Loading Data

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Overview

This tutorial details the process of obtaining EVLA data from the archive and importing the data into CASA measurement set (MS) format. (A similar process for importing EVLA data into AIPS can be found here.) In addition, the implications of data averaging are described so that one may make an informed decision about whether -- and how -- to perform frequency or time-averaging to reduce the size of the dataset.

Obtaining data from the archive

EVLA data are available from the NRAO Science Data Archive. Details and updates regarding the archive can be found on the EVLA/VLA/VLBA Data Archive web page. Here, we choose to download a publicly-available observation; note that if you are downloading proprietary data you will need to either sign into your NRAO account (using the link at the top of the archive page) or obtain the Project Access Key from the NRAO data analysts.

We want to find data associated with the project "TVER0002", so enter this into the "Project Code" under the "General Search Parameters" and submit the query. This will find two archive files; we will download the first by clicking on the checkbox next to the file name "TVER0002_sb2557689_1.55517.018916574074", entering an email address, and selecting "Create tar file". Note that this last step makes data retrieval substantially easier, since the data will comprise multiple files within a directory if no tar bundle is requested.

When the archive process is complete, an email notification is sent out with information about the download directory. Copy the data to a convenient location, and unpack the tar file by typing "tar xvf TVER0002_sb2557689_1.55517.018916574074.tar". This will create the SDM data directory, but will retain the original tar file -- to conserve disk space, you will probably wish to delete the tar file.

Starting CASA and importing the measurement set

Note that a description of importing EVLA data into AIPS can be found here.

First, be sure you have the most recent version of CASA installed. To start CASA, type "casapy"; this will start writing output to a log file called "casapy.log" as well as to the logger window, and will store any command-line input in a file called "ipython.log". (Note that a detailed description of the CASA environment, including relevant information on the Python language, can be found here.)

Initial data flagging: online flags, zero flags, and shadow flags

Choosing to average data