DR1 | EDR

Data Release DR1 | 6 Jun 2017

DOI: 10.4225/41/593620ad5b574

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

First Data Release (DR1) Information

Introduction^ Back to top

The SkyMapper First Data Release provides data from the Shallow Survey across >98% of the southern sky, mostly covering the range from the South Celestial Pole to +2 deg in declination, with some extra coverage reaching +10 deg. Included are fields observed between March 2014 and September 2015, with a number of quality cuts applied. Each visit to a given field includes an exposure in all six filters, uvgriz, although on some fields (12%) not all filter images passed the quality cuts. Measurements from over 2.3 billion detections covering over 20,200 deg2 of sky are available. They correspond to ~300 million unique astrophysical objects from magnitude 8 to 18 (complete to ~17.5 mag, depending on the filter). All magnitudes reported on this site are AB mags.

Full details of the DR1 data, processing, and analysis are presented in Wolf et al. (2017, in prep.).

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

Important: Please make sure you review the data access and publication policies prior to downloading or publishing DR1 data. The research collaborations which existed prior to SkyMapper becoming a national facility have been protected through a list of reserved science projects, with which other teams are prohibited from competing prior to worldwide release. DR1 is expected to be made available to the world in October 2017.

 

What type of data is provided?^ Back to top

DR1 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 DR1 database tables is provided through this website and the Virtual Observatory Table Access Protocol (TAP). 

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

The dr1.master table has been pre-matched to several other photometric catalogues. Currently, we hold local copies of the following tables:

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, APASS, Gaia, GALEX, PS1, and UCAC4 catalogues are noted in the last set of columns in dr1.master. For 2MASS, the PSC and XSC were both queried and the two nearest matches from either 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 dr1.master for the closest SkyMapper source to each DR1 object). The ext schema also contains additional catalogues that have not yet been cross-matched with DR1.

We also provide local copies of several well-known spectroscopic catalogues, which we have pre-matched to the SkyMapper master table with a 15" search radius, noting the nearest SkyMapper source in the dr1_id and dr1_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

DR1 consists of 4,016 SkyMapper fields covering a total area of over 20,200 deg2 (see coverage map below). Approximately, 17,600 deg2 of sky are covered with good images in all six filters. 

The DR1 data were obtained from March 2014 to September 2015. During each visit to a SkyMapper field, the full six-filter sequence of uvgriz was observed, with exposure times of 40, 20, 5, 5, 10 and 20 sec. Due to the high read-out noise (RON) of ~10 e-, short exposures in the u and v filters are affected by RON. Even in full Moon, noise is sky-dominated only at t > 20 sec in the u-band.  Note that the SkyMapper v-band is a violet filter, not a visual filter. 

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:

Only detections with flags<8 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.

Median point source completeness limits in AB mag are (17.75, 17.5, 18, 18, 17.75, 17.5) for (u, v, g, r, i, z). These are the magnitudes where the log(N) number counts start to turn away from a linear increase with magnitude. 

Median seeing in DR1 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" (up to 6" for u and v-bands) and an elongation of up to 1.4.

Below we show an example colour-colour (u-g vs g-i) diagram for stars with good detections: 

DR1 (u-g) vs. (g-i)

 

Data Access^ Back to top

Main Page: How To Access

DR1 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 constraints worth considering to ensure a clean dataset of reliable sources from the dr1.master catalogue include:

Improvements of DR1 over EDR^ Back to top

Several changes in the data processing have improved the quality of the DR1 data products:

The following figure compares two example EDR images with their much cleaner DR1 version. Most of the apparent difference results from the new bias treatment, which needs ten principal components per amplifier to capture the range of possible behaviour.

 

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 DR1 data to be aware of:

Current zero-point uncertainties have been estimated by comparing the DR1 photometry against the SDSS Southern Standards by comparing their measured photometry with a prediction based on our photometry and colour terms discussed below. After subtracting the 0.03 mag zero-point offset in SDSS u' reported by Pickles & Depagne (2010), we find that our photometry is on average 1.0+/-0.1% brighter than the SDSS Southern Standards photometry in all six bands. Among the comparison stars we find an rms scatter between the measured and predicted SDSS photometry and of 3.5% in the bands gr, 5% in uiz and 8% in the narrower v-band.

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

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:

IRAFwcstoolswcslibASTds9AstroPyAstromaticCDS AladinIPAC 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 

The astrometry for DR1 is based upon the UCAC4 catalogue (Zacharias et al. 2013). As validation of the astrometric solutions, the sources from the DR1 master catalogue have been cross-matched against the Gaia DR1 catalogue. For robust DR1 sources (those with flags=0, nimaflags=0, ngood>1, and r_psf<17mag), the distribution of Gaia position offsets is:

Distribution of offsets from Gaia positions

The sky distribution of the mean Gaia offsets is:

Mean position offset from Gaia

The spatial distribution of SkyMapper DR1 sources without Gaia cross-matches closely resembles the distribution of UCAC4 sources without Gaia cross-matches, which arise due to gaps in coverage and regions of low quality astrometic solutions in Gaia DR1 (see Fig. 6 of Arenou et al. 2017).

Photometric Quality

The photometric zero-points for the SkyMapper DR1 images were determined by comparison with the photometry of APASS DR9 (Henden et al. 2016), accounting for colour transformations between the filter systems and reddening (with the reddening maps of Schlegel et al. 1998).

Point Sources

We have compared the SkyMapper DR1 griz photometry to that of Pan-STARRS1 DR1. Without taking account of differences between the bandpasses of the two surveys, the sky distributions of magnitude differences are:

g-band:g-band magnitude differences between SkyMapper DR1 and Pan-STARRS1 DR1

r-band:r-band magnitude differences between SkyMapper DR1 and Pan-STARRS1 DR1

i-band: (Figure corrected 2017-07-07)i-band magnitude differences between SkyMapper DR1 and Pan-STARRS1 DR1

z-band:z-band magnitude differences between SkyMapper DR1 and Pan-STARRS1 DR1

Note that these plots are restricted to SkyMapper DR1 sources with flags = nimaflags = 0, ngood_> 1, x_psf = 14.5 - 17.5mag, Pan-STARRS1 cross-match distances < 2", and Pan-STARRS1 DR1 n_detections > 1. The bright-end limit is imposed to avoid large systematic differences arising from sources above the typical Pan-STARRS1 saturation limit. While not exclusively restricted to point sources, they are the vast majority of sources shown in the figures above.

Extended Sources

For extended sources, we have compared the SkyMapper DR1 Petrosian magnitudes with the final photometry from the 6dF Galaxy Survey of low-redshift galaxies (Jones et al. 2009). The rms in (g_petro-BMAG) or (r_petro-RMAG) shows a strong trend as a function of the mean r-band Petrosian radius (radius_petro in the SkyMapper DR1 master catalogue), with a residual rms of ~0.3mag. The comparison here is limited to SkyMapper DR1 sources with flags = nimaflags = 0 and g_petro < 17.5mag.

 (g_petro - BMAG) vs radius_petro for SkyMapper DR1 and 6dFGS

 

Known Issues | DR1^ Back to top

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

Issue ↕ First affected release ↕ Resolved in ↕ Last modified (UTC) ↕
Corrected description of 'prox' column in master table DR1 DR1 Jun 21 2017, 06:04
Pan-STARRS1 DR1 coverage for cross-matching DR1 Jun 19 2017, 02:43
Spatial variation of zero-point systematics DR1 Jun 19 2017, 02:42
Zero-point calibration EDR Jun 05 2017, 12:10
Fringe correction EDR Jun 05 2017, 12:09
Extended-source photometry EDR Jun 05 2017, 10:36
Spatially varying PSF affecting photometry EDR Jun 05 2017, 05:55
ZIP file downloads in Safari (for info) EDR May 02 2016, 02:21