Image credit: Kwon O Chul

Description

The SkyMapper Southern Survey (SMSS) is a 6-band optical survey conducted with the Australian National University's 1.3m SkyMapper Telescope at Siding Spring Observatory in Australia. The telescope has a 32-CCD mosaic camera, with 268 million pixels, covering 2.4°x2.4°. The SMSS filter set (Bessell et al. 2011) is comprised of u, v, g, r, i, and z, with differences from the SDSS and LSST/VRO bandpasses that facilitate novel scientific applications.

SMSS published its Fourth Data Release (DR4) in February 2024, covering from the South Celestial Pole to Declinations of +16°, with some fields observed up to +28°. Approximately 700 million unique sources have been observed from 13 billion photometric data points measured from over 400,000 images acquired between March 2014 and September 2021.

The typical 10-sigma depths for each field range between 18.5 and 20.5 mag, depending on the filter, but certain sky regions include longer exposures that reach as deep as 22 mag in some filters.

For more information about SMSS and DR4, please visit: https://skymapper.anu.edu.au or consult the SMSS DR4 paper (Onken et al. 2024).

The skymapper_dr4.master table will be crossmatched against our default reference datasets within a 1.5 arcsec radius, nearest neighbor only. These tables will appear with x1p5 in their name in our table browser.

SkyMapper DR4 at a Glance
FilterCentral Wavelength
(nm)
Filter FWHM
(nm)
Typical Saturation
(ABmag)
10-sigma Depth
(ABmag)
Median Seeing
(arcsec)
u350 438.918.63.15
v384 318.218.93.00
g5101579.420.52.80
r6171589.420.22.63
i7791419.519.42.54
z916 849.618.62.49

SkyMapper DR4 Coverage Maps
u-band coverage v-band coverage
g-band coverage r-band coverage
i-band coverage z-band coverage


Data Reduction

Full details of the DR4 data reduction can be found in the SkyMapper DR4 paper (Onken et al. 2024). DOI: 10.25914/5M47-S621.

Notable features of the SkyMapper DR4 data reduction include Gaia-based astrometric solutions derived across each mosaic image; PCA-based fringe removal in i and z bands; and an entirely new photometric zeropoint calibration anchored to synthetic photometry calculated from low spectral resolution Gaia BP/RP spectra.

The Gaia XP synthetic photometry dramatically improves the photometric calibration of the u- and v-band images compared to previous SkyMapper data releases. Careful treatment of the u-band calibration was still required because the XP spectra do not fully cover the wavelength range of the filter, and the systematic uncertainties of the Gaia spectral reconstructions are large at the blue end. Nevertheless, the new photometric calibration naturally resolves trends with reddening and sky location that were identified in prior DRs (e.g. Huang et al. 2021).

PSF photometry is estimated by a series of nested apertures, with aperture corrections fitted to a gradient across each CCD. The photometric uncertainties are estimated from the weighted mean of the outlier-clipped set of measurements. The range of observed magnitudes per filter is also recorded for each source.

Acknowledgment

All users are required to acknowledge the source of the data with an appropriate citation in their publications, using the following text:

"SkyMapper is owned and operated by The Australian National University's Research School of Astronomy and Astrophysics. The SkyMapper Southern Survey has been funded in part through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory. The SkyMapper Southern Survey dataset has been produced with the support of the National Computational Infrastructure (NCI) in Canberra, Australia."