FITS_WRITE

       To write a FITS primary data unit or extension.

       FITS_WRITE, filename_or_fcb, data, [header_in]

       FILENAME_OR_FCB: name of the output data file or the FITS control
               block returned by FITS_OPEN (called with the /WRITE or
               /APPEND) parameters.

       DATA: data array to write.  If not supplied or set to a scalar, a
               null image is written.
       HEADER_IN: FITS header keyword.  If not supplied, a minimal basic
               header will be created.  Required FITS keywords, SIMPLE,
               BITPIX, XTENSION, NAXIS, ... are added by FITS_WRITE and
               do not need to be supplied with the header.  If supplied,
               their values will be updated as necessary to reflect DATA.

       XTENSION: type of extension to write (Default="IMAGE"). If not
               supplied, it will be taken from HEADER_IN.  If not in either
               place, the default is "IMAGE".  This parameter is ignored
               when writing the primary data unit.     Note that binary and
               and ASCII table extensions already have a properly formatted
               header (e.g. with TTYPE* keywords) and byte array data.
       EXTNAME: EXTNAME for the extension.  If not supplied, it will be taken
               from HEADER_IN.  If not supplied and not in HEADER_IN, no
               EXTNAME will be written into the output extension.
       EXTVER: EXTVER for the extension.  If not supplied, it will be taken
               from HEADER_IN.  If not supplied and not in HEADER_IN, no
               EXTVER will be written into the output extension.
       EXTLEVEL: EXTLEVEL for the extension.  If not supplied, it will be taken
               from HEADER_IN.  If not supplied and not in HEADER_IN, no
               EXTLEVEL will be written into the output extension.
       /NO_ABORT: Set to return to calling program instead of a RETALL
               when an I/O error is encountered.  If set, the routine will
               return  a non-null string (containing the error message) in the
               keyword MESSAGE.   If /NO_ABORT not set, then FITS_WRITE will
               print the message and issue a RETALL
       /NO_DATA: Set if you only want FITS_WRITE to write a header.  The
               header supplied will be written without modification and
               the user is expected to write the data using WRITEU to unit
               FCB.UNIT. When FITS_WRITE is called with /NO_DATA, the user is
               responsible for the validity of the header, and must write
               the correct amount and format of the data.  When FITS_WRITE
               is used in this fashion, it will pad the data from a previously
               written extension to 2880 blocks before writting the header.

       MESSAGE: value of the error message for use with /NO_ABORT
       HEADER: actual output header written to the FITS file.

       If the first call to FITS_WRITE is an extension, FITS_WRITE will
       automatically write a null image as the primary data unit.
       Keywords and history in the input header will be properly separated
       into the primary data unit and extension portions when constructing
       the output header (See FITS_READ for information on the internal
       Header format which separates the extension and PDU header portions).

       Write an IDL variable to a FITS file with the minimal required header.
               FITS_WRITE,'newfile.fits',ARRAY
       Write the same array as an image extension, with a null Primary data
       unit.
               FITS_WRITE,'newfile.fits',ARRAY,xtension='IMAGE'
       Write 4 additional image extensions to the same file.
               FITS_OPEN,'newfile.fits',fcb
               FITS_WRITE,fcb,data1,extname='FLUX',extver=1
               FITS_WRITE,fcb,err1,extname'ERR',extver=1
               FITS_WRITE,fcb,data2,extname='FLUX',extver=2
               FITS_WRITE,fcb,err2,extname='ERR',extver=2
               FITS_CLOSE,FCB

       FITS_OPEN, SXADDPAR, SXDELPAR, SXPAR()

       Written by:     D. Lindler      August, 1995
       Work for variable length extensions  W. Landsman   August 1997
       Converted to IDL V5.0   W. Landsman   September 1997
       PCOUNT and GCOUNT added for IMAGE extensions   J. Graham  October 1999
       Write unsigned data types      W. Landsman   December 1999
       Pad data area with zeros not blanks  W. McCann/W. Landsman October 2000
       Return Message='' to signal normal operation W. Landsman Nov. 2000
       Ensure that required extension table keywords are in proper order
             W.V. Dixon/W. Landsman          March 2001
       Assume since V5.1, remove NaNValue keyword   W. Landsman Nov. 2002
       Removed obsolete !ERR system variable  W. Landsman Feb 2004
       Check that byte array supplied with table extension W. Landsman Mar 2004

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