Astrometric calibration

A global (multi-CCD and multi-dither) astrometric calibration is calculated per filter per tile. SCAMP (Bertin 2006, ASP Conf. Series 351, 112) is used for this purpose, with a polynomial degree of 2 over the whole mosaic. The (unfiltered) 2MASS-PSC (Skrutskie et al. 2006, AJ, 131, 1163) is used as astrometric reference catalog. A more detailed description of the astrometric pipeline is in MF13.

SWARP is used to resample all exposures in a tile to the same pixel grid, with a pixel size of 0.2 arcseconds. After background subtraction the exposures are coadded using a weighted mean stacking procedure.

Astrometric quality

The accuracy of the absolute astrometry ("KiDS vs 2MASS") is uniform over a coadd, with typical 2-dimensional RMS of 0.31 arcsec in g, r, and i, and 0.25 arcsec in u. The lower RMS in u-band is most likely due to the fact that in this band on average brighter 2MASS sources are selected as reference sources.

Absolute astrometric quality of one coadd
Absolute astrometric quality of all coadd vs. RA

Absolute astrometric accuracy in KiDS-ESO-DR1, based on the RMS of the 2-dim. positional residual of KiDS – 2MASS source pairings across dithers. Top: across a single KiDS coadd. Plotted is the median of the RMS ensemble of all 50 KiDS tiles per CCD (chip) for u (blue), g (green), r (red) and i (magenta). The typical standard deviation in this RMS ensemble is 0.003 arcsec and is similar for all CCDs and filters. Bottom: for all coadds vs. RA. Plotted is the median RMS for each coadd with colors indicating the filter: u (blue), g (green), r (red) and i (magenta).

The accuracy of the relative astrometry ("KiDS vs KiDS") is also uniform across a single coadd. This is indicated by the 2-dimensional positional residuals of sources across its input dithers. The residuals have a RMS of ~0.029 arcsec for g, r and i and ~0.032 arcsec for u. The slightly larger RMS in u-band is due to the smaller number of available reference sources. Overall, the RMS is ~0.030 arcsec for g, r and i. For u the scatter is larger with maximum RMS ~0.055 arcsec.

Absolute astrometric quality of one coadd
Absolute astrometric quality of all coadd vs. RA

Relative astrometric accuracy in KiDS-ESO-DR1, based on the RMS of the 2-dim. positional residual of source pairings across dithers. Top: across a single KiDS coadd. Plotted is the median of the RMS ensemble of all 50 KiDS tiles per CCD (chip) for u (blue), g (green), r (red) and i (magenta). The typical standard deviation in this RMS ensemble is 0.007 arcsec and is similar for all CCDs and filters. Bottom: for all coadds vs. RA. Plotted is the median RMS for each coadd with colors indicating the filter: u (blue), g (green), r (red) and i (magenta).