TOCats catalogues

26/02/2021 Page maintained by Luciano Nicastro - Last updated on 26/02/2021

Sky view (180 deg – Galactic center marked), with the objects density in the Gaia EDR3 catalogue, with and without the position of the brightest stars marked.

TOCats is a tool for fast access to various object catalogues observed in the optical and infrared band. Users can access them through a web interface or directly from their client application. The project was born within a collaboration between OAS Bologna and Turin Astrophysical Observatory (from which its name) started with the preliminary activities related to the launch of the ESA Gaia satellite.

TOCats uses hierarchical scheme techniques (tree structure) to split the celestial sphere, in particular using the HTM and HEALPix schemes (see references below).
Each catalogue is the collection of several tables: the mother, containing all the objects with all the information, and a set of child tables, obtained, as said, splitting the sky in tiles of various sizes. For each tile the brightest object is selected, becoming its representative, together with the total number of objects present.

In TOCats we chose to use HEALPix starting from an initial sphere split in (typically) 21.5 million tiles (order = 10, Nside = 1,024) up to 768 tiles (order = 3, Nside = 8). In all the tables, including the mother, the objects are indexed using a unique code of the tile they lay in. All the catalogues, and the metadata describing them, are hosted in a relational database (MySQL) that uses the library DIF (Dynamic Index Facility) to be able to swiftly identify the objects falling within a given sky region, independently from the number of objects present into the catalogue (one or more billions is not unusual nowadays).

The catalogues are directly accessible by any script capable to manage the HTTP protocol. Moreover it is possible to host privately accessible catalogues, for example related to research projects that want to make easily visible and comparable the objects under study with respect to those present in the various public catalogues.

For a given catalogue, the TOCats web interface (TOCatsweb) automatically chooses the table to use for the objects search, depending on the sky area displayed: the larger the area, the larger the tiles to use for the search, so to always get a fast search. When the sky area is comparable or smaller than that of the smallest tile used in the sphere pixelisation, the mother table is accessed. This methodology guarantees a constantly short access time, unless in the requested sky area a large number of objects, of the order of tens-hundreds thousand are present. In this case the response time is still almost uniquely due to the data formatting and network transfer.
If the automatic approach is considered not adequate, the user is free to choose the table to query: the mother or any of the child tables.
The interface also offers direct links to archives and data/images as obtained from the sky coordinates one is looking at.

TOCats is VO-compliant, as by default it returns the table with the selected objects as a VOTable; but one can opt for the JSON, TSV or CSV formats.

For further information: