skip to main content

gaia early data release 3 documentation

9.3 External catalogues matched with Gaia EDR3

9.3.9 APASS DR9

Reference paper:
Henden et al. 2016


The AAVSO Photometric All Sky Survey (APASS) project is designed to bridge the gap between the shallow Tycho-2 two-bandpass photometric catalogue that is complete to V=11 and the deeper, but less spatially-complete catalogues like SDSS or Pan-STARRS. It can be used for calibration of a specific field; for obtaining spectral information about single sources, determining reddening in a small area of the sky; or even obtaining current-epoch astrometry for rapidly moving objects.

The survey is being performed from two locations: near Weed, New Mexico in the Northern Hemisphere; and at CTIO in the Southern Hemisphere. Each site consists of dual bore-sighted 20cm telescopes on a single mount, designed to obtain two bandpasses of information simultaneously. Each telescope covers 2.9×2.9 square degrees of sky with 2.57 ′′/pixels, with the main survey taken with B, V, g, r, i filters and covering the magnitude range 10<V<17. A bright extension is under way, saturating at V=7 and extending the wavelength coverage from u to Y. The faint completeness limit is V=16. The APASS Data Release 9 contains approximately 62 million stars, about 99% of the sky. There are some issues in the catalogue which should be taken into account when cross-matching it:

  • APASS team is not providing star IDs until the final product and suggest to identify stars by their RA and DEC.

  • There are a number of duplicate entries. These appear to be caused by the merging process, where poor astrometry in one field may cause two seed centroids to form for a single object.

  • There are a number of entries with 0.000 errors.

  • Centroiding in crowded fields is very poor, blends cause photometric errors as well as astrometric ones.

  • There are saturated stars in the catalogue and the APASS team suggests not to use sources brighter than V=7.

Catalogue cleaning and Join table

The results of a search of the nearest neighbour (neglecting additional neighbours except for the nearest) around each APASS DR9 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. A convenience table is available that can be used to join APASS DR9 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_apassdr9_oid). Both original_ext_source_id and clean_apassdr9_oid are present in the cross-match output tables ( apassdr9_best_neighbour and apassdr9_neighbourhood). However, in case there are suspected duplicates in the external catalogue, different original_ext_source_id will correspond to the same clean_apassdr9_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_apassdr9_oid in the join with the cross-match result tables.

Cross-match algorithm

According to the definition used in this work, APASS DR9 is a dense survey, in the cross-match algorithm Gaia is the leading catalogue, this means that Gaia objects matches are searched for in APASS DR9. 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.