Monday, April 16, 2007

Flux Conservation in Self Calibration?

Any one know what self calibration really does to your flux? I need fluxes of sources that are mostly point-ish, and I like self calibrating because it makes them look more like circles and less like amorphous blobs. But I wonder if I can trust my fluxes after self cal? If I do only phase self cal, should I be ok?

I found in someone's Ph.D. thesis that your fluxes are no longer "absolute" after self calibration. I guess because you're no longer comparing everything with 3C286, you're comparing your target source with itself.

Anyway, when i just do an imean on my images, the total flux density doesn't change a huge amount. Maybe 20% (well, sometimes I get factors of 2, but I avoid those cases). I don't really want to be introducing 20% errors into my flux measurements, though....
Maybe I shouldn't be self caling if I want robust fluxes?

Anyone really understand what cparm(2) does in calib? Is it only important for A&P self cal, or P self cal too?
Squawk!

6 comments:

amanda said...
This comment has been removed by the author.
amanda said...

Did you compare the fluxes after you amplitude self-calibrated or after you just phase self-calibrated? I wonder if there's a difference. Just phase self-calibrating seems to me less risky than amplitude self-calibrating.

Laura said...

You know what? I'm kinda dumb. I was looking at the flux densities generated by imean, but those are confusing because a slight change in the background totally changes the total flux density of the image.
So instead I started looking at clean component flux, and that makes things significantly clearer.

I agree with you very much, that amp and phase self cal is a lot sketchier. When I started looking at the cc fluxes, when I would phase self cal, the fluxes would change reasonable and believably. However, if I then did a A&P self cal, often times the fluxes would decrease by a factor of 2!! (the noise would also decrease significantly, but not by a factor of 2). So for C band VLA data, I definitely am scared of A&P self cal. Am still deciding how sketchy it is at 20 cm.

amanda said...

According to Rick Perley's High Dynamic Range Imaging chapter in Synthesis Imaging in Radio Astronomy II, "When employing amplitude solutions, it is usual and advisable to 'float' the gains -- renomalize the gain solution so that the mean solution is of unit magnitude. This prevesnts the gain solution from being affected by the model having too little flux, thus systematically decreasing the total apparent flux density of the source" (pg. 291) So setting cparm(2) = 1 is a good thing, I think.

Laura said...

I would definitely believe that cparm(2) is good to set, but I don't actually think that it makes a huge difference. My flux changes by a factor of 2 regardless of the value of cparm that I set.

Sam Tun said...

Ok, I think this is the best spot to ask this. I've been working on some self-cal programs for the Owens Valley Solar Array, and I am having similar problems as these. I would want a self-caled map that gives me a good approximation to my CLEAN map fluxes, but no go on that. So, my question is, does anyone know what is done to the clean components to get them to return appropriate visibilities to feed into self-cal? I believe that in AIPS you feed the CLEAN map as a model into calib, but does anyone know the details of what goes on in there (or where to find them)? Gracias.