HASTROM

       Linear transformation of an image to align it with a reference image

       A linear transformation is applied (using POLY_2D) to an image so that
       its astrometry is identical with that in a reference header.  This
       procedure can be used to align two images.

       HASTROM, oldim, oldhd, newim, newhd, refhd, [MISSING =, INTERP = ]
                            or
       HASTROM, oldim, oldhd, refhd, [MISSING =, INTERP ={0,1,2}, NGRID =,
                                      CUBIC =, DEGREE = ]

       OLDIM - Image array to be manipulated.  If only 3 parameters are
               supplied then OLDIM and OLDHD will be modified to contain
               the output image array and header
       OLDHD - FITS header array for OLDIM, containing astrometry parameters
       REFHD - Reference header, containing astrometry parameters.  OLDIM
               will be rotated, shifted, and compressed or expanded until
               its astrometry matches that in REFHD.

       NEWIM - Image array after linear tranformation has been performed.
               The dimensions of NEWIM will be identical to the NAXIS1 and
               NAXIS2 keywords specified in REFHD.  Regions on the reference
               image that do not exist in OLDIM can be assigned a value with
               the MISSING keyword.
       NEWHD - Updated FITS image header associated with NEWIM

       MISSING - Set this keyword to a scalar value which will be assigned
               to pixels in the output image which are out of range of the
               supplied imput image.  If not supplied, then linear
               extrapolation is used.   See the IDL manual on POLY_2D.
               ***NOTE: A bug was introduced into the POLY_2D function in IDL
               V5.5 (fixed in V6.1) such that the MISSING keyword
               may not work properly with floating point data***
       INTERP - Scalar, one of 0, 1, or 2 determining type of interpolation
               0 nearest neighbor, 1 (default) bilinear interpolation,
               2 cubic interpolation.
       CUBIC - a scalar value between -1 and 0 specifying cubic interpolation
               with the specified value as the cubic interpolation parameter.
              (see poly_2d for info).    Setting CUBIC to a value greater
               than zero is equivalent to setting CUBIC = -1.
       NGRID -  Integer scalar specifying the number of equally spaced grid
               points on each axis to use to specify the transformation.
               The value of NGRID must always be greater than DEGREE + 1.
               The default is DEGREE + 2 which equals 3 (9 total points) for
               DEGREE=1 (linear warping).
       DEGREE - Integer scalar specifying the degree of the transformation.
               See the routine POLYWARP for more info.   Default =
               1 (linear transformation) unless polynomial ('SIP') distortion
               parameters are present in either the input or reference FITS
               header.    In that case, the default degree is equal to the
               degree of the distortion polynomial.

       ERRMSG - If this keyword is supplied, then any error messages 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.

       (1) The 3 parameter calling sequence is less demanding on virtual
               memory.
       (2) The astrometry in OLDHD will be precessed to match the equinox
                given in REFHD.
       (3) If an ST Guidestar image is used for the reference header, then the
                output header will be converted to standard astrometry.

       Suppose one has an image array, IM, and an associated FITS header H.
       One desires to warp the image array so that it is aligned with another
       image with a FITS header, HREF.    Both headers contain astrometry info.
       Set pixel values to 0 where there is no overlap between the input and
       reference image, and use linear interpolation (default)
       IDL> hastrom, IM, H, HREF, MISSING = 0

       ad2xy, check_FITS, extast, get_EQUINOX(), gsssextast, hprecess,
       putast, sxaddpar, sxaddhist, sxpar(), xy2ad, zparcheck

       Written  W. Landsman, STX Co.              Feb, 1989
       Updated to CHECK_FITS                      Dec, 1991
       New astrometry keywords                    Mar, 1994
       Recognize GSSS header   W. Landsman        June, 1994
       Added CUBIC keyword     W. Landsman        March, 1997
       Converted to IDL V5.0   W. Landsman        September 1997
       Accept INTERP=0, Convert output GSS header to standard astrometry
                               W. Landsman        June 1998
       Remove calls to obsolete !ERR system variable   March 2000
       Added ERRMSG output keyword  W. Landsman    April 2000
       Need to re-extract astrometry after precession  W. Landsman Nov. 2000
       Check for distortion parameters in headers, add more FITS HISTORY
       information    W. Landsman   February 2005

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