12] Number of detections across all KS4
tiles (n_det)
57- 67 F11.7 deg GLON Galactic longitude (glon)
69- 79 F11.7 deg GLAT [-87.3/34.1] Galactic latitude (glat)
81- 88 F8.5 mag E(B-V) [0.007/16.3] E(B-V) extinction from Schlegel+
(1998ApJ...500..525S 1998ApJ...500..525S) using the dustmap tool
(ebmv_sfd)
90- 101 F12.6 d b_MJD-B ? Minimum Modified Julian Date in band B
(minmjdb)
103- 114 F12.6 d b_MJD-V ? Minimum Modified Julian Date in band V
(minmjdv)
116- 127 F12.6 d b_MJD-R ? Minimum Modified Julian Date in band R
(minmjdr)
129- 140 F12.6 d b_MJD-I ? Minimum Modified Julian Date in band I
(minmjdi)
142- 153 F12.6 d B_MJD-B ? Maximum Modified Julian Date in band B
(maxmjdb)
155- 166 F12.6 d B_MJD-V ? Maximum Modified Julian Date in band V
(maxmjdv)
168- 179 F12.6 d B_MJD-R ? Maximum Modified Julian Date in band R
(maxmjdr)
181- 192 F12.6 d B_MJD-I ? Maximum Modified Julian Date in band I
(maxmjdi)
194- 205 F12.6 d MJD-B ? Mean Modified Julian Date in band B
(meanmjdb)
207- 218 F12.6 d MJD-V ? Mean Modified Julian Date in band V
(meanmjdv)
220- 231 F12.6 d MJD-R ? Mean Modified Julian Date in band R
(meanmjdr)
233- 244 F12.6 d MJD-I ? Mean Modified Julian Date in band I
(meanmjdi)
246- 253 F8.4 mag Bmag [-23.96/26.85]? SExtractor AUTO magnitude in
band B (magautob)
255- 260 F6.4 mag e_Bmag [0/0.5]? Uncertainty on Bmag
(magerrautob)
262- 268 F7.4 mag Vmag [4.6/25.31]? SExtractor AUTO magnitude in
band V (magautov)
270- 275 F6.4 mag e_Vmag [0/0.5]? Uncertainty on Vmag
(magerrautov)
277- 284 F8.4 mag Rmag [-10.96/25.51]? SExtractor AUTO magnitude in
band R (magautor)
286- 291 F6.4 mag e_Rmag [0/0.5]? Uncertainty on Rmag
(magerrautor)
293- 300 F8.4 mag Imag [-18.91/25.7]? SExtractor AUTO magnitude in
band I (magautoi)
302- 307 F6.4 mag e_Imag [0/0.5]? Uncertainty on Imag
(magerrautoi)
309- 316 F8.4 mag Bmag3 [-23.64/26.83]? Aperture magnitude (3 arcsec)
in band B (magaper3b)
318- 323 F6.4 mag e_Bmag3 [0/0.5]? Bmag3 uncertainty (magerraper3b)
325- 331 F7.4 mag Vmag3 ? Aperture magnitude (3 arcsec) in band V
(magaper3v)
333- 338 F6.4 mag e_Vmag3 [0/0.5]? Vmag3 uncertainty (magerraper3v)
340- 347 F8.4 mag Rmag3 ? Aperture magnitude (3 arcsec) in band R
(magaper3r)
349- 354 F6.4 mag e_Rmag3 [0/0.5]? Rmag3 uncertainty (magerraper3r)
356- 363 F8.4 mag Imag3 ? Aperture magnitude (3 arcsec) in band I
(magaper3i)
365- 370 F6.4 mag e_Imag3 [0/0.5]? Imag3 uncertainty (magerraper3i)
372- 379 F8.4 mag Bmag5 ? Aperture magnitude (5 arcsec) in band B
(magaper5b)
381- 386 F6.4 mag e_Bmag5 [0/0.5]? Bmag5 uncertainty (magerraper5b)
388- 394 F7.4 mag Vmag5 ? Aperture magnitude (5 arcsec) in band V
(magaper5v)
396- 401 F6.4 mag e_Vmag5 [0/0.5]? Vmag5 uncertainty (magerraper5v)
403- 410 F8.4 mag Rmag5 ? Aperture magnitude (5 arcsec) in band R
(magaper5r)
412- 417 F6.4 mag e_Rmag5 [0/0.5]? Rmag5 uncertainty (magerraper5r)
419- 426 F8.4 mag Imag5 ? Aperture magnitude (5 arcsec) in band I
(magaper5i)
428- 433 F6.4 mag e_Imag5 [0/0.5]? Imag5 uncertainty (magerraper5i)
435- 442 F8.4 mag Bmag10 [-24/27]? Aperture magnitude (10") in band B
(magaper10b)
444- 449 F6.4 mag e_Bmag10 [0/0.5]? Bmag10 uncertainty (magerraper10b)
451- 457 F7.4 mag Vmag10 [4.8/26.4]? Aperture magnitude (10") in band V
(magaper10v)
459- 464 F6.4 mag e_Vmag10 [0/0.5]? Vmag10 uncertainty (magerraper10v)
466- 473 F8.4 mag Rmag10 [-12/26.5]? Aperture magnitude (10") in band R
(magaper10r)
475- 480 F6.4 mag e_Rmag10 [0/0.5]? Rmag10 uncertainty (magerraper10r)
482- 489 F8.4 mag Imag10 [-18/25.5]? Aperture magnitude (10") in band I
(magaper10i)
491- 496 F6.4 mag e_Imag10 [0/0.5]? Imag10 uncertainty (magerraper10i)
498- 505 F8.3 arcsec FWHM-B [-361.5/4215]? FWHM of the object in band B
(fwhmimageb)
507- 511 F5.3 --- S/G-B [0/1]? Source Extractor stellarity index in
band B (classstarb)
513- 523 E11.5 arcsec Rh-B [-116130/6418.3]? Half-light radius in band B
(fluxradiusb)
525- 530 F6.3 --- Krad-B [0/74.1]? Kron radius factor used for
MAG_AUTO in band B (kronradiusb)
532- 542 E11.5 ct Fmax-B [-39.4/8.3e19]? Peak count above background
of the brightest object pixel in band B
(fluxmaxb)
544- 549 F6.2 deg PA-B [-90/90]? Position angle in band B
(thetaimageb)
551- 555 F5.3 --- Ell-B [0/1]? Ellipticity in band B (ellipticity_b)
557- 564 F8.3 pix Aimg-B [0.49/2088]? Length of object semi-major axis
in pixels in band B (aimageb)
566- 572 F7.3 pix Bimg-B [0.03/632]? Length of object semi-minor axis
in pixels in band B (bimageb)
574- 581 F8.3 arcsec FWHM-V [-104/3976]? FWHM of the object in band V
(fwhmimagev)
583- 587 F5.3 --- S/G-V [0/1]? Source Extractor stellarity index in
band V (classstarv)
589- 599 E11.5 arcsec Rh-V [-467473/7762]? Half-light radius in band V
(fluxradiusv)
601- 607 F7.3 --- Krad-V [0/406]? Kron radius factor used for MAG_AUTO
in band V (kronradiusv)
609- 619 E11.5 ct Fmax-V [-50/1.2e+9]? Peak count above background of
the brightest object pixel in band V
(fluxmaxv)
621- 626 F6.2 deg PA-V [-90/90]? Position angle in band V
(thetaimagev)
628- 632 F5.3 --- Ell-V [0/1]? Ellipticity in band V (ellipticity_v)
634- 641 F8.3 pix Aimg-V [0.49/1746]? Length of object semi-major axis
in band V (aimagev)
643- 649 F7.3 pix Bimg-V [0.01/493]? Length of object semi-minor axis
in band V (bimagev)
651- 658 F8.3 arcsec FWHM-R [-211/8240]? FWHM of the object in band R
(fwhmimager)
660- 664 F5.3 --- S/G-R ? Source Extractor stellarity index in band R
(classstarr)
666- 676 E11.5 arcsec Rh-R [-4.2e+7/6084]? Half-light radius in band R
(fluxradiusr)
678- 684 F7.3 --- Krad-R [0/628]? Kron radius factor used for MAG_AUTO
in band R (kronradiusr)
686- 696 E11.5 ct Fmax-R [-312/1.3e+15]? Peak count above background
of the brightest object pixel in band R
(fluxmaxr)
698- 703 F6.2 deg PA-R [-90/90]? Position angle in band R
(thetaimager)
705- 709 F5.3 --- Ell-R ? Ellipticity in band R (ellipticity_r)
711- 718 F8.3 pix Aimg-R [0.3/2212]? Length of object semi-major axis
in band R (aimager)
720- 727 F8.3 pix Bimg-R [0.03/1070]? Length of object semi-minor axis
in band R (bimager)
729- 736 F8.3 arcsec FWHM-I [-720/4658]? FWHM of the object in band I
(fwhmimagei)
738- 742 F5.3 --- S/G-I ? Source Extractor stellarity index in band I
(classstari)
744- 754 E11.5 arcsec Rh-I ? Half-light radius in band I (fluxradiusi)
756- 761 F6.3 --- Krad-I [0/99.3]? Kron radius factor used for
MAG_AUTO in band I (kronradiusi)
763- 773 E11.5 ct Fmax-I [-330/6.9e+16]? Peak count above background
of the brightest object pixel in band I
(fluxmaxi)
775- 780 F6.2 deg PA-I [-90/90]? Position angle in band I
(thetaimagei)
782- 786 F5.3 --- Ell-I ? Ellipticity in band I (ellipticity_i)
788- 795 F8.3 pix Aimg-I [0.49/1939]? Length of object semi-major axis
in band I (aimagei)
797- 803 F7.3 pix Bimg-I [0.01/542]? Length of object semi-minor axis
in band I (bimagei)
805- 807 I3 --- Flags-B [0/119]? Flags raised by Source Extractor in
band B (0=clean source) (flags_b)
809- 810 I2 --- Ndith-B [0/11] Number of all contributing
single-epoch exposures in band B (ndith_b)
812 I1 --- FlISO-B [0/8] OR-combined flagged pixel types within
the isophotal radius in band B
(0=clean source) (imaflagsisob) (1)
814- 816 I3 --- SsCt30-B [0/331] Spurious source count within 30"
in band B (sscount30b)
818- 819 I2 --- FlSs-B [-1/0]? Spurious source flag in band B
(ssflag_b) (2)
821 I1 --- Edge-B [0/1] Edge flag, proximity of tile boundaries
in band B (edge_b)
823- 825 I3 --- Flags-V [0/117]? Flags raised by Source Extractor in
band V (0=clean source) (flags_v)
827- 828 I2 --- Ndith-V [0/12] Number of all contributing
single-epoch exposures in band V (ndith_v)
830 I1 --- FlISO-V [0/8] OR-combined flagged pixel types within
the isophotal radius in band V
(0=clean source) (imaflagsisov) (1)
832- 834 I3 --- SsCt30-V [0/306] Spurious source count within 30" in
band V (sscount30v)
836- 837 I2 --- FlSs-V [-1/0]? Spurious source flag in band V
(ssflag_v) (2)
839 I1 --- Edge-V [0/1] Edge flag, proximity of tile boundaries
in band V (edge_v)
841- 843 I3 --- Flags-R [0/119]? Flags raised by Source Extractor in
band R (0=clean source) (flags_r)
845- 846 I2 --- Ndith-R [0/12] Number of all contributing
single-epoch exposures in band R (ndith_r)
848 I1 --- FlISO-R [0/8] OR-combined flagged pixel types within
the isophotal radius in band R
(0=clean source) (imaflagsisor) (1)
850- 852 I3 --- SsCt30-R [0/276] Spurious source count within 30"
in band R (sscount30r)
854- 855 I2 --- FlSs-R [-1/0]? Spurious source flag in band R
(ssflag_r) (2)
857 I1 --- Edge-R [0/1] Edge flag, proximity of tile boundaries
in band R (edge_r)
859- 861 I3 --- Flags-I [0/119]? Flags raised by Source Extractor in
band I (0=clean source) (flags_i)
863- 864 I2 --- Ndith-I [0/12] Number of all contributing
single-epoch exposures in band I (ndith_i)
866 I1 --- FlISO-I [0/8] OR-combined flagged pixel types within
the isophotal radius in band I
(0=clean source) (imaflagsisoi) (1)
868- 870 I3 --- SsCt30-I [0/299] Spurious source count within 30"
in band I (sscount30i)
872- 873 I2 --- FlSs-I [-1/0]? Spurious source flag in band I
(ssflag_i) (2)
875 I1 --- Edge-I [0/1] Edge flag, proximity of tile boundaries
in band I (edge_i)
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Note (1): The imaflags_iso captures information from the external pipeline
mask (see Jeong+ 2026JKAS...59..157J 2026JKAS...59..157J) mapped to the source's isophotal
footprint.
Note (2): The ssflag identifies potential spurious detections.
See Section 3.2.1.
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Acknowledgements:
Seo-Won Chang [seowon.chang at snu.ac.kr]
License: CC-BY-4.0
(End) G. Monari, F.-X. Pineau, E. Perret [CDS] 13-Mar-2026