Source-linked AI summary
SkyMapper Southern Survey: First Data Release (DR1)
Christian Wolf, Christopher A. Onken, Lance C. Luvaul, Brian P. Schmidt, Michael S. Bessell, Seo-Won Chang, Gary S. Da Costa, Dougal Mackey, Tony Martin-Jones, Simon J. Murphy, Tim Preston, Richard A. Scalzo, Li Shao, Jon Smillie, Patrick Tisserand, Marc C. White, Fang Yuan
TL;DR
The paper addresses the need for broad, multicolour Southern-sky coverage and presents SkyMapper Southern Survey DR1 as a public imaging data release. It documents the survey strategy, processing, catalogues, and database, yielding a large Shallow Survey dataset with roughly 18-magnitude completeness and quantified photometric performance. The release is useful as a Southern optical reference and for subsequent astronomical investigations, while coverage, crowding, PSF, and illumination limitations remain.
Problem
A complete multicolour Southern-sky inventory to approximately g,r ≈22 is missing, limiting the available ground-based Southern reference coverage.
Method
The paper presents SkyMapper Southern Survey DR1, documenting its Shallow Survey strategy, data processing, catalogue construction, database schema, and public science queries.
Results
DR1 contains over 66,000 images, over 285 million unique objects, and approximately 18-magnitude completeness in all six bands across 17,200 deg^2.
Takeaways & Limitations
DR1 provides a public, broad Southern-sky optical dataset supporting homogeneous reference use and new community-led scientific investigations.
Takeaways & Limitations
DR1 is affected by missing coverage, stellar crowding, PSF variation, and unaccounted illumination effects in some fields or objects.
Abstract
from arXiv · showhide
We present the first data release (DR1) of the SkyMapper Southern Survey, a hemispheric survey carried out with the SkyMapper Telescope at Siding Spring Observatory in Australia. Here, we present the survey strategy, data processing, catalogue construction and database schema. The DR1 dataset includes over 66,000 images from the Shallow Survey component, covering an area of 17,200 deg$^2$ in all six SkyMapper passbands $uvgriz$, while the full area covered by any passband exceeds 20,000 deg$^2$. The catalogues contain over 285 million unique astrophysical objects, complete to roughly 18 mag in all bands. We compare our $griz$ point-source photometry with PanSTARRS1 DR1 and note an RMS scatter of 2%. The internal reproducibility of SkyMapper photometry is on the order of 1%. Astrometric precision is better than 0.2 arcsec based on comparison with Gaia DR1. We describe the end-user database, through which data are presented to the world community, and provide some illustrative science queries.
1 INTRODUCTION
The introduction frames SkyMapper as a Southern, multicolour imaging survey designed to fill missing coverage and open discovery space beyond duplicating northern surveys. It presents DR1 as a large public Shallow Survey release supporting broad astronomical use.
- Survey motivation: The Southern sky lacks a complete multicolour inventory to approximately g,r ≈22, motivating SkyMapper’s survey project.SkyMapper is intended to fill this coverage and depth gap rather than simply reproduce SDSS in another hemisphere.
- Survey motivation: SkyMapper targets Southern science enabled by access to the Galactic bulge, Milky Way centre, Magellanic Clouds, and distinctive stellar populations.Its filters provide sensitivity to stellar surface gravity and metallicity through u−v and v−g colours.
- Survey motivation: The survey also supplies a homogeneous Southern optical reference atlas for radio facilities and supports studies requiring broad sky coverage, including cosmic dipoles and bulk flows.Combining hemispheric datasets can improve constraints on large-scale motions and other sky-wide measurements.
- DR1 contribution: The release emphasizes public data access and long-term legacy use, allowing researchers to ask new questions beyond the projects’ original science plans.The paper describes the DR1 release version and its public availability alongside the catalogue contribution.
- DR1 contribution: DR1 contains approximately 285 million unique astrophysical objects over 20,200 deg^2 and more than 2.1 billion detections from over 66,000 images.The release is largely complete to magnitude approximately 18 in all six filters across 17,200 deg^2.
2 SKYMAPPER
SkyMapper is a robotic 1.35-metre wide-field facility whose Southern Survey combines shallow, six-colour imaging with a planned deeper Main Survey. Its passbands, detectors, calibration procedures, and imaging properties define the DR1 observing system.
- Facility and operations: SkyMapper uses a 1.35-m primary mirror, 32-CCD mosaic, and 2.4 × 2.3 deg^2 field of view at approximately 0.5 arcsec pixel scale.The telescope is fully robotic, with autonomous nightly scheduling for survey operations.
- Facility and operations: The Shallow Survey uses short, dithered, repeated visits and six-colour sequences completed within less than five minutes to cover the Southern hemisphere.Its short exposures provide early full-hemisphere coverage and a bright saturation limit.
- Filters and detectors: SkyMapper’s filters include distinct u and v bands, while griz central wavelengths and widths differ from their SDSS counterparts by up to 40 nm.The u and v filters were designed to support stellar surface-gravity and metallicity information, but u-band red leaks near 700 nm are present.
- Filters and detectors: CCD quantum-efficiency differences at short wavelengths produce subtle uvg passband, zeropoint, and colour-term variations across detectors.These variations are not considered in DR1 and are planned for inclusion in a future release.
- Imaging properties: Table 1 summarizes 10σ limits for catalogue objects and median PSF FWHM, zeropoint, and background values across 66,840 exposures.Most uvgri Shallow Survey images are read-noise limited, while z-band images are mostly background-limited.
3 THE SKYMAPPER SHALLOW SURVEY
The Shallow Survey combined repeated, quality-selected imaging with calibration and masking procedures to build DR1, while retaining documented coverage and image-quality limitations. Its observations span varying visit counts, atmospheric conditions, PSF structure, and detector sensitivity.
- Survey strategy: Three visits per field were initially planned, with five visits per field targeted by the end of regular survey operations in 2020.Additional Shallow Survey imaging continued during bright time around full moon.
- Observations and quality selection: 66,840 images remained in DR1 after deselecting images of insufficient quality.Quality cuts included limits on PSF FWHM and elongation, zeropoints, and calibrator-based zeropoint scatter.
- Image properties: Median PSF FWHM ranged from 2.3′′ in z-band to 3.1′′ in u-band, with median elongation of 1.12 across filters.Most images were read-noise limited, except z-band images, which were mostly background-limited.
- Calibration: Zeropoints were generally around 25.5 mag, and mirror cleaning improved efficiency by approximately 0.35 mag.At a given calendar epoch, most zeropoints for a filter scattered by less than 0.1 mag rms.
- Repeated coverage: Object visit counts generally ranged from 0 to 5, but tails reached 17 visits in g and r and exceptionally 35 visits in u-band.Mean visits per object ranged from 2.0 in v-band to 2.8 in g-band; CCD gaps and field overlaps caused spatially uneven sampling.
- Coverage and limitations: DR1 coverage is incomplete in some regions because of missing good-quality visits or unreliable calibrator stars, especially near the Galactic plane.Stellar crowding also reduced the number of cleanly measured isolated stars in dense fields.
- PSF and photometry: PSF FWHM varied from 1.2′′ to 1.8′′ across an i-band image in good seeing because of the curved focal plane.SkyMapper compensated in point-source photometry using a 1D-PSF estimate based on aperture-magnitude trends across each image.
- Calibration limitations: DR1 did not include illumination correction; estimated corrections were below ±1% over 90% of the mosaic but reached 5% near image corners.A small fraction of corner objects could therefore have unaccounted varying illumination conditions.
4 SCIENCE DATA PIPELINE
The Science Data Pipeline uses a PostgreSQL database to coordinate SkyMapper image ingestion, calibration-frame creation, image reduction, photometric measurement, and calibration. Its workflow is outlined in Figure 5 and the accompanying sections.
- Pipeline overview: The Science Data Pipeline uses PostgreSQL to oversee image ingestion, calibration-frame creation, reduced-image production, photometric measurements, and calibration.The pipeline operates semi-autonomously on each mosaic image.
- Pipeline overview: Figure 5 outlines the Science Data Pipeline flow and the sections that describe it.
4.1 Ingest
The ingest stage transfers and reorganizes raw mosaic data, performs quality checks, and applies initial image corrections and masking. Corrections address interference, overscan and bias structure, crosstalk, saturation, and detector artifacts, while some effects remain uncorrected.
- Raw images are transferred to local storage, reorganized from 64 amplifier extensions into 32 CCD images, and screened by preliminary quality assurance.
- The ingest phase suppresses interference, identifies saturation and blooming, subtracts overscan, corrects crosstalk, creates masks, and restores on-sky image orientation.
- 6–7 pixels is the interference wavelength along each row, and each row is corrected independently with a fitted sine curve; rare residuals remain.
- ∼5 × 10^-4 is the fractional amplitude of the strongest routinely treated adjacent-amplifier crosstalk, while saturated pixels can also cause ringing and additional artifacts.
- PCA models row-oriented time-variable bias, while edge-localized “fingers” remain uncorrected in DR1 and optical reflections from very bright stars are not removed.
4.2 Astrometry
SkyMapper derives per-CCD astrometric solutions tied to UCAC4, fits focal-plane distortions, validates the resulting WCS, and assesses accuracy against Gaia DR1. The solutions are successful for nearly all images and achieve sub-arcsecond positional agreement.
- Each of 32 CCDs is solved independently against UCAC4, with Source Extractor positions used to fit a ZPN projection that is transformed into TPV for pipeline compatibility.
- 98.6% of mosaic images yield valid WCS solutions for all 32 CCDs, while partially successful images average more than 28 valid solutions.
- 96.7% is the WCS success rate in both u- and v-filters, where fewer stars are bright enough for UCAC4 matching.
- 0.16 arcsec is the overall median offset between DR1 sources and their nearest Gaia DR1 matches for separations below 10 arcsec.
- 0.12 arcsec is the median offset for cleanly detected objects with r-band magnitudes between 9 and 14 mag, about one-twentieth of the typical PSF FWHM.
4.3 Calibration Data
Calibration data are assembled into validity-bounded master bias and flatfield frames, with processing choices adapted to detector behavior and observing-era flatfield strategy. DR1 retains several detector limitations, including early-2014 tearing and uncorrected bias fingers.
- Master calibration-frame validity is bounded at detector warming and other significant changes to image properties, including voltage, flatfield-angle, or telescope-optics changes.
- At least 10 bias exposures contribute to each master bias frame, usually from the same night, with adjacent nights used when necessary.
- Night-specific principal components are derived from master-bias residuals to model row-dependent bias structure separately in each CCD half.
- ±10 nights is the nominal flatfield input window, shortened by hard calibration boundaries and followed by CCD-level tolerance filtering.
- 25 November 2014 marked the start of opposing-pair flatfields, introduced to capture and eliminate the field-of-view sky-brightness gradient.
- Approximately 5% of counts were shifted to neighboring columns by early-2014 detector tearing; the effect was left uncorrected in DR1 but eliminated after July 2014 voltage changes.
4.4 Application of Calibrations
Science images receive the calibrated bias and flatfield corrections, cosmic-ray treatment, residual half-CCD sky-level adjustment, WCS insertion, compression, and masking. CCDs lacking a recoverable WCS solution are excluded from further analysis.
- Science frames are fitted with night-specific bias principal components after source masking and background subtraction, then divided by the associated master flatfield.
- Cosmic rays are detected and removed separately for each amplifier using lacosmicx with amplifier-specific read-noise settings and image masks.
- Residual sky-level differences between CCD halves, caused by low-count nonlinearity, are measured from central columns and corrected with an additive offset.
- 16-bit integer conversion uses BZERO 32668 and BSCALE 1, producing a count range of -100 to 65435 before lossless image and mask compression.
- CCD frames without a WCS solution are retried after calibration and discarded from further analysis if the second attempt also fails.
4.5 Photometry
SkyMapper DR1 calibrates photometry against APASS, 2MASS, and PS1-derived transformations, then models spatial zero-point variation across each mosaic. The pipeline derives corrected aperture and PSF magnitudes while applying reddening, airmass, and quality-control adjustments.
- Source photometry: SExtractor detects sources on each CCD and measures photometry in apertures ranging from 4 to 60 pixels.The pipeline uses image masks, local backgrounds, deblending, cleaning, and multiple aperture diameters.
- Aperture corrections: A 2-D linear plane fitted from suitable stars models spatial aperture-correction variation across each CCD.The fit uses at least five stars when necessary and applies four passes of 2.5σ clipping.
- PSF photometry: PSF magnitudes are weighted means of predicted 15′′-aperture magnitudes from seven smaller apertures, with χ2 retained to distinguish spatial profiles.Nearby neighbours can contribute flux within the 15′′ aperture.
- Photometric calibration: Approximately 12.8 million APASS and 2.5 million 2MASS sources provide calibrators for predicted SkyMapper magnitudes derived through PS1-trained transformations.Calibrators are selected using clean flags, magnitude ranges, photometric quality, and neighbour constraints.
- Zero-point calibration: Final zero-point planes fit linear x- and y-gradients with four-pass 2.5σ clipping, and the resulting calibration is applied to all mosaic detections.Gradients are fixed to zero when five or fewer stars remain unclipped; most frames use 300–1000 calibrators.
- Photometric calibration: The u-band magnitudes are adjusted globally by 0.05 mag brighter because of an offset relative to PS1-based predictions.Red-leak effects also make u-band transformations dependent on exposure airmass, so predictions are interpolated between airmasses 1 and 2.
5 SURVEY PROPERTIES
DR1 survey properties are assessed through repeatability, PS1 photometric comparisons, morphology diagnostics, depth counts, and stellar colour spaces. The results quantify band-dependent precision and completeness while showing how SkyMapper colours and morphology indicators support source characterization.
- 5.1 Photometric comparison: Repeat-visit PSF photometry has internal scatter of 8 mmag in griz and 12 mmag in uv bands.The external comparison uses PS1 DR1 measurements transformed into SkyMapper magnitudes.
- 5.1 Photometric comparison: 23 mmag RMS separates measured from PS1-predicted griz magnitudes, with mean offsets generally below 10 mmag.The z-band median difference reaches 0.06 mag at E(B−V) = [0.5, 1.5], especially toward low Galactic latitude.
- 5.1 Photometric comparison: The extrapolated u and v bands each show 0.12 mag RMS scatter against PS1-based predictions.A 0.05 mag u-band offset was corrected globally, while the v-band reddening-dependent shift was left uncorrected and likely reflects metallicity differences.
- 5.2 Morphology indicators: CLASS_STAR favors low-contamination point-source selection, whereas PSF-minus-Petrosian magnitude favors completeness but becomes more contaminated at faint magnitudes.For true point sources, the magnitude difference remains centered near zero despite increasing noise.
- 5.3 Survey depth and number counts: Number counts turn over near 18 mag in most bands, while the narrower v band is complete to 17.5 mag.Galaxy completeness occurs at brighter magnitudes because extended outer regions are lost in Shallow Survey image noise.
- 5.3 Survey depth and number counts: A v-selected stellar population with v < 17.5 is suggested to be largely complete only to z < 15.Horizontal offsets among band-specific number-count curves encode mean stellar colours.
- 5.4 Colour space and colour terms for stars: SkyMapper u−g separates cool dwarfs from cool giants, while u−v separates AF main-sequence from horizontal-branch stars.The u and v filters bracket the Hydrogen Balmer break, whose gravity-sensitive absorption changes v-band flux and u−v colour.
- 5.4 Colour space and colour terms for stars: Synthetic comparisons show the strongest colour term among griz occurs in g, reaching 0.4 mag for cool stars.The comparison includes Pickles stellar spectra and DA/DB white-dwarf models, with a reddening vector shown for AV = 1.
6 DATA ACCESS AND EXAMPLE APPLICATIONS
The DR1 data are publicly accessible through the SkyMapper ASVO Node, with visual browsing, TAP services, and database schemas supporting example scientific queries. Applications include variability studies, changing-look Seyfert searches, and selection of extremely metal-poor stellar candidates.
- Data access: The SkyMapper ASVO Node provides DR1 access through SkyViewer and standards-compliant services, including TAP tables organized by data release and external catalogues.The table layout and column descriptions are documented in the database schema.
- Known variable stars: 256,146 known variable stars were cross-matched with DR1 to measure brightness variations from repeat SkyMapper visits.The analysis excludes measurements affected by flags, bad pixels, or merged child objects.
- Known variable stars: The combined high-amplitude list contains 39 U Geminorum, 2,200 RR Lyrae, 55 Cepheid, and 520 Mira variables, grouped by characteristic time scales.The illustrative plot uses v- and r-band amplitudes for objects differing by more than 1 mag between good-quality detections.
- Changing-look Seyferts: Three of five low-redshift Seyfert candidates changed from type 1 to type 2 after their UV continua and broad hydrogen lines weakened relative to earlier spectra.The candidates were selected by comparing SkyMapper g-band photometry with older Hamburg-ESO Survey BJ magnitudes.
- Metal-poor star candidates: Approximately 25,000 high-quality candidates with v−g colours suggesting [Fe/H] ≤−2.5 were identified from an initial 7.7-million-source sample.The colour selection uses g−i as an effective-temperature proxy and v−g as a metallicity indicator; the candidate distribution includes Galactic-centre concentration and unresolved substructure.
7 SURVEY FUTURE
Future SkyMapper observations emphasize deeper Main Survey imaging, continued Shallow Survey visits, and more homogeneous calibration. Together, the surveys are intended to provide repeated measurements across nearly the full Southern sky.
- Future observing strategy: Since March 2015, SkyMapper observations have focused on deeper Main Survey exposures while Shallow Survey visits continue during very bright moon phases.Additional Shallow Survey passes use larger dithers to support future calibration improvements.
- Future calibration: An übercal approach can use measurements of common objects in overlapping images to fine-tune image zeropoints and improve calibration homogeneity.The additional Shallow Survey passes are designed to assist this calibration strategy.
- Future observing strategy: The Main Survey is planned to be 4 mag deeper than the Shallow Survey, whose images are mostly limited by read-out noise.Main Survey colour sequences are collected within 20 minutes, with additional gr and iz image pairs during dark time and twilight.
- Future survey products: The Shallow Survey will eventually calibrate the Main Survey, so DR1 focuses on instantaneous point-source photometry despite containing many variable objects.This scope reflects the Shallow Survey’s role as a calibration source and its predominance of point sources.
A SOURCE EXTRACTOR PARAMETERS AND TAP TABLE SCHEMA
The appendix documents the parameters measured in DR1 photometry tables and the organization of the SkyMapper ASVO TAP database. Schemas separate data releases and external catalogues while defining source, photometric, detector, and image tables.
- Source extraction: The DR1 photometry tables record the SExtractor parameters used in source measurements.These parameters are listed in Table 4.
- TAP schemas: The SkyMapper ASVO TAP service organizes each data release in its own schema and collects external catalogues in the ext schema.Table 5 summarizes the schemas and contained tables.
- TAP schemas: The dr1 schema points to the latest table versions, while version-specific tables can be accessed with dr1p0_ or dr1p1_ prefixes.The documented example prefixes correspond to DR1.0 and DR1.1.
- Master table: The DR1 master table schema defines source-level quantities including mean magnitudes, measurement errors, shapes, and position angles.The listed columns include r-band shape and position-angle fields together with u-, v-, and g-band photometric quantities.
- Photometry tables: The fs_photometry schema includes aperture fluxes and zero-point-corrected aperture magnitudes with associated errors.Examples include 4 arcsec and 5 arcsec diameter aperture measurements corrected for PSF losses.
- Image-level tables: Separate ccds, images, and mosaic tables document detector- and image-level data within the DR1 database.These structures are represented in the corresponding database-schema tables.