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      Box average an image array and update the FITS header array


      The function BOXAVE() is used. This procedure is recommended for
      integer images when photometric precision is desired, because it
      performs intermediate steps using REAL*4 arithmetic. Otherwise, the
      procedure HREBIN is much faster.

Calling Sequence

      HBOXAVE, Oldim, Oldhd, Newim, Hewhd, box
      HBOXAVE, Oldim, Oldhd, box


      Oldim - the original image array
      Oldhd - the original image FITS header, string array

Optional Inputs

      box - the box size to be used, integer scalar. If omitted, then
              HBOXAVE will prompt for this parameter.

Optional Outputs

      Newim - the image after boxaveraging
      Newhd - header for newim containing updated astrometry info
              If output parameters are not supplied, the program
              will modify the input parameters OLDIM and OLDHD
              to contain the new array and updated header.

Optional Keyword Output

      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 parameters BSCALE, NAXIS1, NAXIS2, CRPIX1, and CRPIX2 and
      the CD (or CDELT) parameters are updated for the new FITS header.


      Compress the image in a FITS file 'image.fits' by a factor of 4 and
      update the astrometry in the FITS header
      IDL> im = readfits('image.fits',hdr) ;Read FITS file into IDL arrays
      IDL> hboxave, im, hdr, 4 ;Boxaverage by 4
      IDL> writefits,'image.fits',im,hdr ;Write a new FITS file
      CHECK_FITS - Check that the FITS header is appropriate to the image
      BOXAVE() - Performs box averaging of an image
      SXPAR(), SXADDPAR - Read and write FITS keyword values

Modification History

      Written, Aug. 1986 W. Landsman, STI Corp.
      IDLV2 changes, sxaddpar format keyword added, J. Isensee, July,1990
      Fix 0.5 pixel offset in new CRPIX computation W. Landsman, Dec, 1991
      Update BSCALE even if no astrometry present W. Landsman, May 1997
      Converted to IDL V5.0 W. Landsman September 1997
      Added ERRMSG keyword, Use double formatting W. Landsman April 2000
      Recognize PC matrix astrometry format W. Landsman December 2001

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