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 876 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- 19 I19 --- CoaddID Unique object identifier (coaddobjectid)
21- 31 F11.7 deg RAdeg Right Ascension (J2000) (ra)
33- 43 F11.7 deg DEdeg [-85.1/-28.7] Declination (J2000) (dec)
45- 53 I9 --- Healpix HEALPix index for sky position (healpix_index)
55- 56 I2 --- Ndet [1/12] Number of detections across all KS4
tiles (n_det)
58- 68 F11.7 deg GLON Galactic longitude (glon)
70- 80 F11.7 deg GLAT [-87.3/34.1] Galactic latitude (glat)
82- 89 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)
91-102 F12.6 d b_MJD-B ? Minimum Modified Julian Date in band B
(minmjdb)
104-115 F12.6 d b_MJD-V ? Minimum Modified Julian Date in band V
(minmjdv)
117-128 F12.6 d b_MJD-R ? Minimum Modified Julian Date in band R
(minmjdr)
130-141 F12.6 d b_MJD-I ? Minimum Modified Julian Date in band I
(minmjdi)
143-154 F12.6 d B_MJD-B ? Maximum Modified Julian Date in band B
(maxmjdb)
156-167 F12.6 d B_MJD-V ? Maximum Modified Julian Date in band V
(maxmjdv)
169-180 F12.6 d B_MJD-R ? Maximum Modified Julian Date in band R
(maxmjdr)
182-193 F12.6 d B_MJD-I ? Maximum Modified Julian Date in band I
(maxmjdi)
195-206 F12.6 d MJD-B ? Mean Modified Julian Date in band B
(meanmjdb)
208-219 F12.6 d MJD-V ? Mean Modified Julian Date in band V
(meanmjdv)
221-232 F12.6 d MJD-R ? Mean Modified Julian Date in band R
(meanmjdr)
234-245 F12.6 d MJD-I ? Mean Modified Julian Date in band I
(meanmjdi)
247-254 F8.4 mag Bmag [-23.96/26.85]? SExtractor AUTO magnitude in
band B (magautob)
256-261 F6.4 mag e_Bmag [0/0.5]? Uncertainty on Bmag
(magerrautob)
263-269 F7.4 mag Vmag [4.6/25.31]? SExtractor AUTO magnitude in
band V (magautov)
271-276 F6.4 mag e_Vmag [0/0.5]? Uncertainty on Vmag
(magerrautov)
278-285 F8.4 mag Rmag [-10.96/25.51]? SExtractor AUTO magnitude in
band R (magautor)
287-292 F6.4 mag e_Rmag [0/0.5]? Uncertainty on Rmag
(magerrautor)
294-301 F8.4 mag Imag [-18.91/25.7]? SExtractor AUTO magnitude in
band I (magautoi)
303-308 F6.4 mag e_Imag [0/0.5]? Uncertainty on Imag
(magerrautoi)
310-317 F8.4 mag Bmag3 [-23.64/26.83]? Aperture magnitude (3 arcsec)
in band B (magaper3b)
319-324 F6.4 mag e_Bmag3 [0/0.5]? Bmag3 uncertainty (magerraper3b)
326-332 F7.4 mag Vmag3 ? Aperture magnitude (3 arcsec) in band V
(magaper3v)
334-339 F6.4 mag e_Vmag3 [0/0.5]? Vmag3 uncertainty (magerraper3v)
341-348 F8.4 mag Rmag3 ? Aperture magnitude (3 arcsec) in band R
(magaper3r)
350-355 F6.4 mag e_Rmag3 [0/0.5]? Rmag3 uncertainty (magerraper3r)
357-364 F8.4 mag Imag3 ? Aperture magnitude (3 arcsec) in band I
(magaper3i)
366-371 F6.4 mag e_Imag3 [0/0.5]? Imag3 uncertainty (magerraper3i)
373-380 F8.4 mag Bmag5 ? Aperture magnitude (5 arcsec) in band B
(magaper5b)
382-387 F6.4 mag e_Bmag5 [0/0.5]? Bmag5 uncertainty (magerraper5b)
389-395 F7.4 mag Vmag5 ? Aperture magnitude (5 arcsec) in band V
(magaper5v)
397-402 F6.4 mag e_Vmag5 [0/0.5]? Vmag5 uncertainty (magerraper5v)
404-411 F8.4 mag Rmag5 ? Aperture magnitude (5 arcsec) in band R
(magaper5r)
413-418 F6.4 mag e_Rmag5 [0/0.5]? Rmag5 uncertainty (magerraper5r)
420-427 F8.4 mag Imag5 ? Aperture magnitude (5 arcsec) in band I
(magaper5i)
429-434 F6.4 mag e_Imag5 [0/0.5]? Imag5 uncertainty (magerraper5i)
436-443 F8.4 mag Bmag10 [-24/27]? Aperture magnitude (10") in band B
(magaper10b)
445-450 F6.4 mag e_Bmag10 [0/0.5]? Bmag10 uncertainty (magerraper10b)
452-458 F7.4 mag Vmag10 [4.8/26.4]? Aperture magnitude (10") in band V
(magaper10v)
460-465 F6.4 mag e_Vmag10 [0/0.5]? Vmag10 uncertainty (magerraper10v)
467-474 F8.4 mag Rmag10 [-12/26.5]? Aperture magnitude (10") in band R
(magaper10r)
476-481 F6.4 mag e_Rmag10 [0/0.5]? Rmag10 uncertainty (magerraper10r)
483-490 F8.4 mag Imag10 [-18/25.5]? Aperture magnitude (10") in band I
(magaper10i)
492-497 F6.4 mag e_Imag10 [0/0.5]? Imag10 uncertainty (magerraper10i)
499-506 F8.3 arcsec FWHM-B [-361.5/4215]? FWHM of the object in band B
(fwhmimageb)
508-512 F5.3 --- S/G-B [0/1]? Source Extractor stellarity index in
band B (classstarb)
514-524 E11.5 arcsec Rh-B [-116130/6418.3]? Half-light radius in band B
(fluxradiusb)
526-531 F6.3 --- Krad-B [0/74.1]? Kron radius factor used for
MAG_AUTO in band B (kronradiusb)
533-543 E11.5 ct Fmax-B [-39.4/8.3e19]? Peak count above background
of the brightest object pixel in band B
(fluxmaxb)
545-550 F6.2 deg PA-B [-90/90]? Position angle in band B
(thetaimageb)
552-556 F5.3 --- Ell-B [0/1]? Ellipticity in band B (ellipticity_b)
558-565 F8.3 pix Aimg-B [0.49/2088]? Length of object semi-major axis
in pixels in band B (aimageb)
567-573 F7.3 pix Bimg-B [0.03/632]? Length of object semi-minor axis
in pixels in band B (bimageb)
575-582 F8.3 arcsec FWHM-V [-104/3976]? FWHM of the object in band V
(fwhmimagev)
584-588 F5.3 --- S/G-V [0/1]? Source Extractor stellarity index in
band V (classstarv)
590-600 E11.5 arcsec Rh-V [-467473/7762]? Half-light radius in band V
(fluxradiusv)
602-608 F7.3 --- Krad-V [0/406]? Kron radius factor used for MAG_AUTO
in band V (kronradiusv)
610-620 E11.5 ct Fmax-V [-50/1.2e+9]? Peak count above background of
the brightest object pixel in band V
(fluxmaxv)
622-627 F6.2 deg PA-V [-90/90]? Position angle in band V
(thetaimagev)
629-633 F5.3 --- Ell-V [0/1]? Ellipticity in band V (ellipticity_v)
635-642 F8.3 pix Aimg-V [0.49/1746]? Length of object semi-major axis
in band V (aimagev)
644-650 F7.3 pix Bimg-V [0.01/493]? Length of object semi-minor axis
in band V (bimagev)
652-659 F8.3 arcsec FWHM-R [-211/8240]? FWHM of the object in band R
(fwhmimager)
661-665 F5.3 --- S/G-R ? Source Extractor stellarity index in band R
(classstarr)
667-677 E11.5 arcsec Rh-R [-4.2e+7/6084]? Half-light radius in band R
(fluxradiusr)
679-685 F7.3 --- Krad-R [0/628]? Kron radius factor used for MAG_AUTO
in band R (kronradiusr)
687-697 E11.5 ct Fmax-R [-312/1.3e+15]? Peak count above background
of the brightest object pixel in band R
(fluxmaxr)
699-704 F6.2 deg PA-R [-90/90]? Position angle in band R
(thetaimager)
706-710 F5.3 --- Ell-R ? Ellipticity in band R (ellipticity_r)
712-719 F8.3 pix Aimg-R [0.3/2212]? Length of object semi-major axis
in band R (aimager)
721-728 F8.3 pix Bimg-R [0.03/1070]? Length of object semi-minor axis
in band R (bimager)
730-737 F8.3 arcsec FWHM-I [-720/4658]? FWHM of the object in band I
(fwhmimagei)
739-743 F5.3 --- S/G-I ? Source Extractor stellarity index in band I
(classstari)
745-755 E11.5 arcsec Rh-I ? Half-light radius in band I (fluxradiusi)
757-762 F6.3 --- Krad-I [0/99.3]? Kron radius factor used for
MAG_AUTO in band I (kronradiusi)
764-774 E11.5 ct Fmax-I [-330/6.9e+16]? Peak count above background
of the brightest object pixel in band I
(fluxmaxi)
776-781 F6.2 deg PA-I [-90/90]? Position angle in band I
(thetaimagei)
783-787 F5.3 --- Ell-I ? Ellipticity in band I (ellipticity_i)
789-796 F8.3 pix Aimg-I [0.49/1939]? Length of object semi-major axis
in band I (aimagei)
798-804 F7.3 pix Bimg-I [0.01/542]? Length of object semi-minor axis
in band I (bimagei)
806-808 I3 --- Flags-B [0/119]? Flags raised by Source Extractor in
band B (0=clean source) (flags_b)
810-811 I2 --- Ndith-B [0/11] Number of all contributing
single-epoch exposures in band B (ndith_b)
813 I1 --- FlISO-B [0/8] OR-combined flagged pixel types within
the isophotal radius in band B
(0=clean source) (imaflagsisob) (1)
815-817 I3 --- SsCt30-B [0/331] Spurious source count within 30"
in band B (sscount30b)
819-820 I2 --- FlSs-B [-1/0]? Spurious source flag in band B
(ssflag_b) (2)
822 I1 --- Edge-B [0/1] Edge flag, proximity of tile boundaries
in band B (edge_b)
824-826 I3 --- Flags-V [0/117]? Flags raised by Source Extractor in
band V (0=clean source) (flags_v)
828-829 I2 --- Ndith-V [0/12] Number of all contributing
single-epoch exposures in band V (ndith_v)
831 I1 --- FlISO-V [0/8] OR-combined flagged pixel types within
the isophotal radius in band V
(0=clean source) (imaflagsisov) (1)
833-835 I3 --- SsCt30-V [0/306] Spurious source count within 30" in
band V (sscount30v)
837-838 I2 --- FlSs-V [-1/0]? Spurious source flag in band V
(ssflag_v) (2)
840 I1 --- Edge-V [0/1] Edge flag, proximity of tile boundaries
in band V (edge_v)
842-844 I3 --- Flags-R [0/119]? Flags raised by Source Extractor in
band R (0=clean source) (flags_r)
846-847 I2 --- Ndith-R [0/12] Number of all contributing
single-epoch exposures in band R (ndith_r)
849 I1 --- FlISO-R [0/8] OR-combined flagged pixel types within
the isophotal radius in band R
(0=clean source) (imaflagsisor) (1)
851-853 I3 --- SsCt30-R [0/276] Spurious source count within 30"
in band R (sscount30r)
855-856 I2 --- FlSs-R [-1/0]? Spurious source flag in band R
(ssflag_r) (2)
858 I1 --- Edge-R [0/1] Edge flag, proximity of tile boundaries
in band R (edge_r)
860-862 I3 --- Flags-I [0/119]? Flags raised by Source Extractor in
band I (0=clean source) (flags_i)
864-865 I2 --- Ndith-I [0/12] Number of all contributing
single-epoch exposures in band I (ndith_i)
867 I1 --- FlISO-I [0/8] OR-combined flagged pixel types within
the isophotal radius in band I
(0=clean source) (imaflagsisoi) (1)
869-871 I3 --- SsCt30-I [0/299] Spurious source count within 30"
in band I (sscount30i)
873-874 I2 --- FlSs-I [-1/0]? Spurious source flag in band I
(ssflag_i) (2)
876 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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History:
05-May-2026: Insert into VizieR
* 11-May-2026: CoaddID corrected by the authors to be unique.
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