Coordinates, units and formats

By default databases are assumed to contain columns of Right Ascension and Declination on which they can be searched. However, some databases do not contain such coordinates, but rather have Cartesian $x,y$ positions. Typically such positions occur in catalogues generated by detecting the objects present in a CCD direct image frame or a digitised photographic plate. The following rules apply.

1. By default databases are assumed to be searchable on columns of Right Ascension and Declination. Further, these columns are assumed to be called `ra' and `dec', respectively. Simply include the tokens %ra and %dec in the query.

2. If a `positional' search (ie. one finding objects in either a circular area or a region bounded by meridians of Right Ascension and parallels of Declination) is to be made on columns of Right Ascension and Declination then the formats and units of the given values should be:

   Right Ascension

   sexagesimal or decimal hours,

   Declination

   sexagesimal or decimal degrees.

   If a sexagesimal value is entered than a colon (`:') should be used as a separator. The coordinates should be for equinox and epoch J2000.

3. If a database is searchable on $x,y$ positions rather than celestial coordinates then the following keywords should be added to its entry in the configuration file:

   ra_col: -1
dec_col: -1
x_col: (sequence number of column holding $x$ coordinate)
   y_col: (sequence number of column holding $y$ coordinate)

   Setting ra_col and dec_col to -1 indicates that columns of Right Ascension and Declination are not available.

4. The units of the $x,y$ positions are simply `pixels', that is, dimensionless numbers. Note however, that they are of type REAL rather than INTEGER; positions can be expressed to a fraction of a pixel.

5. The units in which the radius, height and width of circular and `rectangular' positional searches are expressed also vary depending on whether the database contains celestial coordinates or $x,y$ positions. The tokens affected are:

   %r %r1 %r2 %w %h

   The alternatives for their units are:

   in a search on celestial coordinates:

   decimal minutes of arc,

   in a search on $x,y$ positions:

   decimal pixels.