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!

1 comment:

rob said...

i don't understand why this would help. frazer owen recommends treating RR and LL separately in this situation, maybe even going so far as to split into RR-IF1, RR-IF2, LL-IF1 and LL-IF2.

mind you, i've tried both approaches (averaging LL+RR and treating them completely separately) and neither has helped a great deal!