A priori models of sources can be used to good effect in `CLEAN'. A good example is in the `CLEAN'ing of images of planets. In this case the visibility function of a circular disk can be subtracted from the observed visibilities before making the dirty image. `CLEAN' is then asked only to find the small perturbations from the disk model, so the image quality and speed of convergence can both be improved.
1996 November 4