Friday, September 21, 2007

Clean Components and Self Calibration

Well, I just talked to Juan Uson from NRAO-Charlottesville, and he delivered some useful information for us burgeoning self-cal experts.

He says that you should ALWAYS use all your clean components for self calibration. Don't cut at the first negative! He says that when they were first learning about self cal, they were very timid and therefore advised people to cut at the first negative, but now they know better. You want to use your very best clean component model for self cal, which includes all components!

Here are some reasons why--

a bright source that is centered on the edge between two pixels will actually take an infinite number of clean components to model, because basically the clean algorithm keeps exchanging flux back and forth between the surrounding pixels. so, to model such a source, there are lots of negative clean components required.

if a source has any complex structure at all, you can't really effectively model it without using some negative components.

I told him that occasionally, the word on the street is "Just self cal off of one bright source that you know the structure of, and leave the rest of the flux alone." When I tried to do that once, the self cal created fictional ghost sources in my data and it scared me! Juan was not surprised; he said its a terrible idea to self cal on only a select few sources. Basically what self cal does is divide your real fluxes by your model fluxes, and try to make the residuals look like noise. If there is still real flux in your residuals (because your clean model is based off only a select few sources), then it will try to turn that real flux into noise and crazy things will happen.

So, clean out all your flux, and use all your clean components!


Ian said...

This is interesting, but is it still true in the case of a direct Fourier transform? Does it only arise from FFT gridding...?

Laura said...

Unfortunately, I'm really not sure. Do you choose in IMAGR if you want to do an FFT or direct Fourier transform?

I've never thought about such differences before...