EVLA Wide-Band Wide-Field Imaging: G55.7 3.4

Overview

This CASA Guide describes the imaging of the supernova remnant G55.7+3.4.. The data were taken on August 23, 2010, in the first D-configuration for which the new wide-band capabilities of the WIDAR correlator were available. The 8-hour-long observation includes all available 1 GHz of bandwidth in L-band, from 1-2 GHz in frequency.

The measurement set (MS) which provides a starting point for this Guide has been imported from the SDM file set and time-averaged to 10 seconds. Both of these options are available from the NRAO data archive, and since this operation can be relatively time-consuming, the imported, time-averaged dataset is being provided. The original data set can be found [here]; note that it is 170 GB in raw form.

Averaging to 10 seconds and the removal of some scans which are not used in this tutorial reduces the size of the data set to around 14 GB; the addition of columns for model and corrected data (known as "scratch columns") returns it to 43 GB, which is the size of the MS which will be used here.

[Instructions on locating data at the workshop... and from outside the workshop.]

Preliminary data evaluation

Start CASA by typing casapy-stable on the command line. If you have not used CASA before, some helpful tips are available on the Getting Started in CASA page.

As a first step, use listobs to have a look at the MS:

# In CASA
listobs('G55.7+3.4_10s.ms')

The logger output will look like this:

A priori calibration and flagging

Before we proceed with further processing, we should check the observation log to see if there were any issues noted during the run that need to be addressed. The observing log file is linked to the [archive web page] for this observation (at far right; under "logs etc."). Looking at the log, we can see that antenna ea07 may need a position correction, and antennas ea06, ea17, ea20, and ea26 did not have L-band receivers installed at the time and should be flagged.

Antenna position correction

Correcting the position of an antenna is done with the task gencal. This is important, because the observed visibilities are a function of [math]\displaystyle{ u }[/math] and [math]\displaystyle{ v }[/math]. If an antenna's position is incorrect, then [math]\displaystyle{ u }[/math] and [math]\displaystyle{ v }[/math] will be calculated incorrectly, and there will be errors in any image derived from the data. (These corrections could also be determined later by a baseline-based calibration incorporating the [math]\displaystyle{ b_{ij} }[/math] term from the equation above, but since they are known a priori it makes sense to incorporate them now.)

Any corrections can be ascertained from the EVLA/VLA Baseline Corrections site. For future reference, be sure to read to the bottom of that document to see how to calculate the additive corrections. Fortunately, the current case is simple as there is only a single correction for each antenna. The calculations are inserted via gencal. Currently these must be done by hand, though the plan is for future releases of CASA to have an automated lookup of the corrections.

Automatic RFI excision

Evaluating results & manual flagging

Flagging lowest edge of SPW 0 ...