The catalogue fields are known in DIRA2 by means of an alphanumeric code of eight or less characters. Moreover we have defined a vocabulary of the physical quantities used in astronomy.
The DB_INFO command allows one to know the field name in each catalogue or to search for information in the vocabulary. Moreover, during the execution of the DIRA2 commands, the help structure shows the field names of the selected catalogue, when these are requested.
In DIRA2, no reserved codes exist for the fields, but two fields with the same name can't exist on a catalogue. The field names can contain only alphanumerical characters and no special symbols, the first character must be an alphabetic one.
The sky coordinates have a special role in the astronomical catalogues because they are often used to identify the objects. Historically, the coordinates have been defined with plenty of formats and it was not possible to use general decoding routines without loosing computer speed. DIRA2 does not support coordinates written in more than one field, as HH MM SS or sDD PP SS.ss, but uses this convention in the commands user interface. In the catalogues, coordinates must be written in one of the following formats:
Here longitude and latitude are referred to a generic coordinate system: for example they can be RA and DEC in an equatorial system, or galactic longitude and latitude in a galactic coordinate system and so on.
Please note that formats 1 and 2 represent the coordinates as an integer number of 9 digits. This was the only format adopted for the coordinates in the previous version of DIRA and is similar to the typical format HH MM SS.mmm and sDD MM SS.mmm (without spaces and dots) used in most original catalogues for Right Ascension and Declination. The position of the digit inside the field is mandatory, and one must write the thousandth of seconds (as 000) even if one has a lower coordinates precision.
The formats 5 and 6 are an extension of the format 1 and 2 for data with high precision coordinates as Hipparcos or VLBI catalogues. The other formats do not require a fixed numbers of digits, but need the decimal point.
Internal DIRA2 commands use the coordinates as Real*8 values to avoid arithmetical precision problems. Moreover, most commands take advantage of using catalogues sorted on a coordinate with a direct access on the selected records.
When a catalogue's field contains a coordinate the 'field type' parameter in the system files must be filled with the appropriate code. This parameter can be set with the PDB_MANAG command.
Typically the fields are not normalized to physical units. The original values of the fields are preserved in public catalogues, and the user can write his catalogues using different scale units for the same physical quantity, (if he need it !). The normalization of the fields is required for the coordinates, and for the proper motions values. The allowed coordinates formats are shown in the previous table, the proper motion must be written in arcsec./years units. The values of the proper motion in R.A. must not contain cos () factor.
We adopt these convenctions on the astrometric fields: