Friday, September 21, 2007

Beam Squint and Self Calibration (relevant even if you don't care about polarization!)

Here's another tip from Juan Uson about a self-cal trick to get the very best dynamic range in your images.

The VLA has beam squint in Stokes V. What this means is that away from the center of your field, the RR and LL polarizations separate a bit (I don't really understand the details of this). This leads to an image artifact-- if you have a bright source off the center of your field, it'll seem like you can't clean it very well. Even if you've done your very best job at self calibration and cleaned very deeply, there will still remain rings around the source (which of course drives up your noise).

One easy fix for this problem is to do a normal Phase self calibration, but when it's time for your A&P self calibration, average your RR and LL. I believe the relevant parameter for this in CALIB is APARM(3) > 1. This should give you a self cal that is not affected by the beam squint, and models your bright sources better!

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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!

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Wednesday, September 19, 2007

Filling data that includes midnight

When you get your data from the archive, it's normally in several files. For me, each file is usually a day's worth of data, so I fill and calibrate each data file separately. However, if you have observations that go from say 20:00 on 17-Feb to 6:00 on 18-Feb, the archive creates a separate files for the observations on the 17th and for the observations on the 18th. You need to read in both files to calibrate that series of observations.

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Monday, September 17, 2007

Complications with Shamelessly Raiding the Archive

Amanda and I are spending the week in Socorro, so hopefully we will be learning little new AIPS facts every day! Here's my first one:

Sometimes I have dreams about raiding the VLA archive and combining like 6 different data sets. However, often times the data sets will have slightly different centers-- in my case, often one observer centers on the galaxy's center, while another observer center on a supernova that goes off in the outskirts of the galaxy, some 5 arcminutes away. I was wondering, can you just DBCON these two data sets together, and AIPS can handle it?

The answer is...

DBCON can adjust the phases, so yes, it can image data from two pointings with significantly different centers. What Miller Goss said you have to be wary of is that DBCON doesn't adjust amplitudes. So, if there are two different pointing centers, the center of your galaxy is going to fall in different locations of the primary beam in the different observations. If you are combining two observations separated by 5 arcminutes at 20cm, where the primary beam is 30 arcminutes, this shouldn't be a big issue...probably the primary beam sensitivity of the two pointings will only vary by a few percent. However, if the separation of your pointings starts approaching the size of the primary beam, then you can no longer use DBCON and need to think of some other way to combine the data.

Read more!