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      Modify a FITS header in a file on disk.


      Opens a FITS file, and adds or modifies a parameter in the FITS header.
      Can be used for either the main header, or for an extension header.
      The modification is performed directly on the disk file.




      FILENAME = String containing the name of the file to be read.
      NAME = Name of parameter, scalar string If NAME is already in the
                header the value and possibly comment fields are modified.
                Otherwise a new record is added to the header. If NAME is
                equal to either "COMMENT" or "HISTORY" then the value will be
                added to the record without replacement. In this case the
                comment parameter is ignored.
      VALUE = Value for parameter. The value expression must be of the
                correct type, e.g. integer, floating or string. String
                values of 'T' or 'F' are considered logical values.
  Opt. Inputs :
      COMMENT = String field. The '/' is added by this routine. Added
                starting in position 31. If not supplied, or set equal to ''
                (the null string), then any previous comment field in the
                header for that keyword is retained (when found).


  Opt. Outputs:


      EXTENSION = Either the number of the FITS extension, starting with the
                  first extension after the primary data unit being one; or a
                  character string containing the value of EXTNAME to search
                  for. If not passed, then the primary FITS header is
      BEFORE = Keyword string name. The parameter will be placed before the
                location of this keyword. For example, if BEFORE='HISTORY'
                then the parameter will be placed before the first history
                location. This applies only when adding a new keyword;
                keywords already in the header are kept in the same position.
      AFTER = Same as BEFORE, but the parameter will be placed after the
                location of this keyword. This keyword takes precedence over
      FORMAT = Specifies FORTRAN-like format for parameter, e.g. "F7.3". A
                scalar string should be used. For complex numbers the format
                should be defined so that it can be applied separately to the
                real and imaginary parts.
      ERRMSG = If defined and passed, then any error messages will be
                returned to the user in this parameter rather than
                depending on the MESSAGE routine in IDL. If no errors are
                encountered, then a null string is returned. In order to
                use this feature, ERRMSG must be defined first, e.g.
                      ERRMSG = ''
                      FXHMODIFY, ERRMSG=ERRMSG, ...
                      IF ERRMSG NE '' THEN ...




      This routine can not be used to modify any of the keywords that control
      the structure of the FITS file, e.g. BITPIX, NAXIS, PCOUNT, etc. Doing
      so could corrupt the readability of the FITS file.


      Modify the name 'OBJECT' keyword in the primary FITS header of a FITS
      file 'spec98.ccd' to contain the value 'test domeflat'
      IDL> fxhmodify, 'spec98.ccd', 'OBJECT', 'test domeflat'

Side Effects

      If adding a record to the FITS header would increase the
      number of 2880 byte records stored on disk, then the file is
      enlarged before modification, unless the NOGROW keyword is passed.


      Data Handling, I/O, FITS, Generic.
  Prev. Hist. :
  Written :
      William Thompson, GSFC, 3 March 1994.
  Modified :
      Version 1, William Thompson, GSFC, 3 March 1994.
      Version 2, William Thompson, GSFC, 31 May 1994
              Added ERRMSG keyword.
      Version 3, William Thompson, GSFC, 23 June 1994
              Modified so that ERRMSG is not touched if not defined.
      Version 3.1 Wayne Landsman GSFC 17 March 2006
              Fix problem in BLKSHIFT call if primary header extended
      Version 3.2 W. Landsman 14 November 204
              Allow for need for 64bit number of bytes
      Version 4, William Thompson, GSFC, 22-Dec-2014
              Modified test for keyword EXTEND to only issue warning.


      Version 4, 22-Dec-2014

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