The original catalog consisted in 24 files, one for a 7.5° strip in declination. Each file was extended to a directory, named N000...N8230 and S0000...S8230, i.e. with the same conventions as those used for the GSC Catalog.
In each of these directories, there is one file for 30min (i.e. 7.5° at the Equator) in right asension; the total number of files is therefore 24×48 = 1152 files, with an average number of 20,000 (near the poles) to 800,000 objets per file. In each of these files, the range of the coordinates is then restricted to 7.5°, i.e. a maximal value of 2,700,000 when the coordinates are expressed in their original units of 10mas. The final grouping allowed to reduce one record to 7 or 8 bytes (the mean is close to 7).
The resulting catalog occupies only 3.6Gbytes, including all transformation and query software; the full 526×106 objects are tested in about 45 minutes (i.e. 5µs per object) on a Sparc-20 (72MHz).
A few benchmarks made on a Sparc-20 (72MHz) give the following average elapsed times (between 15 and 70for a search by position on the catalog, keeping the 10 closest stars (actually performed on the USNO-A1.0 which was converted in April 1997 with an almost identical software):
=========================================================================
Search Tested stars Time required Reading time
Radius (') per target (s) for 1 star (microsec)
-------------------------------------------------------------------------
2.5 14153 0.09 6.4
10.0 66394 0.24 3.6
30.0 201351 0.67 3.3
=========================================================================
A client/server access to the PMM USNO-A2.0 Catalog – as well as to other catalogues – is also available via the findpmm2 program which is part of the cdsclient package.
François Ochsenbein, CDS.home