KMTNet Synoptic Survey of Southern Sky III: The First Data Release

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$^{\circ}$ < Decl. < -28.8$^{\circ}$ 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 $\sim$4,000 deg$^{2}$ to enable a broad range of astrophysical research. The release includes deep co-added images reaching median 5$σ$ 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 $+0.054 \pm 0.129$ arcsec in RA and $-0.015 \pm 0.120$ arcsec in Dec relative to Gaia DR3. {\refbf Photometric uniformity for point sources is maintained within $\pm 0.03$ mag 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. All data products are publicly available via the CDS and NOIRLab's Astro Data Lab.

Figures (14)

• Figure 1: Multi-Order Coverage map of the KS4 DR1 sky coverage, presented in a polar projection centered on the South Celestial Pole (Decl. = -90$^\circ$). The grid lines are spaced at 20-degree intervals in RA and 10-degree intervals in Dec. The red-shaded regions indicate the DR1 footprint. The gray-shaded regions represent the full KS4 survey area observed to date, while the blue hatching shows the extent of the Dark Energy Survey footprint. The positions of the LMC, SMC, and the Galactic Bulge are marked with yellow symbols, and a thick dashed line indicates the Galactic Plane.
• Figure 2: Spatial distribution of seeing (FWHM) for the KS4 DR1 co-added images. Values represent the median FWHM per tile, prioritizing the minimum seeing in overlap regions. The color scale is fixed across all panels for comparison. The full observed ranges are: $B$ (1.28--3.54 arcsec), $V$ (1.18--2.94 arcsec), $R$ (1.14--3.02 arcsec), and $I$ (1.10--3.01 arcsec).
• Figure 3: Spatial distribution of $5\sigma$ limiting magnitudes (AB) for the KS4 DR1 co-added images. Values represent the median depth per tile, prioritizing the maximum depth in overlap regions. The color scale is fixed across all panels, with range markers indicating the full extent for each filter. The observed ranges are: $B$ (21.77--23.45 mag), $V$ (21.66--23.40 mag), $R$ (21.65--23.62 mag), and $I$ (21.10--22.80 mag).
• Figure 4: Examples of the spatial distribution of spurious sources in two representative KS4 tiles, 0328 (left) and 2143 (right), highlighting several archetypal artifact patterns targeted by our cleaning pipeline. These patterns include diagonal streaks from satellite trails, vertical bleeding trails from saturated stars, circular halos around bright stars, scattered features caused by stray light from bright stars located outside the field of view, and grid-like concentrations in regions of insufficient dithering.
• Figure 5: Example spatial and temporal coverage patterns for a representative KS4 tile 0487. Top: The spatial distribution of detected sources (black points) within regions covered by at least NDITH exposures, for a tile completed with the four-point dithering strategy. White areas indicate regions where sources are not plotted because those areas received fewer than the specified NDITH exposures. As NDITH increases from 1 to 4, the panels illustrate the source distribution across progressively more uniformly covered areas. Bottom: The same sources colored by their mean observation time (MEAN_MJD). The significant color variation, especially in the NDITH < 4 panels, highlights that some exposures were taken at widely separated epochs, in some cases separated by more than a year.