Astronomical data are frequently organized in tables, for example, a standard source catalog or the results of a series of observations. Another use for tables is to hold information related to observations, such as observing logs and calibration data, which are more conveniently transported in a table accompanying the observations than as keywords. Not surprisingly, the first standard extension developed and endorsed by the IAU was that for tables (Harten et al. 1988). This ASCII table extension uses ASCII records to carry the information.
The data appear as a character array, in which the rows represent the lines of a table and the columns represent the characters that make up the tabulated items. Each member of the array is one character or digit. Each row consists of a sequence of fields. The sequence of fields is the same for all rows, and all rows must have the same length. Each field in the table contains one item in the table, either a character string or the ASCII representation of a number in FORTRAN-77 format. A particular field may consist of many columns, as many as are needed to carry the full string or number.
The field organization is described by a series of keywords. These keywords describe the initial column and number of columns for each field. Other kinds of information that can be listed in the header include the units of quantities in the field, scale factors between the values in the table and the physical quantities they represent--analogous to the BZERO and BSCALE values of Basic FITS--and the string used to represent an undefined value. Fields in the table may be reserved for comments, which would be skipped by automatic decoding software but may be read by simply printing the table. This property allows the transfer of tables that contain a large quantity of comments. Examples 3 and 5 illustrate ASCII table headers.