# 9.3.10 GSC 2.3

Reference paper:

Original catalogue:
R. Smart, private communication

The Guide Star Catalog II (GSC-II) is an all-sky database of objects derived from the uncompressed Digitized Sky Surveys that the Space Telescope Science Institute has created from the Palomar and UK Schmidt survey plates and made available to the community. Like its predecessor (GSC-I), the GSC-II was primarily created to provide guide star information and observation planning support for Hubble Space Telescope. Two catalogues have already been extracted from the GSC-II database and released to the astronomical community.

A magnitude-limited ($Rf=18.0$) version, GSC2.2, was distributed soon after its production in 2001, while the GSC2.3 release has been available for general access since 2007. The GSC2.3 catalogue contains astrometry, photometry, and classification for 945 592 683 objects down to the magnitude limit of the plates. Positions are tied to the International Celestial Reference System; for stellar sources, the all-sky average absolute error per coordinate ranges from 0.2${}^{\prime\prime}$to 0.28${}^{\prime\prime}$depending on magnitude. When dealing with extended objects, astrometric errors are 20% worse in the case of galaxies and approximately a factor of 2 worse for blended images. Stellar photometry is determined to a 0.13-0.22 mag accuracy as a function of magnitude and photographic pass bands ($Rf$, $Bj$, $In$). Outside of the Galactic plane, stellar classification is reliable to at least 90% confidence for magnitudes brighter than $Rf=19.5$, and the catalogue is complete to $Rf=20$.

## Data curation and catalogue preparation

There are 3 350 256 objects in GSC2.3 with RA and DEC errors equal to 0, while it is mandatory to have errors on coordinates in order to run the cross-match. For the sake of completeness, these objects were not deleted, but we assigned to them the largest position error found in the catalogue (i.e. 1.6 arcsec).

## Catalogue cleaning and Join table

The results of a search of the nearest neighbour (neglecting additional neighbours except for the nearest) around each GSC2.3 object are shown in Marrese et al. (2019, Appendix A). The distribution of nearest neighbours (i.e. the source separation distribution) shows the presence of nearest neighbours much closer than the catalogue angular resolution. They are most probably either multiples (objects with exactly the same astrometry) or suspected duplicates (objects much closer than the catalogue angular resolution). It was thus decided not to consider multiple/duplicates when calculating the cross-match. Multiples involving either Tycho-2 or SKY2000 bright objects (which were added to the GSC2.3 catalogue for completeness) were not considered as multiples in the cross-match calculations, given the different resolution of these two catalogues when compared to the GSC2.3 observations. A convenience table is available that can be used to join GSC2.3 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_gsc23_oid). Both original_ext_source_id and clean_gsc23_oid are present in the cross-match output tables ( gsc23_best_neighbour and gsc23_neighbourhood). However, in case there are suspected duplicates in the external catalogue, different original_ext_source_id will correspond to the same clean_gsc23_oid. In the cross-match output table only the original_ext_source_id of the source with the best astrometry among the suspected duplicates is listed. In practice, users may use the original_ext_source_id in the original catalogue to find the matching source with the best astrometry. Users interested to find all matching suspected duplicates should instead use the clean_gsc23_oid in the join with the cross-match result tables.

## Cross-match algorithm

According to the definition used in this work, GSC2.3 is a dense survey, in the cross-match algorithm Gaia is the leading catalogue, this means that Gaia objects matches are searched for in GSC2.3. For this catalogue special treatment was done for a) the sources which are resolved in Gaia, and b) small issues in Gaia astrometry, especially for bright stars.