SkyMapper | Southern Sky Survey

DR4 | DR2 | DR1.1 | EDR

Data Release EDR | 9 May 2016

DOI: 10.4225/41/572FF2C5EBD30

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

Accessibility: World-wide accessible

Release documentation

Early Data Release (EDR) Information

• Introduction
• What type of data is provided?
• Sky Coverage and Data Quality
• Data Access
• Caveats and Known Issues
• World Coordinate System
• SkyMapper Technical Details
• Filter Transformations, Star Colours and Dust Reddening

Introduction^ Back to top

The SkyMapper Early Data Release provides data from the Short Survey across one third of the southern sky. Included are all fields observed between March 2014 and March 2015 where at least two visits of the telescope were made in near-photometric conditions. Each visit includes an exposure in all six filters, uvgriz. Measurements are available for approximately 6,700 sq. deg of sky and for objects from magnitude 8 to 20 (complete to 17-18 mag depending on the filter). All magnitudes reported on this site are AB mags.

While this release is science grade, we anticipate further improvements to the zeropoint calibration and better data quality at the faint end, which we plan to roll out later in the year with Data Release 1. DR1 will be accompanied by a journal paper on the survey and its data processing methods. EDR documentation is available at this site. We encourage all interested parties to use the EDR for their research and give the SkyMapper Team feedback on data quality, completeness and suggestions for improving the access tools. These suggestions will then be incorporated into DR1.

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 EDR data. Australian astronomers will have exclusive access to the EDR, and will have protected access to future DRs for the first 12-18 months after release. Furthermore, ANU protects its considerable investment into SkyMapper through a list of protected science projects for its own research collaborations prior to worldwide release.

What type of data is provided?^ Back to top

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

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

• edr.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 (48,833,849 rows, 60 columns)

• edr.fs_photometry — per image/CCD object detections and corresponding photometric and shape information (503,901,913 rows, 82 columns)

• edr.images — a listing of image coordinates and exposure information (19,147 rows, 12 columns)

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

• ext.allwise — the NASA Widefield Infrared Survey Explorer (WISE) AllWISE data release (747,634,026 rows, 298 columns)

• ext.apass_dr9 — the AAVSO Photometric All Sky Survey (APASS) DR9 (61,176,401 rows, 25 columns)

• ext.twomass_psc — the Two Micron All Sky Survey (2MASS) Point Source Catalogue (PSC; 470,992,970 rows, 60 columns)

• ext.twomass_xsc — the Two Micron All Sky Survey (2MASS) Extended Source Catalogue (XSC; 1,647,599 rows, 389 columns)

• ext.ucac4 — the Fourth U.S. Naval Observatory CCD Astrograph Catalog (UCAC4, 113,780,093 rows, 58 columns)

Matching to the external tables was attempted with a 60" search radius around each SkyMapper source and the nearest AllWISE, UCAC4, APASS and 2MASS source ID and distance is noted in the last set of columns in edr.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 edr.master for the closest EDR source to each SkyMapper object).

We also provide local copies of several well-known spectroscopic catalogues, which we have pre-matched to the SkyMapper master table:

• ext.spec_hesqso — catalogue from the Hamburg/ESO survey for bright QSOs. III. (Wisotzki+, 2000) (415 rows, 11 columns, 170 matches with <2" search radius)

• ext.spec_2qz6qz — catalogue from the 2dF and 6dF QSO Redshift Surveys (49,425 rows, 33 columns, 3,780 matches with <2" search radius)

• ext.spec_6dfgs — catalogue from the 6 Degree Field Galaxy Survey (6dFGS) (124,647 rows, 12 columns, 48,572 matches with <2" search radius)

• ext.spec_2dfgrs — catalogue from the 2 Degree Field Galaxy Redshift Survey (2dFGRS) (245,591 rows, 25 columns, 72,907 matches with <2" search radius)

Matching the spectroscopic tables to SkyMapper was attempted with a 2" search radius and the nearest SkyMapper source has been noted in the final edr_id and edr_dist columns. 

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

The EDR consists of 1,176 SkyMapper fields covering a total area of ~6,700 sq. deg (see map of CCD centres below, 32 CCDs per exposure). Included are all fields in the low-reddening sky that have at least two images in every filter without any signs of transmission gradients across the field-of-view (near-photometric conditions) and seeing <5".

The EDR data were obtained from March 2014 to March 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. 

Median point source completeness limits in AB mag are (17.7, 17.3, 17.7, 17.7, 17.7, 17.5) for the filters (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 the EDR is (2.9", 2.7", 2.5", 2.3", 2.2", 2.1") for the filters (u, v, g, r, i, z).

Data Access^ Back to top

Main Page: How To Access

EDR 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. Your comments on the functionality and stability of these services is especially encouraged as we prepare for DR1.

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 EDR data to be aware of:

• The images show uncorrected bias residuals and fixed-pattern noise varying from image row to image row. A temporally-unstable bias implies that the bias level can vary with moderate freedom between the overscan. This problem has now been solved in the pipeline, and will be fixed in the reprocessed data published later in the year as part of Data Release 1 (DR1). 

• The flatfield is preliminary and may be incorrect by up to 5% in the corners of u and v-band images. No illumination correction has been applied. Illumination correction will be included in DR1, but it may not be possible to fully model the temporal evolution of the flatfields that result from outgassing and condensation onto the mosaic under cooled conditions.

• No fringe correction is applied to Short Survey data. However, the fringe amplitude in i-band and z-band should be mostly below +/-2 counts.

• 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. 

• Zeropoint calibration has not been finalised. The zeropoints are derived through comparison to the AAVSO Photometric All-Sky Survey (APASS), but several improvements will only be implemented for DR1, including improved APASS-to-SkyMapper transformations and improved reddening correction within that transformation.

• Zeropoints in the u-band are not corrected for atmospheric extinction as a function of airmass. Also, the u-band filter has a red leak, the relative strength of which changes with airmass as the main ultraviolet bandpass is suppressed much more by the atmosphere than the red leak; an M0 star at airmass 2 appears 0.4 mag bluer in any colour index of u minus another filter than at airmass 1. For DR1 we may attempt to publish leak-corrected flux estimates for the u-band assuming stellar SEDs.

Current zeropoint uncertainties have been estimated by comparing the EDR photometry against the SDSS Southern Standards. We find zeropoint residuals of <5% in the bands gri and up to 10% in u-band. In DR1 we intend to reduce these tolerances to less than half. 

Click here for reddening corrections and preliminary estimates of the colour transformations between SkyMapper and other standard filters.

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. 

SkyMapper Technical Details^ Back to top

The SkyMapper mosaic camera contains 32 CCDs of 4096x2048 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 sq. deg. field-of-view. Each SkyMapper image is split into its 32 constituent CCDs, which are presented as separate files to the image cutout service.

The SkyMapper filter curves (with atmosphere) are:

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. Additional information on the filter set can be found in Bessell et al. (2011).

Known Issues | EDR^ 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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