8.3.2 SkyMapper DR2
The Second Data Release begins SkyMapper’s exploration of the deep Southern sky. DR2 includes the first release of images from the Main Survey, as well as a larger Shallow Survey dataset.
Data curation and catalogue preparation
The data curation was done following the caveats and known issues described on the SkyMapper
official website and considering the issues which may affect the cross-match.
According to the documentation, while the Right Ascension position uncertainties in the master table include values down to 1 mas, the true minimum uncertainty should be assumed to be 36 mas. A minimum value of 36 mas was thus used for cross-match computations.
In addition, the master table has 2.9 million objects with mean epoch either zero or null.
In order to not exclude those objects from the cross-match computations, a mean epoch corresponding to the average mean epoch was used for the cross-match.
Catalogue cleaning and Join table
When the position errors in the external catalogue are too high, then the cross-match results are very uncertain and almost meaningless, as several Gaia sources can be matched the external catalogues ones. All objects (265869) with position errors, either in Right Ascension or Declination, larger than 1 arcsec where thus excluded from the cross-match computation.
A convenience table is available that can be used to join the SkyMapper DR2 catalogue with the cross-match results. It lists all the SkyMapper DR2 sources which were used in the cross-match.
Subsubsection Cross-match algorithm
According to the definition used in this work, SkyMapper DR2 is a dense survey, in the cross-match algorithm Gaia is the leading catalogue, this means that Gaia objects matches are searched for in SkyMapper DR2.
For this catalogue special treatment was done for a) the sources which are resolved in Gaia, b) the possible underestimation of astrometric errors in SkyMapper DR2, and c) small issues in Gaia astrometry, especially for bright stars.