HEXTRACT

       Extract a subimage from an array and update astrometry in FITS header

       Extract a subimage from an array and create a new FITS header with
       updated astrometry for the subarray

       HEXTRACT, Oldim, Oldhd, [ Newim, Newhd, x0, x1, y0, y1, /SILENT ]
               or
       HEXTRACT, Oldim, Oldhd, [x0, x1, y0, y1, /SILENT, ERRMSG =  ]

       Oldim - the original image array
       Oldhd - the original image header

       x0, x1, y0, y1 - respectively, first and last X pixel, and first and
       last Y pixel to be extracted from the original image, integer scalars.
       If omitted,  HEXTRACT will prompt for these parameters

       Newim - the new subarray extracted from the original image
       Newhd - header for newim containing updated astrometry info
               If output parameters are not supplied or set equal to
               -1, then the HEXTRACT will modify the input parameters
               OLDIM and OLDHD to contain the subarray and updated header.

       /SILENT - If set and non-zero, then a message describing the extraction
               is not printed at the terminal.   This message can also be
               suppressed by setting !QUIET.

       ERRMSG - If this keyword is supplied, then any error mesasges will be
               returned to the user in this parameter rather than depending on
               on the MESSAGE routine in IDL.   If no errors are encountered
               then a null string is returned.

       The FITS header parameters NAXIS1, NAXIS2, CRPIX1, and CRPIX2 are
       updated for the extracted image.

       Read an image from a FITS file 'IMAGE', extract a 512 x 512 subimage
       with the same origin, and write to a new FITS file 'IMAGENEW'
       IDL> im = READFITS( 'IMAGE', hdr )      ;Read FITS files into IDL arrays
       IDL> hextract, im, h, 0, 511, 0, 511    ;Extract 512 x 512 subimage
       IDL> writefits, 'IMAGENEW', im ,h       ;Write subimage to a FITS file

       CHECK_FITS, STRN(), SXPAR(), SXADDPAR, SXADDHIST

       Written, Aug. 1986 W. Landsman, STX Corp.
       Use astrometry structure,   W. Landsman      Jan, 1994
       Minor fix if bad Y range supplied   W. Landsman    Feb, 1996
       Added /SILENT keyword              W. Landsman     March, 1997
       Converted to IDL V5.0   W. Landsman   September 1997
       Added ERRMSG keyword    W. Landsman   May 2000

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