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      CONGRID an image and update astrometry in a FITS header


      Expand or contract an image using CONGRID and update the
      associated FITS header array.

Calling Sequence

      HCONGRID, oldhd ;Update FITS header only
      HCONGRID, oldim, oldhd, [ newim, newhd, newx, newy, /HALF_HALF
                                CUBIC = , INTERP=, OUTSIZE=, ERRMSG=, ALT= ]


      OLDIM - the original image array
      OLDHD - the original image FITS header, string array

Optional Inputs

      NEWX - size of the new image in the X direction
      NEWY - size of the new image in the Y direction
              The OUTSIZE keyword can be used instead of the
              NEWX, NEWY parameters

Optional Outputs

      NEWIM - the image after expansion or contraction with CONGRID
      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 Inputs

      ALT - Single character 'A' through 'Z' or ' ' specifying which astrometry
          system to modify in the FITS header. The default is to use the
          primary astrometry of ALT = ' '. See Greisen and Calabretta (2002)
          for information about alternate astrometry keywords.
      CUBIC - If set and non-zero, then cubic interpolation is used. Valid
              ranges are -1 <= Cubic < 0. Setting /CUBIC is equivalent to
              CUBIC = -1 and also equivalent to INTERP = 2. See INTERPOLATE
              for more info. Setting CUBIC = -0.5 is recommended.
      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.
      /HALF_HALF - Due to edge effects, the default behaviour of CONGRID is
          to introduce a slight shift in the image center. Craig Markwardt
          (http://cow.physics.wisc.edu/~craigm/idl/misc.html) has written
          a modified version of CONGRID called CMCONGRID that when used with
          the /HALF_HALF keyword eliminates any shift. The use of the
          /HALF keyword emulates CMCONGRID and eliminates any shift in the
          image centroid.
      INTERP - 0 for nearest neighbor, 1 for bilinear interpolation
              (default), 2 for cubic (=-1) interpolation.
      OUTSIZE - Two element integer vector which can be used instead of the
              NEWX and NEWY parameters to specify the output image dimensions

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.


      Expansion or contraction is done using the CONGRID function, unless
      HALF_HALF is set.
      The parameters BSCALE, NAXIS1, NAXIS2, CRPIX1, and CRPIX2 and
      the CD (or CDELT) parameters are updated for the new header.


      A FITS header can be supplied as the first parameter without having
      to supply an image array. The astrometry in the FITS header will be
      updated to be appropriate to the specified image size.
      If the FITS header contains astrometry from a ST Guide Star image,
      then the astrometry will be converted to an approximately equivalent
      tangent projection before applying CONGRID.


      Congrid an 512 x 512 image array IM and FITS header H to size 300 x 300
      using cubic interpolation. Use the HALF_HALF keyword to avoid
      a shift of the image centroid
      IDL> hcongrid, IM ,H, OUT = [300, 300], CUBIC = -0.5, /HALF
      The variables IM and H will be modified to the new image size.

Procedures Called


Modification History

      Written, Aug. 1986 W. Landsman, STI Corp.
      Added interp keywords, J. Isensee, July, 1990
      Add cubic interpolation W. Landsman HSTX January 1994
      Recognize a GSSS FITS header W. Landsman June 1994
      Fix case where header but not image supplied W. Landsman May 1995
      Remove call to SINCE_VERSION() W. Landsman March 1996
      Assume since IDL V3.5, add CUBIC keyword W. Landsman March 1997
      Update BSCALE even if no astrometry present W. Landsman May 1997
      Converted to IDL V5.0 W. Landsman September 1997
      Added HALF_HALF keyword W. Landsman February 2000
      Added ERRMSG keyword, use double precision formatting W.L. April 2000
      Recognize PC00n00m astrometry format W. Landsman December 2001
      Now works when both /INTERP and /HALF are set W. Landsman January 2002
      Fix output astrometry for non-equal plate scales for PC matrix or
      CROTA2 keyword, added ALT keyword. W. Landsman May 2005
      Update distortion parameters if present W. Landsman January 2008
      Don't update BSCALE/BZERO for unsigned integer W.Landsman Mar 2008
      Write CRPIX as Double precision if necessary W. Landsman Oct 2012

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