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      Convolution of an image with a Point Spread Function (PSF)


      The default is to compute the convolution using a product of
      Fourier transforms (for speed).
      The image is padded with zeros so that a large PSF does not
      overlap one edge of the image with the opposite edge of the image.
      This routine is now partially obsolete due to the introduction of the
      intrinsic CONVOL_FFT() function in IDL 8.1

Calling Sequence

      imconv = convolve( image1, psf, FT_PSF = psf_FT )
      correl = convolve( image1, image2, /CORREL )
      correl = convolve( image, /AUTO )


      image = 2-D array (matrix) to be convolved with psf
      psf = the Point Spread Function, (size < or = to size of image).
      The PSF *must* be symmetric about the point
      FLOOR((n_elements-1)/2), where n_elements is the number of
      elements in each dimension. For Gaussian PSFs, the maximum
      of the PSF must occur in this pixel (otherwise the convolution
      will shift everything in the image).

Optional Input Keywords

      FT_PSF = passes out/in the Fourier transform of the PSF,
              (so that it can be re-used the next time function is called).
      FT_IMAGE = passes out/in the Fourier transform of image.
      /CORRELATE uses the conjugate of the Fourier transform of PSF,
              to compute the cross-correlation of image and PSF,
              (equivalent to IDL function convol() with NO rotation of PSF)
      /AUTO_CORR computes the auto-correlation function of image using FFT.
      /NO_FT overrides the use of FFT, using IDL function convol() instead.
              (then PSF is rotated by 180 degrees to give same result)
      /NO_PAD - if set, then do not pad the image to avoid edge effects.
              This will improve memory and speed of the computation at the
              expense of edge effects. This was the default method prior
              to October 2009


      When using FFT, PSF is centered & expanded to size of image.


      written, Frank Varosi, NASA/GSFC 1992.
      Appropriate precision type for result depending on input image
                              Markus Hundertmark February 2006
      Fix the bug causing the recomputation of FFT(psf) and/or FFT(image)
                              Sergey Koposov December 2006
      Fix the centering bug
                              Kyle Penner October 2009
      Add /No_PAD keyword for better speed and memory usage when edge effects
            are not important. W. Landsman March 2010
      Add warning when kernel type does not match integer array
            W. Landsman Feb 2012
      Don't force double precision output W. Landsman July 2014

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