;put 0 values around the input image so that output image is
;4*size(input image)
function zero_around, image

nxpix = n_elements(image[*,0])
nypix = n_elements(image[0,*])

res   = dblarr(2L*nxpix, 2L*nypix)
;insert input image
res[nxpix/2L : 3L*nxpix/2L -1L, nypix/2L : 3L*nypix/2L -1L] = image

return, res

end