MRD_HREAD

     Reads a FITS header from an opened disk file or Unix pipe

     Like FXHREAD but also works with compressed Unix files

     MRD_HREAD, UNIT, HEADER  [, STATUS, /SILENT ]

     UNIT    = Logical unit number of an open FITS file

     HEADER  = String array containing the FITS header.

     STATUS  = Condition code giving the status of the read.  Normally, this
                 is zero, but is set to -1 if an error occurs, or if the
                 first byte of the header is zero (ASCII null).

      /SILENT - If set, then warning messages about any invalid characters in
                the header are suppressed.
      /FIRSTBLOCK - If set, then only the first block (36 lines or less) of
                the FITS header are read into the output variable.   If only
                size information (e.g. BITPIX, NAXIS) is needed from the
                header, then the use of this keyword can save time.  The
                file pointer is still positioned at the end of the header,
                even if the /FIRSTBLOCK keyword is supplied.

      The file must already be positioned at the start of the header.  It
      must be a proper FITS file.

       The file ends by being positioned at the end of the FITS header, unless
       an error occurs.

      Written,  Thomas McGlynn                     August 1995
      Modified, Thomas McGlynn              January 1996
         Changed MRD_HREAD to handle Headers which have null characters
          A warning message is printed out but the program continues.
          Previously MRD_HREAD would fail if the null characters were
          not in the last 2880 byte block of the header.  Note that
          such characters are illegal in the header but frequently
          are produced by poor FITS writers.
      Converted to IDL V5.0   W. Landsman   September 1997
      Added /SILENT keyword   W. Landsman   December 2000
      Added /FIRSTBLOCK keyword  W. Landsman   February 2003

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