Introduction
------------

We now are able to provide the USNO A-V1.0 Catalogue on-line through a
web interface and through Skycat. The catalogue itself is described in
detail in the README files contained on the CD-ROM's. These files are
accessible on-line using the following URLs:

http://archive.eso.org/skycat/usno.html

For the web interface use the URL:

http://archive.eso.org/skycat/servers/usnoa

Some technical details
----------------------

The catalogue is kept on-line on a dedicated hard-disk. The server
called behind the scenes is a C-program written at ESO.

The 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 was done in order to allow fast access to any sky region in
short time (about a second).

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. For a description of the contents of the
standard columns and some special values you may find there please
refer to the original description files. 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 width 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
opposite 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).

General remarks
---------------

The USNO A-V1.0 catalogue contains more than 488 million objects,
i.e. more than 11.000 objects per square degree in the mean over the
whole sky! The maximum number of objects that might be retrieved by
a single request is limited to 100.000. For very dense regions this
might cover less than one sq. degree only. Please keep always in mind
that this is really a huge catalogue when requesting data for a larger
area. The term 'large' is mainly dependent on the galactic
latitude and might be as small as some ten square arcmin. Skycat as
well as the web interface allow to limit the number of objects
requested by setting magnitude limits or just limit the absolute
number of objects to be submitted by the server.

The USNO A-V1.0 catalogue is the output of an automatic object
detection process. If you compare the objects contained in the
catalogue with DSS images you will find that the USNO A-V1.0 is really
an excellent piece of work. But every object detection process also
has it's limitations, thus you may find objects in the catalogue where
you hardly could detect anything on a DSS image or a CCD frame. Or
there might be cases where you are wondering about an 'obvious' object
on your image which is not in the catalogue. Please read the
descriptions to learn about the contents and the processing of the
catalogue and it's limitations.

If you benefited from using USNO-A and this service, we ask that you
give the catalog a citation.

Tips for Skycat Users
---------------------

The skycat config file loaded by default from ESO at startup has been
modified to provide direct access to the USNO A-V1.0 catalogue (see
under Catalogs... menu option).

By default, the output of the server is sorted by R-magnitude. This
setting has been chosen in favor of the sorting by distance to the
center, because it assures that at least the brightest stars are
retrieved by the default request for 60 objects.

The default symbols used to plot the objects are circles scaled by the
value of R-magnitude, where objects with magnitude 15 or less get the
smallest possible circle. Up to now the default
symbol used circles to mark GSC objects. Circles are now used for the
USNO catalog, squares are used for GSC.

Contact
-------

Any questions and suggestions should be send to Andreas Wicenec
(awicenec@eso.org).

For the ESO/ST-ECF Science Archive Group
--Miguel