HBOXAVE

       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.

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

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

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

       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.

       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

       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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