Get the data - eakadams/ihow-hi GitHub Wiki

Retrieve data from the archive

To retrieve the raw data, go to the NRAO archive: https://data.nrao.edu/. Click "Show search inputs" and enter appropriate inputs for finding this data. First, use the "resolver" to enter the R.A. and Dec. for Sextans B and use a small search radius of 15arcseconds. Select the telescope as both "VLA" and "EVLA" (this is before and after the major VLA upgrades). Select "L" as the receiver as HI observations are in L-band (this also means it is not necessary to specify a frequency). Finally, select "D" configuration.

After you search, you should see a result with the title "VLA and HST: Star Formation History and ISM Feedback in Nearby Galaxies"; this is the VLA ANGST program. Expand that result by clicking the "+" icon. Select the available dataset for download, and then click the download button. Follow the instructions in the NRAO emails for downloading the data once it is prepared. Generally, using the wget command is the most robust and recommended.

Load data into CASA

As this is older VLA data, we will need to use the task importvla. Check the parameters of this task:

CASA <5>: inp importvla                                                        
# importvla -- Import VLA archive file(s) to a measurement set
archivefiles  = []                      # Name of input VLA archive file(s)
vis           = ''                      # Name of output visibility file
bandname      = ''                      # VLA frequency band name:''=>obtain all bands in the archive file
frequencytol  = '150000.0Hz'            # Frequency shift to define a unique spectra window (Hz)
project       = ''                      # Project name: '' => all projects in files
starttime     = ''                      # Start time to search for data
stoptime      = ''                      # End time to search for data
applytsys     = True                    # Apply nominal sensitivity scaling to data and weights
autocorr      = False                   # Import autocorrelations to MS, if set to True
antnamescheme = 'new'                   # 'old' or 'new'; 'VA04' or '04' for VLA ant 4
keepblanks    = False                   # Fill scans with blank (empty) source names (e.g. tipping scans)
evlabands     = False                   # Use updated eVLA frequencies and bandwidths for bands and wavelengths

And run the command on the CASA command line:

importvla(archivefiles='AO215/observation.54681.7643171/AO215_1_54681.76432_54681.80529.exp',vis='ao215_54681.ms')

Note there are errors about leap second tables. Leap second accuracy is not needed for the resolutions we are working at, but it is possible to update these tables.

Check the content of the dataset with listobs:

listobs(vis='ao215_54681.ms')

This is a simple observation with a primary calibrator, phase calibrator, and single target scan.

Check the antenna layout:

plotants(vis='ao215_54681.ms')

You can see this project was observed in the VLA -> JVLA transition based on the antenna naming scheme; those with an "E" had been upgraded and those with a "V" were classic.