Storing and representing columns of angles

CURSA provides special facilities for representing columns which contain angles. Usually angular columns are used to store celestial coordinates such as Right Ascension and Declination, position angles or small angles such as the great circle distance between neighbouring points or the angular size of extended objects. However, they can contain any angular measure. The basis of these facilities is that columns of angles are stored and manipulated internally in radians, but are automatically displayed by xcatview (see Section ) and catview (see Section ) as hours or degrees, minutes or seconds, optionally formatted as a sexagesimal value.

CURSA recognises a column which is to be treated in this way by a combination of the DTYPE (data type) and UNITS attributes.

• The data type must be DOUBLE PRECISION or REAL,¹⁸

• The units attribute of the column should be set to `RADIANS' followed by an angular format specifier enclosed in curly brackets (`{}'). The simplest forms of this angular format specifier are simply `HOURS' and ` DEGREES' for hours and degrees respectively. Thus, examples of the UNITS attribute are:

  RADIANS{HOURS}

  to display the column in hours,

  RADIANS{DEGREES}

  to display the column in degrees.

  The angular format specifiers are described in full in the following section.

Incidentally, the external display format attribute of the column, EXFMT, must be set to a valid Fortran 77 format specifier corresponding to the data type of the column, because of the way that CURSA represents catalogues as FITS tables.

It is possible to interactively modify the way that a column of angles is displayed by xcatview or catview. This alteration is achieved by supplying a new UNITS attribute for the column. In practice it is only the angular format specifier which is changed; the UNITS must still be of the form

RADIANS{angular format specifier}

The angular format specifier is described in full below.