3.3. Keyword naming convention

Table 2 lists the corresponding set of coordinate system keywords for use with each type of FITS image representation. The data type of the table keyword matches that of the corresponding primary image keyword. The allowed values for these keywords are identical for all three types of images as defined in the main body of this paper. The old and now deprecated keyword CROTAi has been used with tables and is included since readers will need to understand this keyword even if writers should no longer write it. See Paper II for a discussion of this point. To support current usage, the keywords are given in their current form to be used for the primary coordinate representation (a is blank) and a new form to support the new capability to specify alternate coordinates for the same axis (a is A through Z). For new keywords, the two forms are identical and are shown in a single column midway between the columns for old primary and new alternate WCS keywords. The following notes apply to the naming conventions used in Table 2:

• The j, i prefix and suffix characters are integers referring to a pixel and intermediate world coordinate axis number, respectively, of the array. When used as a keyword suffix the image dimension may range from 1 to 99 with no leading 0, but when used as a prefix the integer is limited to a single digit to conform to the 8-character keyword name limit so the image may only contain up to 9 dimensions.
• a is a 1-character coordinate version code and may be blank (primary) or any single uppercase character from A through Z.
• n and k are integer table column number without any leading zeros (1-999).
• m is an integer between 0 and 99 with no leading zero giving the coordinate parameter number. m cannot exceed 9 when n exceeds 99, but see the following Section.
• The guidelines given Sect. 2.3 must be applied to the the value of the CUNITia keyword and its derivatives. In particular the value is restricted to 'deg' when referring to celestial coordinates; see Paper II.