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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^ 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:

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

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:

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:

Flag Bit Meaning
1 The object has neighbors, bright and close enough to significantly bias the photometry, or bad pixels (more than 10% of the integrated area affected).
2 The object was originally blended with another one.
4 At least one pixel of the object is saturated (or very close to).
8 The object is truncated (too close to an image boundary).
16 Object's aperture data are incomplete or corrupted.
32 Object's isophotal data are incomplete or corrupted.
64 A memory overflow occurred during deblending.
128 A memory overflow occurred during extraction.

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

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:

Improvements of DR2 over DR1^ Back to top

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


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:

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:

Typical TPV FITS header
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:

IRAF wcstools wcslib AST ds9 AstroPy Astromatic CDS Aladin IPAC Montage
2.16+ 3.8.4+ 5.0+ 5.7.3+ 7.0+ 1.1+ 3/2014 onwards 9+ 4.0+

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.

Issue ↕ First affected release ↕ Resolved in ↕ Last modified ↕
Extended-source photometry EDR Feb 13 2019, 16:21 AEDT
Zero-point calibration EDR DR2 Feb 13 2019, 16:21 AEDT
Pan-STARRS1 DR1 coverage for cross-matching DR1.1 DR2 Feb 13 2019, 16:20 AEDT
Fringe correction EDR DR2 Feb 13 2019, 16:20 AEDT
Incorrect photometry when merging child objects DR1.1 Sep 13 2017, 12:45 AEST
Spatially varying PSF affecting photometry EDR Jun 05 2017, 15:55 AEST
ZIP file downloads in Safari (for info) EDR May 02 2016, 12:21 AEST