VLA Data Combination-CASA4.6.0

'This tutorial was created and tested using CASA 4.6.0

Introduction

The VLA can be configured into [four principal array configurations, A, B, C, and D.] A is the most extended and D the most compact configuration. Consequently, A configuration data has the highest spatial resolution whereas D delivers the best surface brightness sensitivity and also images the largest scales on the sky distribution. The best possible picture of an object is to combine different array combinations.

In this tutorial, we will combine data from the surroundings of Sgr A*, the central, supermassive black hole of our Milky Way.

Typical Observation times

When an object is being observed by the VLA in different configurations, ideally integration times are matched by their surface brightness sensitivity. Adjacent VLA configurations result in synthesized beams that differ in linear size by are approximately a factor of 3. The beam area is thus about 10 times different and the more extended configuration would ideally need to be 10 times longer than the most compact one. This, however, is frequently not very practical and it turns out that integration times that differ by factors of 3 are delivering data that can be combined quite well.

This rule, however, is only a guidance and any data that is being obtained can be combined. Weighting will then be primarily achieved by the image "Briggs" scheme that produces weights between the "natural" and "uniform" extremes, i.e. between weighting by the number of visibilities that are gridded in each cell and weighting by the cells themselves.

In addition, each visibility exhibits weights that should only depend on integration time, bandwidth, and system temperature. The VLA, however, currently does not measure Tsys and weights between different observations will need to be adjusted as described below.

The data

In the following we will combine three different datasets from the [NRAO Monitoring of the Galactic Center/G2 Cloud Encounter]. We will combine S-band A, B, and C configuration data. At this stage, the data were all calibrated and the science target split out.

The measurement sets can be downloaded here:

[Sgr A* A-configuration] [Sgr A* B-configuration] [Sgr A* C-configuration]

As a first step, let's download the files, then untar:

tar -xzvf VLA-SgrA-Sband-Aconfig.ms.tar.gz
tar -xzvf VLA-SgrA-Sband-Bconfig.ms.tar.gz
tar -xzvf VLA-SgrA-Sband-Cconfig.ms.tar.gz

There will now be three unpacked MeasurementSets, one for each configuration.

Initial Imaging

To start with, we will make first, quick images to check the integrity of the data.

 = False\nFri Aug 9 20:28:30 2013  HISTORY applycal [] calwt       = [True]\nFri Aug 9 20:28:30 2013  HISTORY applycal [] applymode   = "calflag"\nFri Aug 9 20:28:30 2013  HISTORY applycal [] flagbackup  = True\nFri Aug 9 20:28:33 2013  HISTORY imager::data selection [mode=none nchan=[-1] start=[0] step=[1] mstart=\'Radialvelocity: 0\' mstep=\'Radialvelocity: 0\' spectralwindowids=[] fieldids=[] msselect=] \nFri Aug 9 20:28:37 2013  HISTORY imager::defineimage() [Defining image properties:nx=1500 ny=1500 cellx=\'0.689049arcsec\' celly=\'0.689049arcsec\' stokes=I\' mode=MFS nchan=-1 start=0 step=1 spwids=[-1] fieldid=0 facets=1 frame=5 distance=\'0\',  phaseCenter=\'field-0 \' mStart=\'Radialvelocity: 0\' qStep=\'0 \'\' mFreqStart=\'Frequency: 0] \nFri Aug 9 20:28:40 2013  HISTORY imager::weight() [] Weighting MS: Imaging weights will be changed\nFri Aug 9 20:28:40 2013  HISTORY imager::weight() [] Briggs weighting: sidelobes will be suppressed over full image\nFri Aug 9 20:36:25 2013  HISTORY imager::clean() [] Clean gain = 0.1, Niter = 900, Threshold = 0 mJy\nFri Aug 9 20:36:25 2013  HISTORY imager::clean() [] Continuing deconvolution\nFri Aug 9 20:38:35 2013  HISTORY imager::clean() [] Threshhold not reached yet.\nFri Aug 9 20:38:36 2013  HISTORY imager::iClean() [] Restoring Image(s) with the clean-beam']