  The principles of difference imaging outlined and the technique of Alard and
Lupton (1997) is generalised to generate the best possible difference images to
within the limits of measurement error. It is shown how for a large database of
images we can approach diffraction-limited spatial resolution to within
measurement error of a combined image. This is achieved through an iterative
procedure of difference imaging and deconvolution. The design of the ideal
imager to approach this limit is discussed.
