The USNO A-V1.0 Catalogue and this Service
The Catalogue provided here on-line through a web interface is
described in detail in the following README files contained on the CD-ROM's:
Some technical details
The catalogue is kept on-line on a dedicated harddisk. The server
called behind the scenes is a C-program written at ESO. This program
does not use the index files provided by the USNO but uses a single
index file instead, which has been produced at ESO. This index file
contains all information needed to access the complete catalogue in
chunks of 50.000 objects or less. The USNO index in contrary is built
up as a set of files (one for each data file) and they have a
fixed spacing in RA of 0.25 h. The number of stars in a chunk of 0.25
h x 7.5 deg might be rather high at low galactic latitudes and thus
the access in those regions might take a long time just to retrieve
some stars. This was the main reason for us to produce a new index file.
Additions to the data contained in the catalogue
The output of the server is slightly modified with respect to the data
contained in the catalogue. The most obvious difference is the
existence of an ID. This ID-number is an artificial unique number
built up by a zone number and a running number in that zone (similar
to the numbering scheme used with the GSC). The zone numbers are build
as a 4 digit string from 0000 to 1725 representing the distance in
0.1deg to the South Pole (step widht is 75). These zone numbers are
derived from the original splitting of the catalogue. The running
numbers are build as a 8 digit string (with leading zeros), they
represent the number of the object in the respective zone
(i.e. file).
The column 'gsc', if present, contains a '+' for objects which are
also in the GSC and a '-' for all other objects. This is just the
opposit of what is contained in the catalogue, but it appears more
logic.
There are two additional columns at the right of the output table
containing the distance to the center in arcmin and the position angle
(north by east).