CLEAN

The radio world uses CLEAN—and an infinite variety of alternatives and modifications thereto—for building radio interferometry maps. Today I met with Fabian Walter (MPIA), Frank Bigiel (Heidelberg), Tom Herbst (MPIA), and Rix to discuss probabilistic replacements. While we don't have a plan, we were able to come up with the following reasons that CLEAN needs improvement (these all apply to vanilla CLEAN; there are non-vanilla versions that fix some of these issues but not all, to my knowledge): (1) It is some kind of optimizer, but a heuristic optimizer (that is, there are optimizers out there with better properties) and it is not optimizing a well-specified (let alone justified) scalar objective. (2) It requires heuristic decision-making about stopping. (3) The noise in the final map it makes can only be estimated by taking the rms in empty regions; it has no error or noise model and doesn't propagate uncertainties from the fundamental visibilities (let alone calibration uncertainties and so on). (4) The final map it makes has some flux convolved with the clean beam, and some fainter flux convolved with the dirty beam; this means that there is no well-defined point-spread function and the absolute calibration of the map is ill-defined. (5) It provides no mechanism for producing or quantitatively comparing alternative maps; that is, it produces only a point estimate, no judgement of that estimate or sampling around that estimate. (6) It requires binning of the visibility data in the u-v plane. (7) There is no mechanism by which the map it produces is standardly or by convention projected back into the space of visibilities and compared to the raw data to vet or confirm any fundamental or even heuristic noise model.

In addition to all this I could add things about the scene model and priors on the scene model and so on, but these aren't really the problem of CLEAN itself. In other news, Mykytyn continued working on getting large numbers of RC3 galaxies measured by The Tractor and even started writing up some of his work.