FXWRITE

       Write a disk FITS file.

       Creates a disk FITS file and writes a FITS primary header, and
       optionally a primary data array.

       FXWRITE, FILENAME, HEADER [, DATA ]

       FILENAME = String containing the name of the file to be written.
       HEADER   = String array containing the header for the FITS file.

       DATA     = IDL data array to be written to the file.  If not passed,
                  then it is assumed that extensions will be added to the
                  file.

       None.

       None.

       NANVALUE = Value signalling data dropout.  All points corresponding to
                  this value are set to be IEEE NaN (not-a-number).  Ignored
                  unless DATA is of type float, double-precision or complex.
       NOUPDATE = If set, then the optional BSCALE and BZERO keywords in the
                  HEADER array will not be changed.  The default is to reset
                  these keywords to BSCALE=1, BZERO=0.
       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 = ''
                       FXWRITE, ERRMSG=ERRMSG, ...
                       IF ERRMSG NE '' THEN ...

       CHECK_FITS, GET_DATE, HOST_TO_IEEE, FXADDPAR, FXPAR

       None.

       If DATA is passed, then HEADER must be consistent with it.  If no data
       array is being written to the file, then HEADER must also be consistent
       with that.  The routine FXHMAKE can be used to create a FITS header.
       If found, then the optional keywords BSCALE and BZERO in the HEADER
       array is changed so that BSCALE=1 and BZERO=0.  This is so that these
       scaling parameters are not applied to the data a second time by another
       routine.  Also, history records are added storing the original values
       of these constants.  (Other values of BZERO are used for unsigned
       integers.)
       If the /NOUPDATE keyword is set, however, then the BSCALE and BZERO
       keywords are not changed.  The user should then be aware that FITS
       readers will apply these numbers to the data, even if the data is
       already converted to floating point form.
       Groups are not supported.

       None.

       Data Handling, I/O, FITS, Generic.

       W. Thompson, Jan 1992, from WRITEFITS by J. Woffard and W. Landsman.
       Differences include:
               * Made DATA array optional, and HEADER array mandatory.
               * Changed order of HEADER and DATA parameters.
               * No attempt made to fix HEADER array.
       W. Thompson, May 1992, changed open statement to force 2880 byte fixed
                              length records (VMS).  The software here does not
                              depend on this file configuration, but other
                              FITS readers might.
       W. Thompson, Aug 1992, added code to reset BSCALE and BZERO records,
                              and added the NOUPDATE keyword.

       William Thompson, GSFC, January 1992.

       Version 1, William Thompson, GSFC, 12 April 1993.
               Incorporated into CDS library.
       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 4, William Thompson, GSFC, 12 August 1999
               Catch error if unable to open file.
       Version 4.1 Wayne Landsman, GSFC, 02 May 2000
               Remove !ERR in call to CHECK_FITS, Use ARG_PRESENT()
       Version 5, William Thompson, GSFC, 22 September 2004
               Recognize unsigned integer types

       Version 5, 22 Sep 2004

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