SkyMapper | Southern Sky Survey

DR4 | DR2 | DR1.1 | EDR

Data Release DR2 | 27 Feb 2019

DOI: 10.25914/5ce60d31ce759

News about this release | Browse table metadata | Documentation | Known issues with this release

💾 Download the full DR2 catalogue

Accessibility: World-wide accessible

Release documentation

DR2 Documentation

Please read the DR2 paper.
The DR2 paper makes heavy reference to the DR1 paper.

• Introduction
• What type of data is provided?
• Sky Coverage and Data Quality
• Data Access
• Improvements of DR2 over DR1
• Caveats and Known Issues
• SkyMapper Detectors
• SkyMapper Filters
• World Coordinate System
• Astrometric Quality

Introduction^ Back to top

This Second Data Release begins SkyMapper's exploration of the deep Southern sky. DR2 includes the first release of images from the Main Survey (with exposure times between 2.5 and 20 times longer than the Shallow Survey, depending on the filter), as well as a larger Shallow Survey dataset, image processing enhancements, and a refined photometric calibration. Included are fields observed between March 2014 and March 2018, with a number of quality cuts applied. Measurements from over 4.7 billion detections covering over 21,000 deg² of sky are available. They correspond to over 500 million unique astrophysical objects from magnitude 8 to 22. All magnitudes reported on this site are AB mags.

Please address all feedback, suggestions, and bug reports in the first instance to skymapper@anu.edu.au.

What type of data is provided?^ Back to top

DR2 contains reduced images with overscan, bias, flatfield correction and a World Coordinate System (TPV projection) applied, as well as photometric catalogues (both raw detections and object-merged) for objects in each image. Full access to the DR2 database tables is provided through this website and the Virtual Observatory Table Access Protocol (TAP).

Five SkyMapper-specific database tables have been exposed in this data release:

• dr2.master — mean object astrometry, photometry and shape information from all good detections. This table should be used for most photometric queries and is also exposed through the Cone Search service

• dr2.photometry — per image/CCD object detections and corresponding photometric and shape information

• dr2.images — a listing of image coordinates and exposure information

• dr2.ccds — a listing of CCD details used by the Image Cutout service

• dr2.mosaic — a listing of the CCD positions within the SkyMapper mosaic

The dr2.master table has been pre-matched to several other large catalogues:

• dr1.master — SkyMapper DR1

• ext.allwise — the NASA Widefield Infrared Survey Explorer (WISE) AllWISE data release

• ext.refcat2 — The ATLAS All-Sky Stellar Reference Catalog (Tonry et al. 2018)

• ext.gaia_dr2 — the Gaia Mission Catalogue DR2

• ext.galex_guvcat_ais — the All-sky Imaging Survey from the GALEX catalogs of unique UV sources (Bianchi et al. 2017)

• ext.ps1_dr1 — the Pan-STARRS1 (PS1) DR1 Catalog, selected columns, restricted to decMean<20deg and nDetections>1

• ext.twomass_psc — the Two Micron All Sky Survey (2MASS) Point Source Catalog (PSC)

• ext.ucac4 — the Fourth U.S. Naval Observatory CCD Astrograph Catalog (UCAC4)

Matching to the external tables was attempted with a 15" search radius around each SkyMapper source and the ID and distance of the nearest source from the 2MASS, AllWISE, Gaia DR2, GALEX, PS1, and UCAC4 catalogues are noted in the last set of columns in dr2.master. In contrast to DR1, only the 2MASS PSC was queried (instead of both the PSC and XSC), while for Gaia DR2, the two nearest matches from the catalogue are listed. The distance to the 2nd-nearest match may indicate the presence of a close neighbour affecting the photometry of the primary source (also see the prox column in dr2.master for the closest DR2 source to each DR2 object). The ext schema also contains additional catalogues that have not been cross-matched with DR2.

We also provide local copies of several spectroscopic and other catalogues, which we have pre-matched to the SkyMapper master table with a 15" search radius, noting the nearest SkyMapper source in the dr2_id and dr2_dist columns of those tables:

• ext.spec_2dfgrs — the 2 Degree Field Galaxy Redshift Survey (2dFGRS)

• ext.spec_2dflens — the 2dF Gravitational Lens Survey (2dFLenS)

• ext.spec_2qz6qz — the 2dF and 6dF QSO Redshift Surveys

• ext.spec_6dfgs — the 6 Degree Field Galaxy Survey (6dFGS)

• ext.spec_galah_dr2p1 — the GALAH DR2.1 catalogue

• ext.spec_hesqso — the Hamburg/ESO survey for bright QSOs. III. (Wisotzki+, 2000)

• ext.spec_twomrs — the 2MASS Redshift Survey (2MRS)

• ext.milliquas_v6p4b — the Million Quasar catalog, v6.4b

• ext.vsx — the AAVSO International Variable Star Index

To aid in constructing your ADQL queries, you can browse the column metadata of all these tables here.

Sky Coverage and Data Quality^ Back to top

DR2 consists of 4,132 SkyMapper fields covering a total area of over 21,000 deg² (see coverage map below).

The 121,494 images in DR2 are comprised of 70% Shallow Survey (SS) and 30% Main Survey (MS) frames. The maximum photometric zeropoint (the magnitude equivalent to one detected count in the image) is shown in each of the 6 sky maps below (in which darker colours are deeper images), illustrating the relative proportion of Shallow Survey and Main Survey data available for each filter.

As the maps make clear, Main Survey imaging in i- and z-band has covered >80% of the Southern sky, although not yet with the final number of visits. The iz images have largely been obtained in twilight, but the other filters require gray or dark time to limit the effects of sky background in the deeper images of the Main Survey and therefore progress has been slower.

Median point source completeness limits are more difficult to estimate than in DR1, owing to the mixture of exposure times and number of visits for each field. The logarithmic number counts of sources with < 5% errors are shown in the figure below, with the bimodal distributions reflecting the Shallow Survey and Main Survey components.

Median seeing in DR2 is (3.1", 2.9", 2.6", 2.4", 2.3", 2.3") for (u, v, g, r, i, z). We have included images with a PSF FWHM of up to 5" and an elongation of up to 1.4.

All source detections in the photometry table include the Source Extractor flags, complemented by additional flags from our data processing in a higher bit range:

From Source Extractor:

Our post-processing adds the following flags, as needed:

• We set bit value 512 for very faint detections that we consider dubious and a potential source of error. Our approach makes sure that very faint detections in the Shallow Survey are ignored for the benefit of letting the Main Survey alone define their distilled properties, while faint Main Survey-only detections are included into the master table.
• Source detections that appear much more concentrated than PSFs and with small minor axis extend have been flagged in the photometry table (flag value 1024) as they may be affected by uncorrected cosmic rays or transient hot pixels.
• Source detections that are close to bright stars in the ATLAS Refcat2 are flagged in the photometry table (flag value 2048), based on a search radius, in degrees, of

  log10 (radius) < -0.2 * m,

  where m is the PS1-to-SkyMapper transformed magnitude in the band in question, down to limits of (4, 5, 8.5, 8.5, 6, 5) for (u, v, g, r, i, z). These are usually affected by scattered light from the bright star, and have either compromised photometry or are spurious sources.
• Sources that are known to be insufficiently merged as it happen to be near the RA=0/360 border are flagged in the master table with value 4096.

Only detections with flags<4 and nimaflags<5 are used for the distill into the master table. Hence, the averaged magnitudes exclude all measurements with bad flags, and the master source list excludes all objects within the exclusion zones around bright stars. However, detections with bad flags or those close to bright stars can still be found in the photometry table.

Data Access^ Back to top

Main Page: How To Access

DR2 images and catalogues can be accessed via the tools on this website (see How To Access), which are being developed as part of the Australian All-Sky Virtual Observatory. The TAP (Table Access Protocol), SIAP (Simple Image Access Protocol), and Cone Search services can also be accessed through Virtual Observatory-aware software tools like TOPCAT and Aladin. 

For TAP queries, some definitions and constraints worth considering when making selections from the dr2.master catalogue include:

• flags = 0 - no Source Extractor warnings about saturation, close neighbours, edge-of-CCD effects, etc.
• nimaflags = 0 - isophotal aperture clean of bad pixels, saturation, cross-talk, cosmic rays
• ngood > 1 - source well-measured in multiple images (either in the same filter or different filters)
• ngood_min > 1 - source well-measured at least twice in one of the filters (note that bands with x_ngood=0 are excluded from the ngood_min count)
• nch_max = 1 - source never resolved into multiple components (note that for nch_max>1 sources, at least one filter is likely to have no master table photometry because of the unresolved split into multiple child objects)
• x_flags = 0, x_nimaflags = 0, x_ngood > 1 - if only concerned with filter x
• For selecting a very clean sample, one might require that every measurement in every filter was free of Source Extractor warnings or bad pixels:
  • flags=0 and nimaflags=0
• For selecting a more complete sample, one might only require that there was at least one good (flags<4 and nimaflags<5) photometric data point in the filter in question:
  • F_ngood>0 for F in (u,v,g,r,i,z)
• To avoid biased photometry due to neighbours, one can check the distance to the second-closest Gaia DR2 source (gaia_dr2_dist2, which shows values up to 15arcsec), the distance to the next-closest SkyMapper DR2 source (prox, which saturates at 15arcsec), or the flags_psf column (with the bits being set to indicate whether a neighbour is expected to bias the photometry by >1% in each band; bits: 1=z, 2=i, 4=r, 8=g, 16=v, 32=u)

Improvements of DR2 over DR1^ Back to top

Several changes in the data processing have improved the quality of the DR2 data products:

• A principal components analysis (PCA) has been used to defringe the i- and z-band Main Survey images (plus a small portion of the iz Shallow Survey images). Here is an example z-band MS image before and after defringing with 10 PCs.

• The purely internal reproducibility of the photometry for isolated objects without bright neighbours is on the order of 1% rms in uv bands and 0.7% rms in griz bands. 
   
• Photometric zeropoint homogeneity has been improved by referencing the Gaia DR2 dataset. The Gaia photometry is used to predict PS1 griz magnitudes using the transformations of Tonry et al. (2018). Then, PS1-to-SkyMapper transformations are applied using synthetic photometry of the stellar spectral library of Pickles (1998). These lead to enhanced uniformity of the DR2 photometry across the Southern sky. Below we show the griz magnitude differences (after transforming the SkyMapper DR2 photometry to the PS1 system) on a scale of +/- 0.15 mag, as well as the difference in (g-r) on a much finer scale of +/- 0.05 mag. Current zero-point uncertainties have been estimated by comparing the DR2 photometry against PS1 and appear to be <1% (rms) at high Galactic latitudes in griz

Caveats and Known Issues^ Back to top

For an up-to-date list of known issues and their prospects for resolution, please see the Known Issues section at the bottom of this page. There are several important issues with DR2 data to be aware of:

• No illumination correction has been applied, but work is underway to enable such a correction in future data releases.

• Fringe correction is applied to i- and z-band images from the Main Survey (and a small fraction of i- and z-band images from the Shallow Survey).

• The u-band has a ~1% red leak at 700-750 nm wavelength. The relative throughput of the leak is airmass-dependent and can seemingly increase the u-flux of red objects towards higher airmass. 

• Photometry is affected by a spatially-varying PSF across the field-of-view, and the PSF shows trefoil in the corners and generally significant wings. 

• PSF magnitudes are from a fast 1D growth curve fit that is calibrated per image and CCD by bright stars and has parameters as a function of the image x,y coordinates. Sources with neighbours of equal brightness will have their PSF magnitudes biased by >1% if those neighbours are separated by <5 arcsec. Brighter neighbours have effects even at larger separation, up to 15" distance. The column FLAGS_PSF and the DR2 paper give more details.

• Extended-source photometry has not been optimised in DR2, but is a primary goal for DR3.

• The photometric zeropoints are most uncertain for the wavelength-extrapolation required in the SkyMapper u and v filters.

• When one or more filters have no magnitudes in the master table, sometimes ngood_min = 0 and sometimes ngood_min > 0, depending on whether the omitted filters had data that were discarded (for too high of flags or nimaflags) [ngood_min=0] or not [ngood_min>0]

• The RA position uncertainties in the master table (e_raj2000) include values down to 1mas, but the true minimum uncertainty should be assumed to be 36mas.

• Because of the way that flagged measurements were treated, the master table has 2.7 million objects with mean_epoch = 0, and 175k with mean_epoch = NULL

SkyMapper Detectors^ Back to top

The SkyMapper mosaic camera contains 32 CCDs of 4096 x 2048 pixels with a plate scale of ~0.50 arcsec/pixel. There are small gaps between the individual CCDs and the resulting field-of view is 2.37 deg x 2.39 deg. The mosaic fill factor is 91% of a 5.68 deg² field-of-view. Each SkyMapper image is split into its 32 constituent CCDs, which are presented as separate files to the image cutout service.

SkyMapper Filters^ Back to top

The SkyMapper filter curves (with atmosphere) are shown below. We have tabulated colour transformations between SkyMapper and other standard filters, as well as predicted star colours and reddening corrections -- they can be found on the page here.

Note that the u-band filter (ultraviolet) is shortward of the Hydrogen Balmer break, while the v-band (violet) is placed between the Balmer break and the Ca H&K 4000AA-break. The u-filter also has a red leak, which needs an airmass-dependent correction. The two lines shown in the figure below are effective transmission curves including atmosphere for airmasses 1 and 2. The third (inset) panel shows the difference between the apparent u-magnitudes observed at airmass 1 and airmass 2 as a function of star colour. The redder the star the brighter it appears in u-band as it is observed closer to the horizon, because calibration stars get fainter, while the red leak keeps the flux bright. Additional information on the filter set can be found in Bessell et al. (2011).

World Coordinate System^ Back to top

Reduced SkyMapper images provided through this website and the SIAP service have been registered onto the sky using the TPV World Coordinate System (RA---TPV, DEC--TPV). TPV builds on the standard TAN projection by adding a general polynomial distortion suitable for wide-field cameras which is described in a set of additional PVi_m keywords. A typical TPV header is reproduced below:

WCSAXES =                    2 / WCS dimensionality                             
CTYPE1  = 'RA---TPV'           / WCS projection type for this axis              
CTYPE2  = 'DEC--TPV'           / WCS projection type for this axis              
LONPOLE =                180.0 / WCS coordinate rotation: longitude             
LATPOLE =                  0.0 / WCS coordinate rotation: latitude              
CRVAL1  =        276.914401980 / RA of reference point                          
CRVAL2  =         -9.384011927 / DEC of reference point                         
CRPIX1  =         -2.900864031 / X reference pixel                              
CRPIX2  =        -63.233494932 / Y reference pixel                              
CUNIT1  = 'deg     '           / X pixel scale units                            
CUNIT2  = 'deg     '           / Y pixel scale units                            
CD1_1   =         -0.000138209 / WCS transformation matrix                      
CD1_2   =         -0.000000195 / WCS transformation matrix                      
CD2_1   =          0.000000225 / WCS transformation matrix                      
CD2_2   =         -0.000138246 / WCS transformation matrix                      
RADESYS = 'ICRS    '           / WCS reference frame                            
PV1_0   =          0.000042557 / WCS projection distortion parameter            
PV1_1   =          1.000461443 / WCS projection distortion parameter            
PV1_2   =          0.000190561 / WCS projection distortion parameter            
PV1_3   =         -0.000000012 / WCS projection distortion parameter            
PV1_4   =          0.001420356 / WCS projection distortion parameter            
PV1_5   =          0.000230202 / WCS projection distortion parameter            
PV1_6   =          0.000277876 / WCS projection distortion parameter            
PV1_7   =          0.000002261 / WCS projection distortion parameter            
PV1_8   =          0.000006422 / WCS projection distortion parameter            
PV1_9   =          0.000001831 / WCS projection distortion parameter            
PV1_10  =          0.000000521 / WCS projection distortion parameter            
PV1_11  =         -0.000000298 / WCS projection distortion parameter            
PV2_0   =          0.000071788 / WCS projection distortion parameter            
PV2_1   =          1.000381476 / WCS projection distortion parameter            
PV2_2   =          0.000493742 / WCS projection distortion parameter            
PV2_3   =          0.000000007 / WCS projection distortion parameter            
PV2_4   =          0.000440372 / WCS projection distortion parameter            
PV2_5   =          0.000927121 / WCS projection distortion parameter            
PV2_6   =          0.000817100 / WCS projection distortion parameter            
PV2_7   =         -0.000000338 / WCS projection distortion parameter            
PV2_8   =         -0.000001059 / WCS projection distortion parameter            
PV2_9   =         -0.000001035 / WCS projection distortion parameter            
PV2_10  =         -0.000004152 / WCS projection distortion parameter            
PV2_11  =         -0.000000226 / WCS projection distortion parameter             

As TPV is a relatively recent WCS parameterisation you may need to update older WCS libraries and software for best results. We have confirmed that TPV is supported by the following libraries and tools:

Note: older versions of Montage (<=3.3) include wcstools v3.8.1 so this library must be updated to 3.8.4+ prior to compiling in order to support TPV. 

Astrometric Quality^ Back to top

The astrometry for DR2 is based upon the UCAC4 catalogue (Zacharias et al. 2013). As validation of the astrometric solutions, the sources from the DR2 master catalogue have been cross-matched against the Gaia DR2 catalogue. As with DR1, the median offset is 0.16 arcsec. The sky distribution of the median Gaia offsets per deg², on a scale of 0 (white) to 0.25 arcsec (black) is shown below.

Known Issues | DR2^ Back to top

Click on the table headings to sort the table based on that value.

The use of all SkyMapper data is governed by the policies outlined on this website. Please review them prior to downloading or publishing data.
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