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CONVOLVE

CONVOLVE

## Purpose

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

## Explanation

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 )
or:
correl = convolve( image1, image2, /CORREL )
or:
correl = convolve( image, /AUTO )

## Inputs

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

## Method

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

## History

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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