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      Read a FITS (primary or extension) header into a string array.


      HEADFITS() supports several types of compressed files including
      gzip (.gz), Unix compressed (.Z), Bzip2 (.bz2) or FPACK (.fz
      http://heasarc.gsfc.nasa.gov/fitsio/fpack/ )

Calling Sequence

      Result = HEADFITS(Filename/Fileunit ,[ ERRMSG =, EXTEN= , COMPRESS=,
                                            /SILENT ])


      Filename = String containing the name of the FITS file to be read.
                If set to an empty string, then user will be prompted for name.
                File names ending in '.gz' are assumed to be gzip'ed compressed
                and under Unix file names ending in '.Z' are assumed to be
                Unix compressed, and file names ending in .bz2 are assumed to
                be bzip2 compressed. If this default behaviour is not
                sufficient then use the COMPRESS keyword.
      Fileunit - A scalar integer specifying the unit of an already opened
                  FITS file. The unit will remain open after exiting
                  HEADFITS(). There are two possible reasons for choosing
                  to specify a unit number rather than a file name:
          (1) For a FITS file with many extensions, one can move to the
              desired extensions with FXPOSIT() and then use HEADFITS(). This
              is more efficient that repeatedly starting at the beginning of
              the file.
          (2) For reading a FITS file across a Web http: address after opening
              the unit with the SOCKET procedure.

Optional Input Keywords

      EXTEN = Either an integer scalar, specifying which FITS extension to
              read, or a scalar string specifying the extension name (stored
              in the EXTNAME keyword). For example, to read the header of
              the first extension set EXTEN = 1. Default is to read the
              primary FITS header (EXTEN = 0). The EXTEN keyword cannot
              be used when a unit number is supplied instead of a file name.
    COMPRESS - If this keyword is set and non-zero, then treat the file
              as compressed. If 1 assume a gzipped file. Use IDL's
              internal decompression facilities for gzip files, while for
              Unix or bzip2 compression spawn off a process to decompress and
              use its output as the FITS stream. If the keyword is not 1,
              then use its value as a string giving the command needed for
              decompression. See FXPOSIT for more info.
    /SILENT - If set, then suppress any warning messages about invalid
              characters in the FITS file.

Optional Keyword Output

      ERRMSG = If this keyword is present, 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.


      Result of function = FITS header, string array


      Print the main FITS header of a file 'test.fits' into a string
      variable, h
      IDL> print, headfits( 'test.fits')
      Print the second extension header of a gzip compressed FITS file
      'test.fits.gz'. Use HPRINT for pretty format
      IDL> hprint, headfits( 'test.fits.gz', ext=2)
      Read the extension named CALSPEC
      IDL> hprint,headfits('test.fits.gz',ext='CALSPEC')

Procedures Called

      The version of fxposit.pro must be post- May 2009.

Modification History

      adapted by Frank Varosi from READFITS by Jim Wofford, January, 24 1989
      Keyword EXTEN added, K.Venkatakrishna, May 1992
      Make sure first 8 characters are 'SIMPLE' W. Landsman October 1993
      Check PCOUNT and GCOUNT W. Landsman December 1994
      Major rewrite, work for Unix gzip files, W. Landsman April 1996
      Added COMPRESS keyword W. Landsman April 2000
      Added ERRMSG keyword W. Landsman July 2000
      Added /SILENT keyword W. Landsman December 2000
      Option to read a unit number rather than file name W.L October 2001
      Test output status of MRD_HREAD call October 2003 W. Landsman
      Allow extension to be specified by name Dec 2006 W. Landsman
      No need to uncompress FPACK compressed files May 2009 W. Landsman
      Use V6.0 notation W.L. Feb. 2011

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