HFWG Recommendation R13:

    The OGIP Long String Keyword Convention

    (Approved: 1994 Jul 27)

    The following example illustrates the OGIP Long String Keyword Convention (version 1.0):

    BIGSTRNG= 'This is a long string value that is continued &' / Any comments
    CONTINUE  'over three keywords in the&   '                 / may be appended
    CONTINUE  ' FITS header.'                           / after the quoted value.

    The & character is used as the last non-blank character of the string to indicate that the string is (probably) continued on the following keyword (if the next keyword name = CONTINUE as described below).

    Each continuation line has the keyword name = CONTINUE. Since there is no equals sign in column 9, this keyword belongs to the same class of keywords as the COMMENT and HISTORY keywords that do not have a defined value. Under our convention the continuation of the character string value is enclosed in quotes starting in column 11 (or higher) of the 80-character record. Any other characters (e.g. a comment field) may optionally follow the closing quote character on any or all of the keywords.

    Finally, the following keywords should be added to the header of any FITS extension that uses this long string convention (the exact wording may change slightly):

    LONGSTRN= 'OGIP 1.0'           / The OGIP long string convention may be used.
    COMMENT   This FITS file may contain long string keyword values that are
    COMMENT   continued over multiple keywords.  This convention uses the  '&'
    COMMENT   character at the end of a string which is then continued
    COMMENT   on subsequent keywords whose name = 'CONTINUE'.

    The presence of the LONGSTRN keyword serves to indicate that long string keywords may be present in the FITS file. The value of this keyword gives the name and version number of the specific convention that is used, which in our case is the OGIP long string convention, version 1.0. Note that the value of this keyword is a string so that it may be used to give the name of any other convention that the FITS community might eventually adopt.