ATCA Continuum Polarization Tutorial NGC612-CASA4.7

From CASA Guides
Revision as of 22:18, 30 June 2016 by Mark.Wieringa (talk | contribs)
Jump to: navigation, search


This CASA Guide is for Version 4.6 of CASA. If you are using a later version of CASA and this is the most recent available guide, then you should be able to use most, if not all, of this casaguide, as we try to limit script breaking changes in CASA development.

Overview

This CASA guide describes the calibration and imaging of a two-pointing continuum data set taken with the Australia Telescope Compact Array (ATCA) of the nearby radio galaxy NGC612 NGC612. It has been adapted from the 3C391 Continuum Tutorial (CASA 4.6) for the VLA . The data were taken with 2048 MHz of bandwidth in 1 MHz channels, centered at 2.1 GHz, and recorded all polarizations. For ATCA data reduction there are two paths you can follow:

  • If you are already familiar with the Miriad package, you can do the loading and optionally the flagging and calibration of the data in Miriad and then import the data into a CASA MeasurementSet with importmiriad.
  • Alternatively, you can use importatca to load the ATCA archive files directly into CASA.

If this is your first attempt at using CASA for Compact Array data you are encouraged to start with a flagged and calibrated Miriad dataset which will be available here when this guide is complete and then try imaging the data. If you feel you are ready to tackle the full reduction job in CASA switch to [this guide]

Obtaining the Data

For the purposes of this tutorial, we have created a starting data set, upon which several initial processing steps have already been conducted. You may obtain the data set from here: available here when this guide is complete(dataset size: 2.0GB).

The steps taken to produce this dataset were:

  • The archive data was loaded into Miriad
  • Basic data flagging of the calibration source was applied. These data were taken in the 1-3 GHz band, which has a lot of interference, especially in the lower part of the band.
  • Bandpass, polarization leakage and fluxscale calibration was done using an observation of 1934-638
  • Phase calibration was solved for on a secondary calibrator
  • All the calibration was applied to the target source observations
  • Further flagging was applied to the target

Once the download is complete, unzip and unpack the file (within a working directory, which you will then run CASA):

# In a Terminal:
tar xzvf ngc612.tgz

How to Use This CASA Guide

Here are a number of possible ways to run CASA, described in more detail in Getting Started in CASA. In brief, there are at least three different ways to run CASA:

  • Interactively examining task inputs. In this mode, one types taskname to load the task, inp to examine the inputs, and go once those inputs have been set to your satisfaction. Allowed inputs are shown in blue and bad inputs are colored red. The input parameters themselves are changed one by one, e.g., selectdata=T. Screenshots of the inputs to various tasks used in the data reduction below are provided, to illustrate which parameters need to be set. More detailed help can be obtained on any task by typing help taskname. Once a task is run, the set of inputs are stored and can be retrieved via tget taskname; subsequent runs will overwrite the previous tget file.
  • Pseudo-interactively via task function calls. In this case, all of the desired inputs to a task are provided at once on the CASA command line. This tutorial is made up of such calls, which were developed by looking at the inputs for each task and deciding what needed to be changed from default values. For task function calls, only parameters that you want to be different from their defaults need to be set.
  • Non-interactively via a script. A series of task function calls can be combined together into a script, and run from within CASA via execfile('scriptname.py'). This and other CASA Tutorial Guides have been designed to be extracted into a script via the script extractor by using the method described at the Extracting_scripts_from_these_tutorials page. Should you use the script generated by the script extractor for this CASA Guide, be aware that it will require some small amount of interaction related to the plotting, occasionally suggesting that you close the graphics window and hitting return in the terminal to proceed. It is in fact unnecessary to close the graphics windows (it is suggested that you do so purely to keep your desktop uncluttered).

If you are a relative novice or just new to CASA, it is strongly recommended to work through this tutorial by cutting and pasting the task function calls provided below after you have read all the associated explanations. Work at your own pace, look at the inputs to the tasks to see what other options exist, and read the help files. Later, when you are more comfortable, you might try to extract the script, modify it for your purposes, and begin to reduce other data.

Importing the Miriad data

Getting the data into a MeasurementSet is simple using the importmiriad task. If your observation has a large range of system temperature you may want to use tsys=True to use the Tsys for the visibility weights.

# In CASA
importmiriad(mirfile='ngc612.uv',vis='ngc612ms')


Applying Parallactic Angle Calibration

Multi-scale Polarization Clean

Image Analysis and Manipulation

Constructing Polarization Intensity and Angle Images

Questions about this tutorial? Please contact the NRAO Helpdesk.

CASAguides


Last checked on CASA Version 4.6.0