15.3.3 SDSS DR 13
The SDSS imaging camera took its first science quality data the night of September 19, 1998, and was the world’s most productive wide-field imaging facility until its last night of science quality data on November 18, 2009. In between it took a total of around 35 000 square degrees of images, covering a unique footprint of 14 055 square degrees of sky. The r photometric CCDs serve as the astrometric reference CCDs for the SDSS. That is, the positions for SDSS objects are based on the r centroids and calibrations. The r CCDs are calibrated by matching up bright stars detected by SDSS with the UCAC astrometric reference catalogues. Stars detected on the r CCDs are matched directly with stars in the United States Naval Observatory CCD Astrograph Catalog (UCAC2, Zacharias et al. 2004), which has a precision of 70 mas at its catalogue limit of , and systematic errors of less than 30 mas. UCAC2 extends up to around a declination of 41 degrees north. Outside the UCAC2 area SDSS DR9 uses an ‘internal’ UCAC data release known as ‘r14’. Together UCAC2 and r14 cover the whole sky. SDSS should be complete to magnitude .
The SDSS DR13 provides more robust and precise photometric calibrations.
For cross-match purposes primary objects only as defined by the SDSS were used. SDSS DR13 sources were extracted from DR13.PhotoPrimary table.
Data curation and catalogue preparation
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 (870000) with position errors, either in Right Ascension or Declination, larger than 1 arcsec where thus excluded from the cross-match computation.
Catalogue cleaning and Join table
As can be seen in the results of a search of the nearest neighbour (neglecting additional neighbours except for the nearest) around each SDSS DR9 object shown in Marrese et al. (2019, Appendix A), SDSS DR9 has no issue with suspected duplicated sources. We recall here that astrometry was not updated from SDSS DR9 to SDSS DR13, only the photometry was updated. While the photometry update can have an impact on the definition of primary object and can thus change the list of sources flagged as primary which are used for cross-match purposes, it does not affect the considerations made above on duplicates. On the contrary, there are a few (269) multiple sources (objects with exactly the same astrometry) in SDSS DR13. It was thus decided not to consider duplicates when calculating the cross-match. A convenience table is available that can be used to join SDSS DR13 catalogue with the cross-match results. The table links the external catalogue original sourceId (original_ext_source_id) to the corresponding additional numerical identifier (clean_sdss_dr13_oid). Both original_ext_source_id and clean_sdss_dr13_oid are present in the cross-match output tables (sdssdr13_best_neighbour and sdssdr13_neighbourhood). However, in case of multiples, different original_ext_source_id will correspond to the same clean_sdss_dr13_oid.
In the cross-match output table only the original_ext_source_id of one of the duplicated sources is listed.
In practice, users may use the original_ext_source_id in the original catalogue to find the matching distinct sources. Users interested to find all matching sources including multiples should instead use the clean_sdss_dr13_oid in the join with the cross-match result tables.
According to the definition used in this work, SDSS DR13 is a dense survey, in the cross-match algorithm Gaia is the leading catalogue, this means that Gaia objects matches are searched for in SDSS DR13. For this catalogue special treatment was done for a) the sources which are resolved in Gaia, b) the possible underestimation of astrometric errors in SDSS DR13, and c) small issues in Gaia astrometry, especially for bright stars.