FITS_OPEN

       Opens a FITS (Flexible Image Transport System) data file.

       FITS_OPEN, filename, fcb

       filename : name of the FITS file to open, scalar string
                  FITS_OPEN can also open gzip compressed (.gz) file *for
                  reading only*, although there is a performance penalty

       fcb : (FITS Control Block) a IDL structure containing information
               concerning the file.  It is an input to FITS_READ, FITS_WRITE
               and FITS_CLOSE

       /APPEND: Set to append to an existing file.
       /HPRINT - print headers with routine HPRINT as they are read.
               (useful for debugging a strange file)
       /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.    (For backward compatibility, the obsolete
               system variable !ERR is also set to -1 in case of an error.)
               If /NO_ABORT not set, then FITS_OPEN will print the message and
               issue a RETALL
       /UPDATE Set this keyword to open an existing file for update
       /WRITE: Set this keyword to open a new file for writing.

       MESSAGE = value: Output error message.    If the FITS file was opened
               successfully, then message = ''.

       The output FCB should be passed to the other FITS routines (FITS_OPEN,
       FITS_READ, FITS_HELP, and FITS_WRITE).  It has the following structure
       when FITS_OPEN is called without /WRITE or /APPEND keywords set.
           FCB.FILENAME - name of the input file
               .UNIT - unit number the file is opened to
               .NEXTEND - number of extensions in the file.
               .XTENSION - string array giving the extension type for each
                       extension.
               .EXTNAME - string array giving the extension name for each
                       extension. (null string if not defined the extension)
               .EXTVER - vector of extension version numbers (0 if not
                       defined)
               .EXTLEVEL - vector of extension levels (0 if not defined)
               .GCOUNT - vector with the number of groups in each extension.
               .PCOUNT - vector with parameter count for each group
               .BITPIX - BITPIX for each extension with values
                                  8    byte data
                                16     short word integers
                                32     long word integers
                               -32     IEEE floating point
                               -64     IEEE double precision floating point
               .NAXIS - number of axes for each extension.  (0 for null data
                       units)
               .AXIS - 2-D array where axis(*,N) gives the size of each axes
                       for extension N
               .START_HEADER - vector giving the starting byte in the file
                               where each extension header begins
               .START_DATA - vector giving the starting byte in the file
                               where the data for each extension begins
               .HMAIN - keyword parameters (less standard required FITS
                               keywords) for the primary data unit.
               .OPEN_FOR_WRITE - flag (0= open for read, 1=open for write,
                                                2=open for update)
               .LAST_EXTENSION - last extension number read.
               .RANDOM_GROUPS - 1 if the PDU is random groups format,
                               0 otherwise
               .NBYTES - total number of (uncompressed) bytes in the FITS file
       When FITS open is called with the /WRITE or /APPEND option, FCB
       contains:
           FCB.FILENAME - name of the input file
               .UNIT - unit number the file is opened to
               .NEXTEND - number of extensions in the file.
               .OPEN_FOR_WRITE - flag (1=open for write, 2=open for append
                                       3=open for update)

       Open a FITS file for reading:
               FITS_OPEN,'myfile.fits',fcb
       Open a new FITS file for output:
               FITS_OPEN,'newfile.fits',fcb,/write

       HPRINT, SXDELPAR, SXPAR()

       Written by:     D. Lindler      August, 1995
       July, 1996      NICMOS  Modified to allow open for overwrite
                               to allow primary header to be modified
       DJL Oct. 15, 1996   corrected to properly extend AXIS when more
                       than 100 extensions present
       Converted to IDL V5.0   W. Landsman   September 1997
       Use Message = '' rather than !ERR =1 as preferred signal of normal
           operation   W. Landsman  November 2000
       Lindler, Dec, 2001, Modified to use 64 bit words for storing byte
             positions within the file to allow support for very large
             files
       Work with gzip compressed files W. Landsman    January 2003
       Fix gzip compress for V5.4 and earlier  W.Landsman/M.Fitzgerald Dec 2003
       Assume since V5.3 (STRSPLIT, OPENR,/COMPRESS) W. Landsman Feb 2004
       Treat FTZ extension as gzip compressed W. Landsman Sep 2004

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